Gaëtan Guignard
email: guignard@univ-lyon1.fr

Thirty-three years (1986-2019) of fossil plant cuticle studies using transmission electron microscopy: a review

Keywords: fossil plant cuticles, ultrastructure, transmission electron microscopy TEM, taxonomy, palaeoenvironment, evolution

Although the cuticle is a very thin layer of a few micrometers, it is one of the main remains found among fossil plants. Apparently very fragile, ultrathin cuticle sections of few dozen nanometers thick are studied by using TEM studies and statistical measurements. The ultrastructure details yielded are of significance not only for taxonomy, but also for palaeoenvironment and evolution of fossil plant. After an historical introduction and a review of the methods to obtain and observe the ultrathin sections, although data are still not enough

Introduction

Historical insights

Our knowledge of the plant cuticle, including its naming, spelling, and wording (coticola, cuticula, kutikula, cuticule, cuticle, etc.), progressed very slowly throughout centuries and countries. It began with the term 'cuticle' used in a very wide sense for plants and/or humans and animals, including formerly for plant cellular layer(s) beneath (epidermis and others…), towards the sensu stricto 'plant cuticle' used in a modern sense as an extracellular membrane over (or mixed with) the epidermis cell wall. The primary research to date on the subject, besides probable omissions, has been discussed here.

The term cuticle originates from Latin (cuticula = cutis + cula = thin skin or small skin).

Identification of the outer parts of plants, or more precisely, allusions to the cuticle exist from ancient Greek and Latin periods; however, it was not named 'cuticula'. The French term 'cuticule' was cited by Amigues's Greek-French translation (1989) for the outermost part of the oak acorn base part (pro parte for the Greek verb παραλιθάζειν, which means 'make stony around') Luccioni (pers. com.), in the research on Theophrastus Historia plantarum p. 22; see also [START_REF] Amigues | Les traités botaniques de Théophraste[END_REF]. The English term 'cuticle' was not found in the Greek-English translation of [START_REF] Hort | Theophrastus Enquiry into plants and minor works on odours and weather signs. 2 volumes[END_REF], where 'become stony at one end or the other' was used.

However, in this English translation, Theophrastus' Greek term derma (δέρμα, = skin) has been translated to 'skin' when referring to the outer part of the olive (p. 399, Theophrastus Book IV XIV 9-10), whereas it has been translated to 'outer covering' for the outer parts of fruits in the general presentation of plants (p. 21, Theophrastus Book I 2 6; Luccioni pers. com.). The particular impermeable property of the cuticle (without naming it) is clearly noted in Book VII 141, for the frond of a fern (οἷον τὸ ἐπὶ τοῦ ἀδιάντου συμβαῖνον·νοὐδὲ γὰρ ὑγραίνεται τὸ φύλλον βρεχόμενον, ὃ δὴ ἐπίδηλόν ἐστι διὰ τὸ μὴ τὴν νοτίαν ἐπιμένειν, ὅθεν καὶ ἡ προσηγορία. γένη δὲ αὐτοῦ δύο, τὸ μὲν λευκὸν τὸ δὲ μέλαν, χρήσιμα δ' ἀμφότερα πρὸς ἔκρυσιν κεφαλῆς τριχῶν ἐν ἐλαίῳ τριβόμενα). This text is translated into English as '…for instance that which we find in wet-proof (maidenhair); the leaf does not even get wet when it is watered, nor does it catch the dew, because the dew does not rest on it...' (Hort, 1916, Vol 2 p. 135), whereas the French translation of [START_REF] Amigues | Théophraste Recherches sur les plantes : Livres III-IV, Livre VII-VIII[END_REF] is slightly different (Luccioni pers. com): '…l'humidité n'adhère pas à sa surface…' (= humidity does not adhere to its surface); however, both translations imply the outermost part of the frond has impermeable properties. [START_REF] Amigues | De la botanique à la poésie dans les Idylles de Théocrite[END_REF] commented on this fern, also used in Theocritus' Idylls XIII poem (see also [START_REF] Banks | The idylls of Theocritus, Bion, and Moschus, and the warssongs of Tirtaeus[END_REF], and explained that the Greek name ἀδίαντον means 'not wet' and uses the French words 'cuticule cireuse' (= waxy cuticle) in the comments. Although this is not directly related to our subject, this plant seems somewhat difficult to identify as several fern taxa have been proposed by the different translators; however, the species is very probably Adiantum capillus-veneris L. (Luccioni pers. com.). This taxon is among the rare ferns where the cuticle has been analysed using freeze-fracture [START_REF] Wada | Freeze-fracture observations on the plasma-membrane, the cell-wall and the cuticle of growing protonemata of Adiantum-capillus-veneris l[END_REF].

The term 'cutis' (= skin) has been used in ancient Latin texts: e.g. Pliny the Elder's Historia naturalis [Latin-French translation by [START_REF] Littré | Histoire naturelle de Pline. 2 volumes[END_REF]] used this term for the outermost part of the oak acorns (Book XVI 8 2): 'Distant inter se magnitudine, et cutis tenuitate' (they differ between them according to their size and the fineness of their skin). Columella [Latin-French translation of De re rustica by Du Bois (1844), Lib. III], from that period too, also used this term for the outermost part of a variety of grapes: '…nam quia minuti acini, et durae cutis uvas habent…' (because they have grapes with small grains and a thick skin). The oldest Latin lexicons [START_REF] Furno | De l'ELEMENTARIUM au THESAURUS : l'émancipation des lexiques latins monolingues au XV e et XVI e siècles[END_REF]) from the Roman period or a little later [Varron, from 1 BC (see the edition of [START_REF] Goetz | Terenti Varronis De lingua latina quae supersunt; recensuerunt Georgius Goetz et Fridericus Schoell; accedunt grammaticorum Varronis librorum fragmenta[END_REF]; Festus and Festo, from the II nd and VIII th century taking in account the text of Flaccus 'De uerborum significatu' from 1 BC (see Lindsay edition of 1913); Marcellus, from the IV th century (see the edition of [START_REF] Lindsay | Nonius Marcellus De compendiosa doctrina[END_REF]; Isidore of Seville, from the VI th century (see the edition of Lindsay 1911)], do not cite 'cuticula'. For instance, the text of Isodore of Seville contains a whole section on plants, but only mentions 'cutis' (Liber XVII-VIII 7 and IX 104, De rebus rusticis -De aromaticis arboribus and De herbis aromaticiis).

From the XI th century, with a great increase from the end of the XV th to the XVI th century, 'cuticula' appeared increasingly in the first general dictionaries of various languages in western Europe (or glossaries, lexicons, or equivalents), but is mostly linked with 'cutis' and human and/or animal skin, whereas it was used more accurately for botanical purposes in more specialised texts. In the 'Elementarium' of Papias from the XI th century (see the edition of Mombritius 1485 or older manual versions), 'cuticula' was noted as a diminutive of 'cutis', with a human and animal skin meaning. In 1286 (see an edition of 1460, by Gutenberg?), Balbus used the same meaning. In one of the most widespread dictionaries of that period, 'cuticula' was cited by [START_REF] Calepino | Dictionarium. Ambrosii calepini bergomatis eremitani dictionarium[END_REF] as a diminutive of 'cutis pellis' [among numerous uses of this text through almost three centuries, for instance in 1588 an English translation of 'cuticula' was provided in eight languages ('a little or thinne skinne'; p. 176); in the ninth and last edition of this text (Facciolatti 1778), 'cuticula' was still cited (p. 197) with a human meaning]. In the Thesaurus linguae latinae of Estienne (1532, also edited in 1573), 'cuticula' was also noted (p. 707) for a diminutive of 'cutis', cutis referring in one case to plants ('cutis arboris'). The Latin word 'cutis' was cited once in the index of the Greek-Latin translation of Theophrastus Historia plantarum by Theodorus Gaza (1552 edition, from the end of the XV th to the beginning of the XVI th century). Scaliger (1644, p. 878) in the Greek-Latin commentary on Theophrastus Historia plantarum, in the text corresponding to the fern cited above for its impermeability, cited Theodorus Gaza's translation (similar sentence in Gaza 1552 p. 153): "Adianti, id est, capilli Veneris, folium numquam madescit, nec quicquam adhaesisse humoris constat (= Adiantus, i.e. Capillus veneris, never gets wet, and it is well-known that no liquid is adhering to it"; Luccioni, pers. com). In the early XVII th century, the diminutive cuticula appeared in other western Europe dictionaries as a synonym of 'coticola' in the Italian 'vocabolario degli accademici della crusca' [(1612), p. 150 of the 'indice delle voci e locuzione latine' part (index of the Latin voices and locutions)]. 'Cuticula' was used several times by [START_REF] Scaliger | Theophrasti Eresii De historia plantarum liber decem, Graecè et latinè[END_REF], but as a wide definition of the outer part of plants, particularly for the outer part of the fruit of Paliurus spina-christi (= garland thorn,Rhamnaceae;p. 266,comm. on Theophrastus Book III 17;Luccioni pers. com). 'Cuticle' in English appeared in one of the first English dictionaries (1658, 'The New World of English Words, or, a general dictionary') but as an anatomical term for the human (or animal?) skin outerpart, and appeared more than 10 times in [START_REF] Grew | The Anatomy of Vegetables Begun[END_REF], apparently referring to different external parts of plants. 'Cuticula' was cited about 20 times in [START_REF] Malpighi | Anatome Plantarum. Cui fubjungitur Appendix, Iteratas & Auctas Ejufdem Authoris de ovo Incubato Obfervationes Continens[END_REF]also, cuticulam, cuticulae, cuticulâ) apparently connected with different outer parts of plants, but specifically with 'epidermide' (epidermis) and "folii" (leaf) in some parts of the book. In other western European languages, the French dictionary of Furetière (1690) cited 'cuticule', but as a synonym of the epidermis of animals. In the Portuguese dictionary of [START_REF] Bluteau | Vocabulario portuguez e latino[END_REF]p. 648), although it was partly devoted to botany, 'cuticula' was a synonym of 'cutîcula' and described anatomically as human epidermis. Similarly, in 1729 'cuticula' and 'cuticular' appeared in the in the Spanish 'Diccionario de Autoridades' with the same meaning.

The plant cuticle has been described increasingly precisely and in detail through time; in Ludwig (botanist and doctor, 1742), the plant cuticle was cited several times and described in Part II Consideratio vegetabilium physica (Chapter II paragraph 315: 'est lamina densa, tenuis, pellucida… et totam plantae superficiem obtegit… haec cuticula plantarum dicitur; it is a dense, thin, transparent layer… and it covers the whole plant surface… it is called plant cuticle'; partly Luccioni (pers. com). It was compared with animal cuticles (paragraph 316), and also quoted in the index. In other paragraphs (431 and 432), maceration experiments on various taxa were described, separating the cuticle from parenchyma tissue. Some years later, [START_REF] Ludwig | Institutiones historico physicae regni vegetabilis[END_REF], described the cuticle slightly differently and also included maceration, which possibly represents one of the first experiments on plant cuticles, but the term was still apparently used in a wide sense ("Omnem plantae superficiem cingit lamina tenuis et pellucida, quae cuticula dicitur, haec densa est et in maceratione plantarum non dissoluitur…"; paragraph 327; "the whole plant surface is surrounded with a thin and transparent layer, called cuticle, it is dense and not soluble by plant maceration…"; partly Luccioni pers. com), further in this paragraph the cuticle was precisely restricted to some parts of plants. It appears several times in this 1757 text, also compared with human skin in the paragraph 328 ("cuticula plantarum 327. Huius et humanae comparatio 328" as written in the index).

Linked to more precise techniques and experiments, the early XIX th century was the starting point for various steps recognizing the plant cuticle as it is currently defined. Treviranus (1821) provided transversal sections ('durshnitt') of various extant plants, but unfortunately with low magnifications where the cuticle was difficult to distinguish from the cell wall of the epidermis (his Tab. I F. 1, 4, 13; his Tab. II F. 14 and 21). However, this was an important step for observing details of the different parts of the outer part of the epidermis, and the term 'cuticula' was used, but not in its modern sense. In 1827, De Candolle used the French term 'cuticule' in Chapter V of his book ('de la cuticule et de l'épiderme' p. 66-77; = about cuticle and epidermis), where he referred to transversal and longitudinal sections of Tritomia varia (his Planche 2 Figs. 3 and4). In Figure 3, he clearly called cuticule (named 'c' on the figure) the epidermal cells surrounded with a thick cell wall, the cuticle thickening not visible. In the text, he included epidermal cells in the 'cuticule', but cited other authors suggesting that the cuticle seems to depend on the type of organ and taxon observed. Brongniart (1830) used the same technique and provided, at a much higher magnification, very precise light microscope figures of extant plant leaves with transversal sections (also called perpendicular sections: 'coupes transversales', 'coupes perpendiculaires à la surface… des feuilles') obtained with very sharp and delicate instruments ('…avec des instrumens très-tranchants et très délicats…'), where the cuticle seems to be drawn; however it is not clear for some cases if he drew the cuticle or cell wall. Although the term cuticle was not used, it was drawn quite precisely in the plates with extant Lilum album (Pl. 8 Fig. 3) and Iris germanica (Pl. 9 Fig. 2), showing a thicker thickening over the upper epidermis, whereas it was thinner in the lower epidermis. As other palaeobotanists of that period, Brongniart was very sensible to the quality of the illustrations (Cleal et al. 2005), which were accurately observed and illustrated, as he also researched extant immerged plants, such as extant Potamogeton perfoliatum and Ranunculus aquatilis that have almost no thickenings above the epidermal cells, which is well-known for plants in these environments. Brongniart referred to a continuous membrane ('membrane continue'), except in the middle part of the stomata, which was obtained with extant Brassica leaves, after maceration in water for a few months. [START_REF] Henslow | On the examination of a hybrid Digitalis[END_REF] published some clear cuticle preparations (without using this term) in extant Digitalis (Scrofulariaceae) hybrid organs (his Plate XVII 6-7; transversal sections in Plates XVII-XVIII also show cuticles over the epidermis) using nitric acid (an efficient technique still currently used for preparations of cuticles), and wrote: "I am however quite positive upon another point which has been a subject of dispute among physiologists; I mean the existence of a delicate homogeneous membrane investing this epidermis. Such a membrane may be distinctly separated by the action of nitric acid, from the epidermis of the corolla, filament, and style. It is faintly marked by parallel longitudinal stria… and appears to coat over the whole surface of these organs…" Brongniart (1834) cited Henslow in his text related to the Brassica experiment, but more accurately described the possibility of chemical maceration to detach a thin 'pellicule sus-épidermique' (= film over the epidermis) from the epidermis (= "cuticule" in his text); according to his text he observed this in many other taxa. Lindley (1835), published a whole part (Chapter II Section I 1; see also in the second edition of a botany book dedicated to 'cuticle', and summarised the research he questioned about the cuticle as follows: "There is some reason to suppose that there is occasionally present, on the outside of the cuticle, a transparent, very delicate membrane, having no organic structure, as far as can be discovered with the most powerful microscopes".

Something similar was noticed by Brongniart in the cabbage leaf; an analogous structure was commented on by Henslow in the Digitalis leaves; "and I have found it very conspicuous on the upper side of the leaves of Dionaea muscipula… Brongniart has paid some attention to this subject… and finds the pellicle by no means uncommon; he even thinks it is present in submersed leaves, and imagines that it covers the stigma in some plants. I, however, find nothing very definite in regard to this, except that the pellicle often exists, and that it does not cover the stomates". The different editions of this book are of interest as they represent an accurate reflection of the research regarding cuticles of that time. The first edition also contained a 'cuticle' part [START_REF] Lindley | An introduction to Botany[END_REF], which was not very detailed. In the third edition (Lindley 1839) the 'cuticle' part was removed and replaced by an 'epidermis' part, where the cuticle was described to be 'organic mucus'. His Plates (I Fig. 2 and II Fig. 1) contained some transversal sections but with the cuticle very simply drawn on the outer part of the epidermis.

Von [START_REF] Mohl | Ueber die cuticula der gewächse[END_REF]Mohl ( , 1847a) made a considerably more precise new finding, showing the different parts of the cuticle (especially the presence of a 'cuticle proper', as cited by [START_REF] Holloway | Structure and histochemistry of plant cuticular membranes: an overview[END_REF] and simultaneously used the German term 'cuticula' (= kutikula) in its modern sense. The translation of the 1842 article in French (Von Mohl 1843) seems to be the first appearance of the French term 'cuticule' in its modern sense, and contained a detailed illustration (Plate 6) with various coloured sections of cuticles, two of them being clearly bilayered (his Fig. 7 of Viscum album, his Fig. 10 of Agave lurida), other figures being from mono-to multi-layered; interestingly for Coniferales, which are now among the well studied fossil plant groups using transmission electron microscopy (TEM), the cuticle of extant gymnosperm Taxus baccata was mentioned (as similar to his Figure 8). [START_REF] Hartig | Beiträge zur Entwickelungsgeschichte der Pflanzen[END_REF] published extant cuticles schemes of sections with 1 and 2-3 zones (his Pl. Figs. 1 and2), and a whole chapter entitled 'cuticule' in the translation in French of this article [START_REF] Hartig | Premier chapitre concernant la formation des cellules et l'épiderme des plantes[END_REF]). In the French translation (Von Mohl 1847b) of the article of Von Mohl (1847a), there was a description of the use of transversal sections to determine the chemical reactions of the cuticle. However, these new cuticle findings (several parts for the cuticle) seemed to have a variable impact according to the authors. From Lindley's botany book, the fourth edition (1848) contained 'back' the 'old wide sense' of cuticle in much more detail than the first and second editions, including the current research of this time, and the cuticle was compared with the 'skin or shell of Crustaceae'. With Henfrey's English translation (1852) of von Mohl (1851) the English term cuticle in its modern sense ('… mass secreted on the outside of the cells, the true cuticle… in which no cellular has been yet shewn to exist') appeared seven times. Transversal sections were provided (Figs. 17,37,(40)(41)(42)48) where the cuticle was indicated in the legends by 'a' (Figs. 37,41,and 48 of extant Helleborus,Hoya,and Cereus).

Moreover, his Fig. 41 was particularly informative, as the cuticle was very clearly shown as detached from underlying layers, and corresponded to the cuticle proper in its modern sense, the cuticular layer in its modern sense just underneath being named 'b' ('wollen, laminated layers of the epidermis cells'); the cell wall was present all over the cell and very precisely drawn.

Bornemann (1856) published some transversal sections ('Querschnitt') of various extant cycads (his Tafel XI 18-19; his Tafel XII 15-18 with two parts in Section 17 above the epidermis cells, one for the cell wall, and one for the cuticle with a clear anticlinal wall between the cells). In the same year [START_REF] Schacht | Lehrbuch der Anatomie und Physiologie der Gewächse[END_REF] provided coloured transversal sections (his Tafel III) of extant plants with cuticles showing various thicknesses, distinct from the cell wall just beneath in another colour; in one drawing the cuticle had two parts, an outermost dark-brown and an inner yellow part. In addition, also in the same year [START_REF] Prillieux | De la structure des poils des Oléacées et des Jasminées[END_REF] published (his Pl. 2 and 3) some sections of various Oleaceae and Jasmineae, where the distinction between the cell wall of the epidermis and cuticle was not very clear. [START_REF] Unger | Vortrag: Beiträge zur Physiologie der Pflanzen[END_REF] provided a transversal section of extant Richardia aethiopica (his Taf. I 4) with a cuticle apparently drawn in a darker colour than the cell wall, but in a very low magnification, not showing any details of the cuticle. Besides, much earlier, this author had published the same results for extant Victoria regia leaf (Unger 1853, his Figs. 2-3). [START_REF] Wolf | Beiträge zur Entwicklungsgeschichte der Orchideen-Blüthe[END_REF] provided transversal sections of the epidermis of Orchis mascula and Trichopilia suavis (his Tafel XVI 3 with 'cuticula' mentioned in the same page of the text, and Tafel XVIII 5-6, respectively) with the cuticle drawn but not mentioned in the captions. Similarly, [START_REF] Kraus | Ueber den Bau der Cycadeen fiedern[END_REF] observed the cuticle of Cycas revoluta, however provided more precise observations of delicately ornamented cuticles (his Tafel XIX 5-6 and Tafel XX 11), even showing the differences in thickness between the upper and lower cuticles; the term 'cuticle' was used about 20 times in this paper. However, [START_REF] Sachs | Lehrbuch der Botanik[END_REF] showed a transversal section of the epidermis of extant Ilex aquifolium (his Fig. 37) with a thick cuticle made of two parts [outer part labelled 'a', devoid of colour according to his text ('farblos'); inner part labelled 'b', with yellow colour ('gelb')]. 1 on Psilotum), which were apparently influenced by his previous extant plant research: he showed clear epidermis more or less thickened in their outermost parts (as cuticles…), whereas the first volume of this fossil plants book [START_REF] Brongniart | Histoire des végétaux fossiles, ou recherches botaniques et géologiques[END_REF], before his description of cuticles in extant plants, did not contain these details. Renault (1879) followed the same approach with carboniferous material, similarly to 'silicified magma' as he described in the introduction, where Cordaites leaves (petrological thin sections, his Plate 16 Figs. 234567) with epidermis cells were very precisely drawn (using a camera lucida of a light microscope) and clearly different from other types of cells, where the upper epidermis was clearly thicker than the lower epidermis; the cuticle also seemed different in thickness. These very delicate and precise drawings of Cordaites were partly reproduced later in several Palaeobotany books [START_REF] Potonié | Lehrbuch der Paläobotanik[END_REF], Scott 1923). Some years later, Renault (1893Renault ( -1896) ) provided some transversal sections of silicified Poroxylon incertae sedis (included in 'genres à place indéterminée', incertae sedis genera) with clear thickenings over the epidermis (Planche LXXV Figs. 56), but without naming 'cuticle' in the captions.

These light microscopy section techniques also overlapped with thin sections of organic remnants containing cuticles, according to drawings or descriptions provided, but they were quite difficult to obtain because of their fragility. According to [START_REF] Stace | Cuticular studies as an aid to plant taxonomy[END_REF], [START_REF] Brodie | Notice on the occurrence of plants in the Plastic clays of the Hampshire coast[END_REF] is apparently the first scientist who mentioned the fossil cuticle by distinguishing it from impressions, but did not use the term 'cuticle' ('the epidermis of the fossil frequently peels off… these remains… have been deposited tranquilly between the waters'), which might suggest organic remnants. Among other authors known so far, the term 'cuticule' (= cuticle) was used in French by Zeiller in 1882, associated with an extraction method from the sediment to obtain clean material observable with the microscope. He also provided seven precise transversal cuticle sections of different thicknesses and shapes, some of them showing various stomata (his Plate 9 Fig. 8 with Callitris, Plate 10 Fig. 14 with Bothrodendron, Plate 11 Figs. 7-10 with Frenelopsis). De Saporta, in spite of having studied cuticles of extant plants, among his numerous publications, published surprisingly few transversal sections of fossil plant cuticles. However, in 1894 his Plates 5 and 11 contained transversal sections of Rhizocaulon roots with a clear outer dark layer of cells, but unfortunately did not include detailed captions, and five sections of Rhizocaulon leaves, also with a darker outermost layer of cells like an epidermis (where cuticle is not visible at all). Later on, [START_REF] Zeiller | Eléments de paléobotanique[END_REF] published a transversal section of Cordaites (his Fig. 141), possibly from permineralized material but where the thickening over the epidermis was clear. In 1912, Thompson provided some light microscope photos and drawings of transversal sections of fossil cuticles (his plates V-VI). In an article with both extant and fossil cycadean plants, [START_REF] Thomas | On the cuticles of some Recent and fossil Cycadean fronds[END_REF] published some transversal sections of extant cycadean cuticles, which were very clearly drawn in a darker colour than the cell wall (his plates 17-18). [START_REF] Halle | Some xerophytic leaf-structures in Mesozoic plants[END_REF] provided transversal sections of the fossils, Pseudocycas and Sciatopitytes (his Pl. 12 4-6 and 27-29), including a whole leaf section. Seward's book on fossil plants (1919) contained a transversal section of Nilssonia brevis (his Fig. 620 B after Nathorst), which was unfortunately not detailed, whereas that of Ctenis sp. (his Fig. 625 B) had a clear darkened cuticle above the epidermis. [START_REF] Potonié | Lehrbuch der Paläobotanik[END_REF] published some transversal sections of a Nilssonia leaf (his Fig. 234 c), with a very homogeneous cuticle and devoid of cell remnants. [START_REF] Florin | Studien über die Cycadales des Mesozoikums : Kungl[END_REF][START_REF] Florin | Die fossilen Ginkgophyten von Franz-Joseph-Land, nebts Erör terunger über vermeintliche Cordaitales mesozoischen Alters-I, Spezieller Teil[END_REF] provided fossil transversal sections ('Quershnitte') of cuticles of Ginkgoales (his Tab. 8 and 20) and Cycadales (from Tab. 2 to Tab. 13).

Following this period of sections in light microscopy through the XX th century (see [START_REF] Stace | Cuticular studies as an aid to plant taxonomy[END_REF] for a detailed chronology of cuticle findings), the first fossil cuticle sections of material embedded in resin and observed using ultrastructural TEM began in 1986 with the genera Tarphyderma and Ticoa (Archangelsky and Taylor 1986;Archangelsky et al. 1986), after TEM images of Miocene leaves (angiosperm Zelkova, [START_REF] Niklas | Ultrastructure and cytochemistry of Miocene angiosperm leaf tissues[END_REF]; see also Fig. 1. 24 of [START_REF] Taylor | Palaeobotany the biology and Evolution of fossil plants[END_REF] were published, where the 'cuticular membrane' appeared to be illustrated (their Fig. I,K), but was not described in detail.

Various aspects of plant cuticle ultrastructure

These fossil studies contained several important topics and contents quite classical in palaeobotany that emerged from the beginning of this research and are still developing. The first topic discussed in papers from 1986 concerned taxonomy. Although [START_REF] Holloway | Structure and histochemistry of plant cuticular membranes: an overview[END_REF] proposed six types of cuticles for living plants, grouping very different taxa in each type [from Selaginella to angiosperms] from different parts of plants (leaves, fruits, ovules, stems, trichomes, stigma…), he was very possibly conscious of the importance of cuticles in taxonomy as he concluded: 'Because the CM of plants is so heterogeneous in structure it is dangerous to oversimplify and generalize about its morphology and construction -there is no typical CM. Consequently, each species must be considered individually…' Although in living plants, until recently, TEM details of cuticles have seldom been provided, they are largely provided in fossil plant taxonomical studies, and still prove that 1) one cuticle may refer to several types of those described by [START_REF] Holloway | Structure and histochemistry of plant cuticular membranes: an overview[END_REF] and many overlaps occur, and therefore the six described types are not enough; 2) in one single type each species has its own identity with specificities in thicknesses and/or proportions of layers and/or sub-layers. In a study on Ticoa, Archangelsky et al. (1986) stated that 'the structural and perhaps histochemical complexities of fossil plant cuticles will require some further division of these layers', enhancing all the future detailed taxonomical articles developed.

Palaeoenvironment is the second topic discussed in term of number of papers. This topic has been developed regularly widely in various fossil plant groups (Kerp andBarthel 1993, Krings and[START_REF] Krings | An improved method for obtaining large pteridosperm cuticles[END_REF]. More precisely for TEM, in a study on Tarphyderma, Archangelsky and Taylor (1986) suggested '…additional insights into ecological parameters...', '…response to the habitat...', and '…various structural features were evolving in other major groups of plants in response to the habitat...' Additionally, Archangelsky et al. (1986) pointed out that "… the presence or absence of specific cuticle layers in the fossils may be directly associated with the environment in which the plant was living…"

The third topic that has been up to now very rarelly documented, concerns evolutionary aspects. In the Ticoa study (Archangelsky et al. 1986), when emphasized the importance comparisons between fossil and living plants cuticles, these authors pointed out comparisons between fossil and living plants cuticles ('subsequent work with fossil plant cuticle ultrastructure will consider both the taxonomic application and evolutionary significance of these features'. In a study on Tarphyderma, Archangelsky and Taylor (1986), suggested 'adaptations' and more precisely 'structural adaptations in fossil plants'. Two other points have not been included in this review, as they have not been documented in detail. The first concerns ontogeny, which is classical if working on features of organisms, evocated also by Archangelsky et al. (1986), who suggested that "…the presence or absence of specific cuticle layers in the fossils… may reflect a particular level of organ development at the time of fossilisation". This has been studied in living plants (Sphagnum, moss, Jeffree 2006;algae, Jeffree 2006;Picea, gymnosperm, Jeffree 2006;Hedera helix L., angiosperm, Viougeas et al. 1995;Utricularia, angiosperm, Jeffree 2006;Solanum lycopersicum, angiosperm, Dominguez et al. 2011band 2012[START_REF] Segado | Ultrastructure of the Epidermal Cell Wall and Cuticle of Tomato Fruit (Solanum lycopersicum L.) during Development[END_REF]Arabidopsis, angiosperm, Nawrath 2006[START_REF] Fabre | The ABCG transporter PEC1/ABCG32 is required for the formation of the developing leaf cuticle in Arabidopsis[END_REF]; see also Fernandez et al. 2016; see also the reviews of Yeats and Rose 2013 and Ingram and Nawrath 2017); however, until now, no fossil studies have been carried out to the best of our knowledge in this regard, owing to the extreme rarity of different stages of the same material found in the fossil record. The second concerns diagenesis and TEM, carried out from 1986 (Archangelsky et al. 1986). Although this is fundamental for the study of fossil plants, the analysis is difficult to carry out. At least within one genus, TEM data comparing fossil (170 millions of years) and living ginkgos, [START_REF] Guignard | Comparative studies of leaf cuticle ultrastructure between living and the oldest known fossil ginkgos in China[END_REF] found very few differences in the cuticles within this genus, although there was a considerably minimised effect of diagenesis.

The details of the methods used to obtain the sections of cuticles are somewhat variable, related to different chemicals used, also depending on the material (preservation and others…) and taxa, the technique of the laboratories.. However, the methods for this especially difficult preparation [START_REF] Taylor | The ultrastructure of fossil cuticle[END_REF], in a special chapter on the ultrastructure of fossil cuticles) followed a similar general scheme as summarised here. According to all the studies selected in Table 1 where the main differences in methods are listed, the material could be extracted from the sediment treatments (or not) with several chemicals during variable times. Moreover, the removal of minerals and sediments if necessary (in order to not destroy the glass or diamond knife when cutting the sections) is also summarised: hydrofluoric acid, followed by then Schulze's reagent (nitric acid and potassium chlorate; [START_REF] Gray | Extraction techniques[END_REF][START_REF] Traverse | Paleopalynology[END_REF], and finally dilute ammonia. Although the same method should be used for effective comparisons between cuticles, the use of these treatments produced very damaged areas in some cuticles (holes, breakings, and missing parts), not allowing detailed observations, especially collecting of enough statistic data possible. In some cases, untreated cuticles provided much longer areas of entire cuticles, which were very comparable with treated material, and are more suitable for observations and photography. Apparently no (or very few) changes are noted in all taxa where it is used [START_REF] Guignard | Comparative studies of leaf cuticle ultrastructure between living and the oldest known fossil ginkgos in China[END_REF][START_REF] Del Fueyo | Leaf cuticle anatomy and the ultrastructure of Ginkgoites ticoensis Archang. from the aptian of patagonia[END_REF][START_REF] Guignard | Insights into the leaf cuticle fine structure of Ginkgoites skottsbergii lund. from the Albian of Patagonia and its relationship within Ginkgoaceae[END_REF][START_REF] Yang | Suturovagina intermedia (Cheirolepidiaceae) from the upper Lower Cretaceous Dalazi Formation of Wangqing, Northeast China: cuticle ultrastructure and palaeoenvironmental insights[END_REF], Carrizo et al. 2019a, this issue;Guignard et al. 2019, this issue).

TEM methods (Appendix A of MethodX article) begin (or not) with the fixation of cuticles (in glutaraldehyde or paraformaldehyde), then a postfixation (or not) in Osmium tetroxide, followed by dehydration in degraded solutions of alcohol allied with graded percentages of resin for its impregnation in the cuticle (Spurr, Epon, Araldite). Finally, a polymerisation of the blocks of resin (at 37, 56, 60 or 70 °C) is achieved depending on the type of resin, during variable times. The majority of research referred to only one kind of resin for each paper, this homogeneity allows comparisons, since some differences are found with various uses of resin for the same taxon (extant Eucalyptus, Populus, Pyrus;Guzman et al. 2014).

From this step, two kinds of sections, using an ultramicrotome, can be obtained from the blocks of resin embedding the cuticles (Appendix B of MethodsX article), usually cut with a diamond knife, but more rarely with a glass knife: 1) rarely mentioned but very useful are thin sections (1 µm) mounted on glass slides and stained (in blue colour) using the methods of [START_REF] Richardson | Embedding in epoxy resins for ultrastaining sectioning in electron microscopy[END_REF], observed with a light microscope, enabling to the quality of the cuticle to be analysed and determine the best sections with the maximum data (containing not only ordinary epidermal cell cuticle, but also potentially stomata and the base of trichomes and papillae); 2) ultrathin sections of 60-70 nm (silver to pale gold), observed using TEM, deposited on uncoated or Formvar-coated 200-300 Mesh copper grids (or other meshes, other metals, even with one slot grids in rare cases). The majority of sections are transversal to the cuticle (i.e. perpendicular to the leaf length), but some are longitudinal (i.e. parallel to the leaf length) to illustrate the homogeneity of the material through these two orientations.

The last step is staining, to obtain high contrasts of the different materials comprising the cuticles. This step shows high variations of techniques, not only in the chemicals (potassium permanganate, uranyl acetate, lead citrate), but especially in combinations of durations and dilutions. Although most papers use only one method, some articles refer to several attempts for the same material: in fossil gymnosperms, [START_REF] Taylor | Comparative ultrastructure of fossil and living gymnosperm cuticles[END_REF] provided TEM photos of cuticle with different staining times (60-20-10 minutes, or 25-25-17, or 20-20-10) used for the three chemicals, respectively. In Umkomasiales [START_REF] Labe | Etudes ultrastructurales de la cuticule de Préspermatophytes fossiles du Jurassique[END_REF], the staining was adjusted (in times and dilutions) according to the material. In Czekanowskiales [START_REF] Zhou | Leaf cuticle ultrastructure of two czekanowskialeans from the Middle Jurassic Yima Formation of Henan, China[END_REF], five methods of staining were used with the three chemicals (or two or even just one), with various times and percentages of solutions. Their best staining is indicated with uranyl acetate (20 min) and lead citrate (20 min). As the type of molecules stained in TEM is questionable by some authors, through the photos provided in the papers for one taxon ( = same type of material), apparently it is difficult to state if different molecules are stained with different stains; only lower or higher contrasts seem to be relevant, but exercising caution is recommended.

As an example, the technique of [START_REF] Lugardon | Contribution à la connaissance de la morphogénèse et de la structure des parois sporales chez les Filicinées isosporées[END_REF] is provided here (Appendix A of MethodsX article), enabling very clean and highly contrasted details of cuticle sections. This technique was formerly used for spores (living and fossil plants) with Spurr and/or Epon resin (as in Marattia spores with Lugardon as a co-author, [START_REF] Wang | Ultrastructure of in situ Marattia asiatica (Marattiaceae) spores from the Lower Jurassic of Hubei, China[END_REF], and adapted for cuticles with Epon resin. Compared with other embedding techniques of 1-2 days very common for TEM studies of living organisms, this one is particularly long (one week of procedure, from day 1 to day 8), which is considered very useful for a low risk of breakings of the cuticle when obtaining thin or ultra-thin sections. Osmium tetroxide postfixation and staining with both uranyl acetate and lead citrate (Appendix B of MethodsX article) allow a very satisfying contrast of the contents of cuticles, enabling high magnifications > 100,000 ×.

In this review, the methods of the three first aspects (taxonomy, palaeoenvironment, evolution) were carried out, and multifactorial analyses were applied, using XLSTAT 2019.1 and XLSTAT3D Plot (Addinsoft (2019); XLSTAT statistical and data analysis solution. Long Island, NY, USA. https://www.xlstat.com.), for chi-square automatic interaction detector (CHAID), factorial component analysis (FCA), and hierarchical components (HCA).

Major results and discussion

3.1 Significances and application in fossil plant taxonomy 3.1.1. Detailed description of different parts of fossil plant cuticles.

Although in living plants mechanical, physical and chemical aspects are searched extensively (Dominguez et al. 2015, and2017 with more than ten articles of a special issue entitled "The plant cuticle, old challenges, new perspectives"), allowing Cohen et al. (2017) to write that "A search in Web of Science… shows that the number of published articles associated with the cuticle gradually increased each decade starting in the 1950s up until the late 1990s…", detailed description of the different layers of the cuticle is seldom given. However they do exist as in living Ficus elastica (Guzmán-Delgado et al. 2015), in Arabidopsis thaliana [START_REF] Mazurek | Connecting the Molecular Structure of Cutin to Ultrastructure and Physical Properties of the Cuticle in Petals of Arabidopsis[END_REF][START_REF] Berhin | The Root Cap Cuticle: A Cell Wall Structure for Seedling Establishment and Lateral Root Formation[END_REF], in orchids (Bulbophyllum weberi and B. cumingii; [START_REF] Kowalkowska | Floral structure of two species of Bulbophyllum section Cirrhopetalum Lindl.: B. weberi Ames and B. cumingii (Lindl.) Rchb. f. (Bulbophyllinae Schltr., Orchidaceae)[END_REF], in tea (Camellia sinensis cv Fuyun; [START_REF] Zhu | Tender leaf and fully-expanded leaf exhibited distinct cuticle structure and wax lipid composition in Camellia sinensis cv Fuyun 6[END_REF]Opuntia (Agüero et al. 2018). In fossil cuticles, mainly leaf cuticles being reviewed here [although cuticles are also present in fruits, stems, flowers and embryos as revealed in living plants [START_REF] Heredia | A Biophysical and biochemical characteristics of cutin, a plant barrier biopolymer[END_REF], Heredia-Guerrero et al. 2017[START_REF] Ingram | The roles of the cuticle in plant development: organ adhesions and beyond[END_REF], all these organs being very fragile for preservation], descriptions are very frequent, possibly because cuticles are often the only remains of fossil plants. The description of the fossil cuticle (= cuticular membrane CM) was first proposed by Archangelsky et al. (1986), following the scheme of [START_REF] Holloway | Structure and histochemistry of plant cuticular membranes: an overview[END_REF] for living plant cuticles, who summarised detailed cuticle sections from von 167). This scheme (Fig. 1 and Plate I for illustrations) with two parts in a fossil cuticle is still used, completed by sublayers of A1 and A2, following the studies of [START_REF] Guignard | Comparative studies of leaf cuticle ultrastructure between living and the oldest known fossil ginkgos in China[END_REF] and [START_REF] Nosova | First data on the ultrastructure of the leaf cuticle of a Mesozoic conifer, Mirovia Reymanówna[END_REF]. This scheme is not as definite as that of [START_REF] Holloway | Structure and histochemistry of plant cuticular membranes: an overview[END_REF], for extant plants, "…it is always inadvisable to present a single generalised scheme for the construction of the plant CM". The cuticular membrane (appendix A) is divided into two regions: an outer cuticle proper (= CP = A) directly in contact with the atmosphere in its outermost part, and an inner cuticular layer (= CL = B) in contact with the cell wall of the subjacent cell (mostly absent in fossil material) in its innermost part. The outer cuticle proper A can be divided in two layers: an outermost A1 polylamellate layer subdivided in some cases into A1U (= upper zone or sublayer, which can be straight or wavy) and A1L (= lower zone or sublayer), then an inner A2 granular layer, divided in some cases into A2U (= upper zone or sublayer) and A2L (lower zone or sublayer). The inner cuticular B can be divided in two layers: fibrilous or spongy B1 in its outer part and granular B2 in its innermost part. This scheme is very synthetic and generalized, and some taxa may have only part of these layers and /or sublayers.

3.1.2. At the level of order and/or family: qualitative descriptions According to the presence-absence of layers and/or sublayers (zones) of cuticles, which are variable in fossil plants studied to date (mainly leaf cuticles), for the ordinary epidermal cuticle cell a simple formula can be proposed, from the simplest one (A2) to complex ones (for instance A1U + A1L + A2U + A2L + B1). Two syntheses are proposed here, one with the presence or absence of A1, A2, B1, B2, selected successively (Fig. 2), one with an HCA developed by software using the combination of all these characters (Fig. 3). In the potential ultrastructural dichotomous key developed 'manually' (Fig. 2), in some cases, the words 'wavy, spongy, fibrilous' were added to this formula to enable the distinction from other cuticles. Moreover, it is noticeable that although the number of combinations of layers and sublayers is relatively high, the actual number of cuticle formulae is relatively low (11) and corresponds to taxonomic groups, where two kinds of groups occur: homogeneous and heterogeneous groups, making this formula an excellent tool for taxonomy.

As a result of Fig. 2, in homogeneous groups (5), each order (or family) contains genera with only one cuticle formula: Coniferales Cheirolepidiaceae, comprising Pseudofrenelopsis-Suturovagina-Hirmeriella, all with a wavy A1-A2-B1-B2 formula; Coniferales Miroviaceae, comprising Mirovia, with a A1U-(A1L)-A2U-(A2L)-B1 formula; Coniferales Araucariaceae, comprising Nothopehuen-Araucaria-Brachyphyllum, all with a A2-spongy B1 formula;

Caytoniales, comprising Ruflorinia, with a A2-B1-B2 formula different between upper and lower cuticles; Czekanowskiales comprising Arctobaiera-Phoenicopsis, also with A2-B1-B2 formula different between upper and lower cuticles.

In heterogeneous groups (3), each order contains two groups of formulae (making a total of six): Ginkgoales, comprising Baiera-Sphenobaiera with A2-B1 (type I) and Ginkgo-Ginkgoites-Pseudotorellia (type II) with A1U-A1L-A2-B1; Pteridospermales, comprising Dichopteris-Komlopteris with A2 (type I), and Pachypteris with A1-A2-B1 (type II) showing differences between upper and lower cuticles; Cycadales comprising Pseudoctenis-Ticoa (type I) with A1-A2-B1 and Restrepophyllum with A1-B2 (type II). If the ultrastructural unity of cuticles of one order (or family) is reinforced if they have just one cuticle formula, for the separation of an order in two ultrastructural cuticle groups the relationship between cuticle ultrastructure and gross morphology is unclear. Actually, even with two groups it makes such heterogeneity taxonomically inconclusive and it would be effective to increase the number of taxa to accurately determine these formulae. For instance, in Ginkgoales (Guignard et al. 2019, this issue), the authors relate that Ginkgo and Ginkgoites are relatively close in gross morphology (which suits their ultrastructural affinities), while Pseudotorellia is different in gross morphology, however the three taxa have the same cuticle ultratructural formula. It is also the case for Baiera which is different from Sphenobaiera in gross morphology, but they have the same cuticle ultrastructural formula.

Using the HCA of the same data and calculated by using data 0-1(Fig. 3; Appendix A of data in brief article), 52 ordinary epidermal cell cuticles from more than 25 taxa have been considered. As these cuticles are considered individually when lower and upper cuticles are available for one taxon, if their cuticle formula is different in one single taxon, it makes some separations in these taxa. However, the identification of orders-families is similar to Fig. 2. This combination of the presence-absence of layers and sublayers by HCA resulted in 9 groups (instead of 11 in Fig. 2; the two 'missing' groups corresponded to 2 orders allied with 5 other cuticles from other orders), namely: Cycadales type II, Coniferales (Cheirolepidiaceae, Miroviaceae, Araucariaceae), Ginkgoales type I and II, Pteridospermales type I and II, Czekanowskiales and Cycadales type I, plus some other cuticles of the former groups.

So far as the former studies revealed where the stomatal apparatus could be observed, subsidiary cell cuticles are usually similar to ordinary epidermal cell cuticles in ultrastructure and give the same formula classification. It is not the case for guard cell cuticles, which are usually different from the cuticle of ordinary epidermal cells and subsidiary cells. Although they are rarely observed, owing to their fragility, the guard cell cuticle is of great taxonomic importance as a multidimensional CHAID classification of the taxa could be also proposed. (Fig. 4; Appendix B), using another order of characters than for ordinary epidermal cell cuticles formerly developed in this paper (Fig. 2 with the following order: A1 or not A1, A2 or not A2, B1 or not B1, B2 or not B2). For guard cell cuticles, firstly the cuticle proper A is involved in the presence/absence of the A1U sublayer, then two possibilities occur (presence/or not of the A2 layer, or presence/or not of the A1L sublayer). The cuticular layer B is then involved, with the presence/or not of B1 and B2 layers. In the few taxa where cuticle guard cell details could be observed, Coniferales (Araucariaceae, Cheirolepidiaceae; Carrizo et al. 2019a this issue, [START_REF] Mairot | Cuticle micro-and ultrastructure of Suturovagina Chow et Tsao (Cheirolepidiaceae): taxonomic and palaeoecological implications[END_REF][START_REF] Yang | Suturovagina intermedia (Cheirolepidiaceae) from the upper Lower Cretaceous Dalazi Formation of Wangqing, Northeast China: cuticle ultrastructure and palaeoenvironmental insights[END_REF][START_REF] Yang | Leaf cuticle ultrastructure of Pseudofrenelopsis dalatzensis (Chow et Tsao) Cao ex Zhou (Cheirolepidiaceae) from the Lower Cretaceous Dalazi Formation of Jilin, China[END_REF][START_REF] Guignard | Cuticle ultrastructure of Pseudofrenelopsis gansuensis: Further taxonomical implications for Cheirolepidiaceae[END_REF], and Pteridospermales [START_REF] Guignard | Ultrastructure of Early Jurassic fossil plant cuticles: Pachypteris gradinarui POPA[END_REF], Thévenard et al. 2005) only one type of guard cell cuticle formula can be distinguished. Ginkgoales [START_REF] Guignard | Comparative studies of leaf cuticle ultrastructure between living and the oldest known fossil ginkgos in China[END_REF][START_REF] Guignard | Insights into the leaf cuticle fine structure of Ginkgoites skottsbergii lund. from the Albian of Patagonia and its relationship within Ginkgoaceae[END_REF][START_REF] Del Fueyo | Leaf cuticle anatomy and the ultrastructure of Ginkgoites ticoensis Archang. from the aptian of patagonia[END_REF][START_REF] Wang | Cuticular Anatomy of Sphenobaiera huangii (Ginkgoales) from the Lower Jurassic of Hubei, China[END_REF], Guignard et al. 2019 this issue) showed two possibilities (group 1 with A2 and B1 layers, group 2 with A1U and A1L, A2, B1), and it is noticeable that two groups could be divided as those with ordinary epidermal cell cuticles OEC (Baiera and Sphenobaiera, Ginkgo-Ginkgoites-Pseudotorellia) (Guignard et al. 2019, this issue). As only one taxon (Ginkgoites skottsbergii which is referred to a form genus of Ginkgoales) does not follow this scheme (it is grouped with Baiera and Sphenobaiera), attention should be given to guard cell cuticles in the future. Although many other papers do not include fine ultrastructural details in the definition of a taxon, these papers enhance their importance for taxonomy in the 'results' or 'discussion' sections. This was the case for two Czekanowskiales [START_REF] Zhou | Leaf cuticle ultrastructure of two czekanowskialeans from the Middle Jurassic Yima Formation of Henan, China[END_REF], with details of their affinities and specificities. For Pachypteris indica and Komlopteris indica [START_REF] Bajpai | Ultrastructure of the leaf cuticle of Pachypteris indica and its comparison with that of Komlopteris indica[END_REF], the ultrastructure was described in 'observations' and 'comparisons' sections. For Dichopteris (Pteridosperm; [START_REF] Thévenard | Reappraisal of the ill-defined Liassic pteridosperm Dichopteris using an ultrastructural approach[END_REF], the 'cuticle microscopy' occurred in the 'descriptions' section, whereas it was placed in 'results' section for Sphenobaiera huangii (Ginkgoales; [START_REF] Wang | Cuticular Anatomy of Sphenobaiera huangii (Ginkgoales) from the Lower Jurassic of Hubei, China[END_REF]. This was also the case for Pachypteris [START_REF] Guignard | Ultrastructure of Early Jurassic fossil plant cuticles: Pachypteris gradinarui POPA[END_REF] Third, the use of ultrastructural comparisons between several taxa of the same group also exist in single papers, with specificities and/or affinities enhanced, at various taxonomic levels. [START_REF] Barale | L'ultrastructure de la cuticule de quelques Bennettitales du Crétacé inférieur d'Argentine[END_REF] compared the ultrastructure of five species of Bennettitales; for four Umkomasiales [START_REF] Labe | Etudes ultrastructurales de la cuticule de Préspermatophytes fossiles du Jurassique[END_REF], two species belonged to the same genus (Pachypteris); taxonomy was then developed in their discussions, and the homogeneity at the order level was enhanced, as well as specificities. Specificities within one genus were also developed between two species of Dicroidium (Corystospermaceae) (Maheswari and Bajpai, 1996a). In 2003, descriptions of six Bennettitales taxa were published, including TEM details, and three ultrastructural groups were proposed (Villar de Seoane 2003).

In some studies, comparisons with living taxa were developed. Early in 1986, the Ticoa harrisii cuticle ultrastructure characters were included in a description of this taxon and compared with those of extant plants, mainly following the six types of Holloway (1982) (Archangelsky et al. 1986). In 1989, two fossil cuticles (Ginkgoites tigrensis and Karinopteris) were described and compared with the cuticle of living Ginkgo biloba [START_REF] Taylor | Comparative ultrastructure of fossil and living gymnosperm cuticles[END_REF]). This living taxon was also compared with Ginkgoites tigrensis (Villar de Seoane for about 33% (8 of them: CM mean -A2 mean, A1U straight mean -A2L mean, A2L mean -B1 fibrilous mean, A2U mean -B2 mean, OL mean -TL mean, A% -B%, A2U % -B1 fibrilous %, A2L % -B1 fibrilous %), and the remaining cuticles (18) having complex and polynomial correlations (even at the order 6 for A1L % -A2U %, CM mean -A1U straight mean). This is a demonstration of the complexity of the relations between cuticle characters as shown by their fine details. In only 33% of the 41 cuticles, one character can be deduced easily from another one, so although a part of the details are very simply (in mean and %) correlated, most of them have very complex relations. The FCA developed with these 41 cuticles and 23 characters (Figs. 5 and6; Appendix B of data in brief article) showed that, with a very satisfying 93.54% of information on the three first axes (1, 2, 3), six main characters are involved in the distinction between the taxa, i.e. firstly the cuticular layer B (Fig. 5; B %, fibrilous B1%, B2 mean), then the cuticle proper (A % and A2U %), and finally the total mean of the cuticular membrane (characters numbers in Fig. 5: 17, 21-22, 3, 13, 1).

At a much lower degree of importance, some other characters are relevant: A mean, A2U mean, A2L mean and % of the cuticle proper; B mean, fibrilous B1 mean, B2% of the cuticular layer (characters numbers in Fig. 5 analyses revealed to be a good taxonomic tool and various points could be enhanced (Appendix D). First, the correlations between the curve tendencies of all Cheirolepidiaceae cuticle characters (Appendix E), are different and more particular to this family than those of all 41 cuticles from various families: for these Cheirolepidiaceae cuticles, among 17 correlations, they were simple and linear (y = ax + b) only for 3 of them (A2 mean -B2 mean, A% -B%, A2% -B1%), most of them having complex and polynomial correlations (even at the order 6 for A1% -A2%, OL mean -TL mean, A1 mean -A2 mean, CM mean -A1 mean). This is a demonstration of the peculiarities of this family (in comparison, the curve tendencies made with all four types of cuticles show eight linear correlations; Appendix C) in which one cuticle character cannot be deduced very easily from another one, except for very few of them (16.7%), and though each cuticle has its own equilibrium of details (in mean and %), enhancing the interest of very delicate observations in this family. Second, from the FCA developed with nine cheirolepidiaceous cuticles (ordinary epidermal cell and subsidiary cell cuticles; 99.44% of information on the first 3 axes; Fig. 7; Appendix F), the importance of 15 characters (Fig. 7a) was ordered as follows: percentages are more important for the construction of the first and main axis 1 (especially A% and A2% of the cuticle proper, B% and B1% of the cuticular layer, all with more than 0.2 of contribution on axis 1), whereas means (except CM mean for axis 2) and other percentages are less or not important. Third, the thickness mean of opaque or translucent lamellae of the A1 wavy layer is not important at all for the construction of the axes (contribution of 0 on axis 1). Concerning the relative positions of the nine cuticles, except one (subsidiary cell cuticle of less xerothermic Suturovagina intermedia), each of the four types was identified relatively clearly by HAC (Fig. 8).

Similarly, for the FCA drawn with the ellipse confidences (Fig. 7b), which was more subtle in the relations between the cuticles as some of them had a higher variability in their absolute values and % than did the others. It is interesting to note that confirming the originality of guard cell cuticles GC discussed formerly, the addition in these multidimensional analyses of three guard cell cuticles (HCA with 12 cuticles; Fig. 8b) increased the heterogeneity of the scheme slightly as guard and subsidiary cell cuticles of less xerothermic S. intermedia are more clearly separated from the ordinary epidermal cell cuticles of the same type of cuticles and make then a stronger group. This was also true for the FCA with 12 cuticles (with added GC) with slightly less information than the FCA with 9 cuticles (95.22% of information on the 3 first axes; Appendix G), where the weight of the characters inducing the axes was also different: the cuticle proper was the most informative for axis 1 (50.5% of information) with B and B1 means, B% and B1%; the cuticular layer contributed to axis 2 (34.46% of information) with A and A2 means, A1%.

Recently, Energy Dispersive Spectroscopy (EDS) element analyses were added to ultrastructural features and included in the taxonomic value: Ginkgoites (Ginkgoales, the putative influence of the environment on cuticles has been envisaged; however, at different levels and various degrees of precision. In their TEM study of Tarphyderma, Archangelsky and Taylor (1986) discussed 'xeromorphic features'; however, unfortunately did not include high enlargements. In the same year, Archangelsky et al. (1986), in Ticoa, considered the 'A2 sublayer' could have 'some ecological significance'. [START_REF] Taylor | Comparative ultrastructure of fossil and living gymnosperm cuticles[END_REF] suggested that 'the overall cuticle thickness differences could be attributed to a number of factors, the most probable of which are ecological…' in their Ginkgo study. Moreover, they began to enhance fine details to determine the possible significance of the lucent lamellae thickness of the A1 layer, evocating 'ecological' significance among other potential interactions. [START_REF] Archangelsky | Ultrastructural studies in fossil plant cuticles[END_REF] discussed the interest of ultrastructural studies of fossil cuticles in strata of different ages and varied 'palaeoecological settings', but unfortunately did not provide details.

For the first time, [START_REF] Barale | L'ultrastructure de la cuticule de quelques Bennettitales du Crétacé inférieur d'Argentine[END_REF], reported in their TEM study on the cuticle ultrastructure of a bennettitalean fossil, a possible relation between cuticle details (their Fig. 8) and xerophytism; however, it did not directly concern the whole cuticle but a zone between the polylamellate layer of the cuticle and outermost wax material. Several authors have carried out similar studies: Maheswari and Bajpai (1996b) reported the biochemical degradation of the cuticle of Thinnfeldia owing to fungous contamination using TEM analysis. In the TEM study of Cycas circinalis [START_REF] Bajpai | Ultrastructure of the leaf cuticle in Cycas circinalis Linn[END_REF], the possible influence of the environment on the 'outer polylamellate and the inner reticulate zones' of the material was discussed. For Pachypteris indica, Bajpai and Maheswari (2000) suggested the possible 'definite' influence of 'ecological conditions'. Villar de Seoane ( 2001) noted that the TEM cuticle details of some Bennettitales (with 'a compact and uniform layer, and a thick lower layer that may be further divided in two sub-layers') could be related to the volcanic activity).

Passalia et al. ( 2010) stated that "regardless the cuticular membranes thickness, most of them were adapted to low rates of transpiration because they have an outer lamellate layer…" in studying a fossil cycad (Restrepophyllum).

With the increasing number of studies, comparisons among genera of the same taxa (family or orders) began to emerge and provided higher degrees of precision about the influence of the environment on the cuticle details. These kinds of comparisons had been clearly enhanced early (Tarphyderma study; Archangelsky and Taylor, 1986), where the authors suggested to "...compare epidermal and cuticular features from similar taxa that grew in different environments…" The studies of two groups can be evocated here. The first group is Cheirolepidiaceae (Coniferales). All the following cheirolepidiaceans studied in ultrastructure:

Hirmeriella muensteri (Guignard et al. 1998 their Fig. 4A-4C), showed fibrils of the B1 cuticular layer becoming parallel to the cell surface, interpreted as the volcanic environment effect, as a result of comparisons with living Pinus cuticles in the same environment. This was also the case for electron-transparent areas (their Figs. 4E, 4F, 7H) in the cuticle, related with oxalate calcium crystal deposits of volcanic area in the same living Pinus material. A last aspect does not directly consider cuticle details, but 'epicuticular and epistomatal waxes', which seem to 'undergo fusion as they are visible in G. tigrensis (Villar de Seoane 1997; their Pl. 1, Fig. 5, Pl. 2, Fig. 1)', also from volcanic environments. In Ginkgo, conversely to Ginkgoites, the potential effect of the environment on the leaf cuticle ultrastructure seems to be not sensitive, as comparisons between living G. biloba and the oldest species G. yimaensis in China (Lower Middle Jurassic) concluded that 'the consistent leaf cuticle ultrastructure of both fossil and living ginkgos indicates that environmental conditions have also little effect' [START_REF] Guignard | Comparative studies of leaf cuticle ultrastructure between living and the oldest known fossil ginkgos in China[END_REF]. In Baiera and Sphenobaiera [START_REF] Guignard | this issue. Cuticle ultrastructure of Baiera-like leaves: taxonomic and palaeoecological implications[END_REF] Theoretically, the most accurate comparisons occur within the same species from different palaeoenvironments, representing extremely rare fossil material; however the characteristics of the palaeoenvironment could be determined through two different studies. These studies could answer at least partly to the questions of [START_REF] Bajpai | Ultrastructure of the leaf cuticle of Pachypteris indica and its comparison with that of Komlopteris indica[END_REF], wondering whether species differences (between Pachypteris indica and Komlopteris indica) were actually species or genus differences, as in their study only one species in each genus was reported.

The first example is the comparative studies made between specimens of the same species (Suturovagina intermedia), from two different localities, and referring to two different macropaleoenvironments ( = palaeoenvironments of the localities sensu lato), versus micropalaeoenvironment which is the environment just around a plant or a leaf (e.g. differences of sun and wind and rain between upper par and lower part of leaf; differences between the top and the bottom of the plant, between the center and the outer part of a plant especially for plants with many branches like tress… One is more xerothermic [START_REF] Mairot | Cuticle micro-and ultrastructure of Suturovagina Chow et Tsao (Cheirolepidiaceae): taxonomic and palaeoecological implications[END_REF]) and the other is less xerothermic [START_REF] Yang | Suturovagina intermedia (Cheirolepidiaceae) from the upper Lower Cretaceous Dalazi Formation of Wangqing, Northeast China: cuticle ultrastructure and palaeoenvironmental insights[END_REF], as evidenced by the associated floristic elements and the EDS data of the matrix. Although the cuticles are generally similar in microstructure, significant differences in ultrastructure (their Table 3) were observed between them. Those of the leaves from the less xerothemic locality (Yang et al;2018, their Fig. 12), show not only a reduction of the thickness, but also different proportions of the layers (wavy A1, A2, B1, and B2) of ordinary epidermal and subsidiary cell cuticles. Based on statistics, the authors proposed 12 ultrastructural characters of ecological significance for this species. Although data are obtained from very limited observations, a potential palaeoenvironment significance of FCA axes (from Fig. 7; Appendix G) was proposed here (Fig. 9), taking account of the cuticle ultrastructure of Suturovagina from two localities. Among the 15 characters of these analyses, the main characters responsible in construction of the FCA axes are A% and A2% of the cuticle proper, B% and B1% of the cuticular layer. An increase in thickness of the cuticular membrane also indicates a more xerothermic environment (South East China), and it is further related to the thickness proportion of A2 and A1 and also the thickness of B2 layers, whereas B1% decreased as compared with that of less xerothermic (North East China) cuticles (Fig. 9). Furthermore, although they are not important for the construction of the FCA axes, more xerothermic (South East China) palaeoenvironments are related to thicker opaque lamellae of the A1 layer and thinner translucent lamellae. This is a demonstration of the signature of environment, moreover it is also of interest for the probable role(s) and equilibrium played by each layer for the elaboration of the cuticular membrane. It would be very enlightening to obtain more material from the same species in other types of palaeoenvironments.

The second study is different as it compared two kinds of leaves of the same species of Komlopteris nordenskioeldii (Nathorst) Barbacka [START_REF] Guignard | Sun and shade leaves? Cuticle ultrastructure of Jurassic Komlopteris nordenskioeldii (Nathorst) Barbacka[END_REF]) from one single locality. Compared with some living leaf plants related to this fossil taxon, fossil sun and shade leaves (different in their macro-morphologies) were selected, possibly referring to potentially high sensibility to micro-palaeoenvironmental influences within each sun and shade leaf two types (in macro-and microscopy). The sun leaf cuticles of ordinary epidermal cells (more exposed to wind, rain…) were thicker than those of the shade leaf cuticles (less exposed to wind, rain…). Moreover, the sun leaf upper cuticle was more exposed than the lower cuticle. Similar is the case with the shadow leaf. In both types of leaves, therefore, the upper cuticle was thicker than the lower cuticle. In total, there are four types of leaf cuticles with different thickness (sun upper and lower leaf cuticles, shadow upper and lower leaf cuticles; Fig. 10). Under TEM, these cuticles were considerably more elaborate in their ultrastructure. Fibrils, pillar-shape structures made of condensed fibrils, granules, and amorphous material are the constituent parts (their Fig. 1). The proportions of the four types of material show distinct changes in the four types of cuticles, demonstrating the great sensibility of this taxon to micro-palaeoenvironment differences. From shaded lower cuticles (less exposed) to sun upper cuticles (most exposed), a gradual decrease of fibrils (and pillarshape structure), and gradual increase of amorphous material can be observed. The intermediate cuticles (sun lower and shade upper cuticles) contain more granules, which are fewer or absent in the extreme cuticles (sun upper and shade lower cuticles).

Finally, ultrastructural studies of cuticles of living plants (Coniferales, genus Pinus, [START_REF] Bartiromo | Influence of volcanic gases on the epidermis of Pinus halepensis Mill. in Campi Flegrei, Southern Italy: A possible tool detecting volcanism in present and past floras[END_REF]; Angiosperms, genus Erica, [START_REF] Bartiromo | The cuticle micromorphology of in situ Ericaarborea L. exposed to long-term volcanic gases in Phlegrean Fields, Campania, Italy[END_REF], also from the same species living in two different environments (volcanic/non-volcanic areas) could be significant for comparisons with fossil materials, as the precise data of these environments can be analysed (soil and atmospheric CO2 concentrations, wind direction…). In Pinus halepensis, the cuticular membrane CM thickness with different zones of B1 increased in the volcanic area, and the different zones of B1 also varied. However, the cell wall (CW) could be measured for this living material (it is rarely the case with fossil material as it is more or less damaged or absent in most cases). The CM was balanced by a thinner CW, making the whole thickness of CM + CW significantly equal between the two environments. In Erica arborea from the same localities, significant differences of CM + CW were found between the two environments, also between the two CW and the two A2 layers. With these two types of living plants, it is clear that the CW also participates in the isolation of the plant. As the CW is rarely observed or entirely preserved in fossils, most often the rare parts with CW do not allow for statistical measurements, so extant cuticle studies provide a great advance in the knowledge of cuticles. In studying cuticle ultrastructure of these living plants, [START_REF] Bartiromo | Influence of volcanic gases on the epidermis of Pinus halepensis Mill. in Campi Flegrei, Southern Italy: A possible tool detecting volcanism in present and past floras[END_REF][START_REF] Bartiromo | The cuticle micromorphology of in situ Ericaarborea L. exposed to long-term volcanic gases in Phlegrean Fields, Campania, Italy[END_REF] mentioned that 'a tendency to a reduction in the fibrillar structure followed by an increase of the granular component has been noted between non-fumigated and fumigated leaves'. Such relation between granules-fibrils and the environment was cited and discussed in the paper of fossil Ginkgoites skottsbergii [START_REF] Guignard | Insights into the leaf cuticle fine structure of Ginkgoites skottsbergii lund. from the Albian of Patagonia and its relationship within Ginkgoaceae[END_REF]) (having a thicker granular A2 layer and thinner B1 fibrilous layer), which shows a different equilibrium of the layers, compared with the more xerothermic G. ticoensis (having a thinner A2 granular layer and a thicker B1 fibrilous layer).

Evolution implications of fossil plant cuticles

Plant cuticle evolution is more and more enhanced for various aspects [START_REF] Niklas | The evolution of hydrophobic cell wall biopolymers: from algae to angiosperms[END_REF][START_REF] Petit | Breeding for cuticle-associated traits in crop species: traits, targets, and strategies[END_REF][START_REF] Renault | A phenol-enriched cuticle is ancestral to lignin evolution in land plants[END_REF]and 2018;especially in tomato, Yeats et al. 2012 andYeats and[START_REF] Yeats | The formation and function of plant cuticles[END_REF], and as written by Cohen et al. ( 2017) "Certainly, the development of plant lipophilic barriers had an immense impact on life on earth and thereby is often considered as one of the most significant evolutionary events" (Kenrick and Crane, 1997;Bateman et al., 1998). However, fossil remains are still scarcely taken in account. When working on plant cuticle ultrastructural features, evolution has been a very attractive topic, enhanced by authors studying fossil plants, from the beginning of TEM papers in fossil cuticles: in their last paragraph of their Tarphyderma study, Archangelsky and Taylor (1986) suggested that 'various structural features were evolving in other major groups of plants in response to the habitat'. The understanding of evolution was also enhanced through the analysis of chemical compounds [START_REF] Taylor | The ultrastructure of fossil cuticle[END_REF], for the 'colonization of the terrestrial environment' [START_REF] Edwards | The cuticle of early vascular plants and its evolutionary significance[END_REF], and conservation of 'biosynthetic pathways' for the 'evolutionary radiation' of 'basal land plants' [START_REF] Jeffree | The fine structure of the plant cuticle[END_REF], and more particularly related to a thickened cuticle in Cactaceae evolution [START_REF] Ogburn | Anatomical variation in Cactaceae and relatives: trait lability and evolutionary innovation[END_REF]. This topic is potentially promising as in living plants recent molecular biology studies in very various taxa enhanced more and more genes related with cuticles, in agronomic plants (fleashy fruits and cereals; [START_REF] Taketa | Barley grain with adhering hulls is controlled by an ERF family transcription factor gene regulating a lipid biosynthesis pathway[END_REF][START_REF] Alkio | Identification of putative candidate genes involved in cuticle formation in Prunus avium (sweet cherry) fruit[END_REF][START_REF] Lashbrooke | Genome investigation suggests MdSHN3, an APETALA2-domain transcription factor gene, to be a positive regulator of apple fruit cuticle formation and an inhibitor of russet development[END_REF]D'Angeli et al. 2016;[START_REF] Wang | Regulation of cuticle formation during fruit development andripening in 'Newhall' navel orange (Citrus sinensis Osbeck) revealed by transcriptomic and metabolomic profiling[END_REF][START_REF] Bi | Wheat drought-responsive WXPL transcription factors regulate cuticle biosynthesis genes[END_REF][START_REF] Xu | Current Advances in Molecular Basis and Mechanisms Regulating Leaf Morphology in Rice[END_REF][START_REF] Zhang | Maize male sterile 33 encodes a putative glycerol-3-phosphate acyltransferase that mediates anther cuticle formation and microspore development[END_REF][START_REF] Shi | The roles of AaMIXTA1 in regulating the initiation of glandular trichomes and cuticle biosynthesis in Artemisia annua[END_REF][START_REF] Belge | Postharvest heat and CO2 shocks induce changes in cuticle composition and cuticle-related gene expression in 'OctoberSun' peach fruit[END_REF]Cohen et al. 2019;[START_REF] Fasoli | Active rearrangements in the cell wall follow polymer concentration during postharvest withering in the berry skin of Vitis vinifera cv[END_REF][START_REF] Wang | Characterization of CRISPR Mutants Targeting Genes Modulating Pectin Degradation in Ripening Tomato[END_REF], and model plants [START_REF] Xu | Overexpression of RcLEC1-B, a HAP3 transcription factor of PLB from Rosa canina, increases the level of endogenous gibberellin and alters the development of cuticle and floral organs in Arabidopsis[END_REF], Zhao et al. 2019). Until recently, although cuticles have been known from very old periods and in various taxa [from terrestrial algae [START_REF] Good | The ultrastructure of Phycopeltis (Chroolepidaceae: Chlorophyta). 1. Sporopollenin in the cell walls[END_REF] to angiosperms], and suggested to have evolutionary significance [START_REF] Edwards | The cuticle of early vascular plants and its evolutionary significance[END_REF][START_REF] Riederer | Introduction: biology of the plant cuticle[END_REF][START_REF] Niklas | The evolution of hydrophobic cell wall biopolymers: from algae to angiosperms[END_REF], there are still limited evolutionary data available in fossil plants using fine cuticle details, mainly owing to limited taxa studied; however, some points can be developed.

Through all orders, considering all the data available for comparisons in the cuticle formulae (Figs. 2 and3), from the most simple formula to the most complex one, a potential evolution of formula through the orders is not clear at all. For instance, Cycadales (basal Gymnosperms compared with Coniferales) have a simple formula, Ginkgoales (also basal Gymnosperms)

have a quite complex formula, while Coniferales have different types of formulae.

However, there are some interesting clues recently revealed by cuticle ultrastructure studies of fossil cuticles as, at the level of the order, Coniferales can be interestingly evocated. Extant

Coniferales are studied more and more (Leslie et al. 2015;[START_REF] Rothwell | Tree of death: The role of fossils in resolving the overall pattern of plant phylogeny[END_REF] and3; appendix C), some remarks can be done.

Araucariaceae have the simplest formula (A2 and spongy B1) and they emerged in Palaeozoic [START_REF] Taylor | Palaeobotany the biology and Evolution of fossil plants[END_REF], Triassic [START_REF] Kunzmann | Araucariaceae (Pinopsida): Aspects in palaeobiogeography and palaeobiodiversity in the Mesozoic[END_REF][START_REF] Stockey | The Araucariaceae: An evolutionary perspective[END_REF][START_REF] Stockey | Mesozoic Araucariaceae: Morphology and systematic relationships[END_REF] putatively in Late

Triassic [START_REF] Axsmith | Two rare fossil cones from the Upper Triassic Chinle Formation in Petrified Forest National Park, Arizona, and New Mexico[END_REF] while unambiguously in Early Jurassic [START_REF] Leslie | An overview of extant conifer evolution from the perspective of the fossil record[END_REF]).

Cheirolepidiaceae have a more complex cuticle (wavy A1, A2, B1, B2) and they emerged widely speaking later (in Mesozoic, [START_REF] Taylor | Palaeobotany the biology and Evolution of fossil plants[END_REF]), more precisely in Late Triassic to Late Cretaceous [START_REF] Yang | Suturovagina intermedia (Cheirolepidiaceae) from the upper Lower Cretaceous Dalazi Formation of Wangqing, Northeast China: cuticle ultrastructure and palaeoenvironmental insights[END_REF]). Miroviaceae are the most complex in their cuticle formula (with A1U (+ A1L for some cuticles), A2U (+ A2L for some cuticles), B1 for Mirovia sibirica) and this species is the youngest, dated to the Early Cretaceous. It seems that,

comparing the cuticles at the same age (Early Cretaceous, the age of the localities studied for these three Families), although we have to be very cautious a potential relation between the degree of simplification and emergence period exists, the oldest family having the most simple cuticle and the youngest family having the most complex cuticle. The threedimensional FCA (Fig. 11), with the combination of 3 axes, made with the absolute values and % of characters (also their absence as evaluated with values "0"), shows clearly with almost 83% data characteristic that the three families are in accord with their emergence period (Fig. 11b; the oldest is on one side of the 3D scheme, the youngest on the opposite side), while Cheirolepidaceae and Miroviaceae are partly merged for some cuticles because of similar values and % of layers along the three axes of the FCA. Although the number of cuticles studied for their fine ultrastructural details is different in each family, they show great differences in their variability of cuticle characters (means and % of characters; appendix B of data in brief article). Araucariaceae are the most stable in their characters values, moreover this stability is reinforced by the other Araucariaceae studied in TEM [Del Fueyo 1991, Del Fueyo andArchangelsky 2002) are from the same age (Lower Cretaceaous)], which show the same cuticle formula. Cheirolepidiaceae cuticles show some variations in their cuticular ultrastructural characters values, they are from the same age but quite related with palaeoenvironment at least for some of them. Miroviaceae cuticles show the greatest variations in their character values, they are from different ages and show different cuticle formulae enabling some remarks detailed further for the potential evolution among this family.

In constructing the preliminary tentative evolutionary scheme of the cuticle ultrastructure in Coniferales, 8 characters (among a total of 23) are much more informative than the others (Fig. 11a). Firstly, spongy B1 layer of the cuticular layer (its mean and %, numbered 18-19 in the figure) is the main distinguishing character among taxa, making mainly the F1 Axis (the most important in significance, 39.9%). Secondly, A1L (its mean, numbered 8) and A2L (its mean and %, numbered 14-15) of the cuticle proper, making mainly the Axis 2 (second in importance, 26%) are also of evolutionary significance. Finally, A2U (its mean, numbered 12) and B2 (its mean and %, named 22-23) of the cuticle proper make mainly the Axis 3 (the third in importance, 16.6 % ).

In this study, the 15 remaining characters (for instance the total cuticular membrane thickness) are located around the axes and not so important to the evolution scheme tentative of cuticle ultrastructure. Till now very few studies have been carried out on the evolution of the cuticle ultrastructure.The present study on the Coniferales is the first attempt to response to [START_REF] Taylor | The ultrastructure of fossil cuticle[END_REF] early assertion, writing that "…the cuticle ultrastructure in palaeobotany may also provide an indirect method of determining how cuticles have evolved.". Similar evolutionary trend in cuticle structure was observed in another Order, Umkomasiales [START_REF] Labe | Etudes ultrastructurales de la cuticule de Préspermatophytes fossiles du Jurassique[END_REF], where the authors noticed an increase in the complexity of cuticle ultrastructure of 4 taxa from Early to Late Jurassic, but without statistical data.

At the level of the family, the cuticle ultrastructure is variable among genera and species.

However, potential evolutionary trends are also visible. In Miroviaceae [START_REF] Nosova | First data on the ultrastructure of the leaf cuticle of a Mesozoic conifer, Mirovia Reymanówna[END_REF], three species of the genus Mirovia from different ages were studied by using TEM: M. kazachstanica from Early Jurassic (Toarcian), M. macrophylla from Middle Jurassic (Bajocian) and M. sibirica from Early Cretaceous (Berriasian-Valanginian). Although it is not at all the remarks of these authors, as for the three Families of Coniferales just evocated, the complexity of the cuticle ultrastructure of these three Miroviaceae species is increasing through time: A1, A2 and B layers visible in the oldest species; A1, A2U and A2L, B in the younger Middle Jurassic species; A1U and A1L, A2U and A2L, B in the youngest species.

Secondly, another very different Family case concerns Ginkgos, where macromorphological (gross morphology…) characters were studied in phylogeny [START_REF] Hasebe | Molecular phylogeny of Ginkgo biloba: close relationship between Ginkgo biloba and Cycads[END_REF]De Franceschi and Vozenin-Serra 2000), more precisely for TEM ultrastructure of reproductive organs [START_REF] Rohr | Ultrastructure of Ginkgo biloba[END_REF]. In 2003, allied with some studies [START_REF] Zheng | A new Mesozoic Ginkgo from eastern Liaoning, China and its evolutionary significance[END_REF]) comparing reproductive characters (ovules) and vegetative characters (leaves) between several gingkos from different ages (from 170 million years ago to living Ginkgo biloba), Zhou and Zheng related a possible case of peramorphosis, which is a case of heterochrony. Using the same material than [START_REF] Zhou | The missing link in Ginkgo evolution[END_REF], cuticle ultrastructural details (the different layers, their values and percentages), allied with macromorphological characters of ginkgos, give some fruitful details whithin Ginkgo [START_REF] Guignard | Comparative studies of leaf cuticle ultrastructure between living and the oldest known fossil ginkgos in China[END_REF]. As result of comparisons between the oldest Ginkgo in China (G. yimaensis,170 millions of years) and living G. biloba, male and female), if the cuticle ultrastructure of these leaves show the same layers of cuticles (A1U + A1L, A2, B1) allowing a characteristic of this genus, between the two taxa separated by 170 millions of years, only few differences in the thicknesses and % of each layer were revealed by statistical measurements, showing a high stability and though a very small degree of potential Evolution compared with the changes of macromorphology of the leaves in the same period. It is interesting to note that, for the same type of organ, i.e. leaves, macromorphological characters (as surfaces and lobes of the leaves) did not follow the same scheme than some micromorphological characters (cuticle ultrastructural characters). Further works are, therefore, needed to study the cuticles of other fossil species of Ginkgos from the Cretaceous and younger strata, such as G. apodes, G. jiayinensis, G. adiantoides and G. cranei [START_REF] Zhou | The missing link in Ginkgo evolution[END_REF][START_REF] Zhou | Tertiary Ginkgo ovulate organs with associated leaves from North Dakota, USA and their evolutionary significance[END_REF][START_REF] Zhou | An overview of fossil Ginkgoales[END_REF]Quan et al., 2010;Quan and Zhou, 2010), to check if there are any changes in ultrastruture of the layers.

Last point concerns the "adaptation" topics, frequently used in fossil cuticle fine detail studies and a component of evolution. Generally speaking, cuticles are known to be related with the arrival of plants on lands: in the introduction of the book on biology of the plant cuticle, [START_REF] Riederer | Introduction: biology of the plant cuticle[END_REF] enhanced in the "evolution of the plant cuticle" part "… the simultaneous appearance of cuticles and stomata as evidence for the physiological adaptations to the colonization of the land…". In extant plants (Mesembryanthemaceae), Ihlenfeldt and Hartmann (1982) considered "fine structure of the epidermal wall" (actually including cuticle according to their figures) as one of the characters to "describe variation in the xeromorphic type of epidermis". In fossil Bennettitales TEM cuticle study [START_REF] Barale | L'ultrastructure de la cuticule de quelques Bennettitales du Crétacé inférieur d'Argentine[END_REF], "adaptation" in a very wide sense is cited.

According to [START_REF] Taylor | Palaeobotany the biology and Evolution of fossil plants[END_REF] in their fossil plants book's glossary, adaptation is "a structure or feature that performs a particular function and which reveals in increased survival or reproduction". In extant organisms the definition is quite similar, enhancing the nonreversibility of adapted features and the transmission to next generations. Adaptation is usually opposed to accommodation, which is also a change but related with the phenotypic plasticity, reversible and not transmitted to the next generations. It is clear that in fossil plants studies these two aspects (adaptation -accommodation) cannot be distinguished as conversely to extant plants neither changes of environment for one single plant nor reversibility and transmission to the next generations can be experimented. For instance in living plants where two different environments for one taxon were studied among two species [START_REF] Bartiromo | Influence of volcanic gases on the epidermis of Pinus halepensis Mill. in Campi Flegrei, Southern Italy: A possible tool detecting volcanism in present and past floras[END_REF][START_REF] Bartiromo | The cuticle micromorphology of in situ Ericaarborea L. exposed to long-term volcanic gases in Phlegrean Fields, Campania, Italy[END_REF], although the authors observed significant differences between the cuticles of volcanic and non-volcanic environments, very cautiously they did not use at all "adaptation", as neither non-reversibility of the changes nor transmission to the next generations was experimented for an eventual adaptation and/or accommodation case.

In TEM studies of fossil cuticles "adaptation" or "adapted" is actually used particularly in a very wide sense of unusual features, especially for extreme palaeoenvironments like xeromorphic or volcanic ones, where unusual cuticle fine details are observed. Haworth and[START_REF] Haworth | Reply to the comment on ''Hot, dry, wet, cold or toxic? revisiting the ecological significance of leaf cuticular micromorphology[END_REF]) concentrated on the critics of "ecological functions of "xeromorphic"

adaptations…" including of course cuticles but unfortunately no ultrastructural fine details.

Villar de [START_REF] Villar De Seoane | Otozamites ornatus sp. nov., a new bennettitalean leaf species from Patagonia, Argentina[END_REF], in a Bennettitales TEM study, enhanced "…strategies of adaptation to the extreme environment…" (volcanic), not especially for the cuticle details but for the stomatal apparatus characters. Interestingly, ultrastructural cuticle fine details are included in the remark of Thevenard et al. (2005) about a Pteridospermales taxon (Dichopteris), for non-xeromorphic characters considering this cuticle with no unusual details. It is clear that several "evolutionary" terms are used in the papers, more or less mixed and apparently in various meanings, also very linked with palaeoenvironmental considerations. With the increase of data, these evolutionary aspects seem to become clearer, however as a synthesis of the use of these terms seems irrelevant, following papers are cited just to illustrate these remarks.

Particularly for volcanic palaeoenvironment, in Cycadales Ticoa study Archangelsky et al. (1986) enhanced for the cuticle ultrastructure to consider "…evolutionary significance of these features". In Cycadales too, Pseudoctenis TEM cuticle details [START_REF] Archangelsky | Cuticular characters adapted to volcanic stress in a new Cretaceous cycad leaf from Patagonia, Argentina. Considerations on the stratigraphy and depositional history of the Baqueró Formation[END_REF]) is related potentially with the stress in volcanic areas. In Cycadales (Villar de Seoane 2005), xeromorphy is used for volcanic palaeoenvironment. In Coniferales, Tarphyderma study (Archangelsky and Taylor, 1986) showed a very special stomatal apparatus and "adaptations" and "structural adaptations in fossil plants" are mentioned. In Bennettitales Pseudofrenelopsis studies [START_REF] Yang | Leaf cuticle ultrastructure of Pseudofrenelopsis dalatzensis (Chow et Tsao) Cao ex Zhou (Cheirolepidiaceae) from the Lower Cretaceous Dalazi Formation of Jilin, China[END_REF][START_REF] Mairot | Cuticle micro-and ultrastructure of Suturovagina Chow et Tsao (Cheirolepidiaceae): taxonomic and palaeoecological implications[END_REF][START_REF] Guignard | Cuticle ultrastructure of Pseudofrenelopsis gansuensis: Further taxonomical implications for Cheirolepidiaceae[END_REF][START_REF] Yang | Suturovagina intermedia (Cheirolepidiaceae) from the upper Lower Cretaceous Dalazi Formation of Wangqing, Northeast China: cuticle ultrastructure and palaeoenvironmental insights[END_REF]) also cite these words.

Conclusions and perspectives

Besides Although of less statistical interest, opaque lamellae OL and translucent lamellae TL of the A1 layer are also represented in these two schemes.

To appear in colour

  Other authors very rarely figured transversal sections, except for extant reproductive organs(De Saporta 1873-1891); however, De Saporta and Marion (1881-1885) published figures of transversal sections of extant stems (dicotyledons Nelumbium, Tradescantia, and Dracaena) and leaflets (Cycadales Encephalartos and Bowenia (their Fig. 49-50, 108-109) with clear cuticle differences of thickness between them. Schimper (1874) provided some Equisetum transversal sections, with limited details of the outermost part, whereas 10 years later De Bary (1884) used largely transversal sections of cuticles in his figures, some of which were extremely precisely drawn (Figs. 25-27 Aloe, Ilex, Pinus; Fig. 222 Betula, Fig. 220 Ribes). Applying the above technique to fossil plants are apparently overlapped in time with extant plant observations, sections of fossil plant cuticles have been documented using light microscopy, and usually with limited details. A few years later after Brongniart's work on extant plants in 1830 and 1834 containing clear descriptions of the cuticle, in the second volume of his fossil plants book, Brongniart (1837) published some transversal sections of permineralized material (petrological thin sections, Pl. 10 Fig. 2 and 3 on lycopods; Pl. 11 Fig.

  Mohl's description (1847; see particularly the distinct Figure 4 in this 1847 volume, showing two clearly distinguished parts of the cuticle, above the cell wall; see also von Mohl 1842, Geneau de Lamarlière 1906, Roelofsen 1952 for his photos and schemes, Roelofsen 1959 for his Fig.

  3.1.3. At the level of the genus and species: quantitative description and statistical values Cuticle ultrastructures and measurements are related to the creation of a taxon, its description, and distinction from related taxa where, generally, values are largely involved. According to the International Code of Nomenclature for Algae, Fungi, and Plants [2018 Shenzen; see also /www.palaeobotany.org/page/nomenclature/nomenclature-typification-and-correctdescription-of-fossil-plants, points 2. Correct description of new (fossil) plant taxa (genera, species) and new combinations 2.1 General rules], a new taxon name must be accompanied by a description or diagnosis (or by a reference to a previously published description or diagnosis) (Art. 32.1d), a diagnosis of a taxon being 'a statement of that which in the opinion of its author distinguishes the taxon from other taxa' (Art. 32.2). In the 'recommendations', three points are of great interest as they state the level of importance attributed to the characters used for a new taxon: 1) the diagnosis should be concise and based on the most important characters only; 2) the description should include all additional data of secondary importance (i.e. not included in the diagnosis), such as measurements; 3) a discussion with comparison to other species; this should provide the differential characters in which the new species differs from other species, at least in the same genus. Furthermore, a fourth point should be added to these three formal points: in Palaeobotany the use of diagnosis emendation also occurs, corresponding (in the case of cuticle ultrastructure) to a change of definition thanks to newly observed characters compared with the original 'older' diagnosis, related to the use of new methods of study. Although there are different opinions as regard to the taxonomic value of fossil plant cuticle ultrastructural features, which may include simple evaluations of layer and sublayer thickness, their percentages, to more elaborate measurements with statistical evaluations (confidence interval and other tests) based on a high number of values (up to 30 so far known in this review), recent studies have become more and more clearly shown that the ultrastructural features of fossil cuticles may be considered as significant in taxonomy, as any other character. First, cuticle ultrastructure characters were included in the diagnosis with the creation of the genus Squamastrobus by Archangelsky and Del Fueyo (1989). After that, such important characters have been given in the diagnosis of two conifers (Morenoa fertilis, Del Fueyo et al. 1990; Glenrosa nanjinensis, Zhou et al. 2000), a new Araucariaceae taxon (Nothopehuen brevis; Del Fueyo 1991) or with araucarian affinity (Brachyphyllum garciarum, Carrizo et al, 2019 this issue), three new cycads (Pseudoctenis ornata, Archangelsky et al. 1995; Mesosingeria oblonga and Ticoa lanceolata, Villar de Seoane 2005), three new Bennettitales (Otozamites ornatus, Villar de Seoane 1999; Otozamites patagonicus, Villar de Seoane 2001; Ptilophyllum eminelidarum Carrizo et al. 2019b), and one Pteridospermatophyta (Ruflorinia orlandoi; Carrizo et al. 2014). Not only cuticle ultrastructure characters of vegetative organs, but also those of the reproductive material (megaspore membranes) have also been incorporated into the diagnosis (Karkenia of Ginkgoales, Zhou et al. 2002; Del Fueyo and Archangelsky, 2001). Second, the description of a new taxon: a new genus (Tarphyderma; Archangelsky and Taylor 1986) was created, including after the diagnosis a description of the 'cuticle ultrastructure'. Artabe et al. (1991) included the cuticle ultrastructure of Kurtziana brandmayri in their description; the description was used in 1992 for 'complementary details' of Squamastrobus tigrensis (Barale et al. 1992); for a new combination (Pachypteris bagualensis) a cuticle ultrastructural description was included (Baldoni and Barale 1996); Passalia et al. (2010), created a new genus with a new species (Restrepophyllum chiguoides), and the cuticle ultrastructure was included in their 'description of the leaf cuticular membrane ultrastructure'.Description was also detailed with three species of Miroviaceae (Coniferales;[START_REF] Nosova | First data on the ultrastructure of the leaf cuticle of a Mesozoic conifer, Mirovia Reymanówna[END_REF]. Connected with the diagnosis of Ptilophyllum eminelidarum(Carrizo et al. 2019b), the authors also provided a precise description of the layers of the cuticles.

  1997) and fossil Ginkgo yimaensis[START_REF] Guignard | Comparative studies of leaf cuticle ultrastructure between living and the oldest known fossil ginkgos in China[END_REF]. Similarly, Mesodescolea plicata (fossil Cycadales) was compared with the living Stangeria paradoxa[START_REF] Artabe | Las cycadales Mesodescolea Archangelsky emend. Archangelsky y Petriella 1971 ([END_REF], and Restrepophyllum chiguoides (fossil Cycadales) was compared with living Zamia[START_REF] Passalia | An Early Cretaceous zamiaceous cycad of South West Gondwana: Restrepophyllum nov. gen. from Patagonia, Argentina[END_REF].Fourth, an emendation of the diagnosis of Ginkgoites tigrensis was published, immediately before the description, with the cuticle ultrastructure (Villar de Seoane 1997). The following year, Tomaxellia biforme (Cheirolepidiaceae) and Athrotaxis ungeri (Taxodiaceae) were compared with two living Taxodiaceae, and an emended diagnosis including ultrastructure was proposed (Villar de Seoane 1998). This was also the case for Araucaria grandifolia (Araucariaceae; Del Fueyo and Archangelsky 2002), Ptilophyllum valvatum and Cycadolepis coriacea (Bennettitales; Villar de Seoane 2001), and the new combination of Ginkgoites patagonica with an 'emended species diagnosis' (Villar de seoane et al. 2015). Based on data of 41 fossil cuticles studied with TEM, of which absolute values and percentage of characters (23 in total) could be analyzed [selected from the 52 cuticles used in the 0-1 data matrix (Appendices A and B of data in brief article], and their curve tendencies (Appendix C), the correlations between some of them (26) are simple and linear (y = ax + b)

  figure) showed importance for the distinction between cuticles in this FCA. Concerning the distinction between cuticles (disposed according the absolute values and % of characters; Fig.6), all groups of the HCA (made with qualitative 0-1 presence-absence; Fig.3) are also relevant (except Cycadales type II, which could not be considered in this FCA): Coniferales (Cheirolepidiaceae, Miroviaceae, Araucariaceae), Ginkgoales type I and II, Pteridospermales type I and II, and Czekanowskiales. For some groups, the distinction is still clear (Coniferales Araucariaceae, Pteridospermales type I, Ginkgoales type I) and explained by the homogeneity and small variability of the absolute values and % of thickness of the constituents in each group. However, this FCA is fundamental as the values and % of the cuticles of the other groups are much more heterogeneous, demonstrating a higher variability, and the positions of the cuticles are finer and inter-linked (especially Ginkgoales type II and Coniferales Cheirolepidiaceae, at a lesser degree Coniferales Miroviaceae, Czekanowskiales, Pteridospermales type II), revealing some possible affinities between these groups.

(

  Villar de Seoane 2001), «…xeromorphic characters…» are used to discuss in seven species study including other features than TEM details. Carrizo et al. (2014) wrote about the "great success at adapting to the different stressed environments" for a new Ruflorinia (Pteridospermales), through various features including TEM. Recently (Passalia et al. 2010) used the term "clade" for the study of a new Cycadales, and suggest a "Chigua clade" in "plant evolution".In Cheirolepidiaceae where special characters are usually observed (wavy A1 layer and other fine details…), xeromorphic features and adaptations are largely cited. Villar de Seoane (1998), comparing extant taxodiaceean and fossil cheirolepidiacean taxa, asking in the discussion for the "interpretations of the phylogenetic relationships of the Cheirolepidiaceae with other conifer families…", suggested in the "relationship" conclusion about "a close relationship between leaves of the Cheirolepidiaceae and Taxodiaceae". Suturovagina and

  Figure 1. OEC = ordinary epidermal cell cuticle, GC = guard cell cuticle.

Figure 4 .

 4 Figure 4. CHAID made with guard cell cuticles features.To appear in colour

Figure 5 .

 5 Figure 5. FCA three-dimensional representation of 23 cuticle characters, linked with figure 6.To appear in colour

Figure 6 .

 6 Figure 6. FCA three-dimensional representation of 41 cuticles, linked with Figure 5.To appear in colour

Figure 7 .

 7 Figure 7. Two-dimensional FCA with 15 characters (a) and 9 cuticles from 4 taxa (b), belonging to 2 types: epidermal and subsidiary cells. Axes 1 and 2.To appear in colour

Figure 10 .

 10 Figure 10. Diagram of reconstruction of the four types of cuticles of Komlopteris according to their ultrastructures.To appear in colour

Figure 11 .Coniferales

 11 Figure 11. Three-dimensional FCA, potential evolution scheme among Coniferales cuticles.To appear in colour

F1

  , Pseudofrenelopsis, and Hirmeriella (Cheirolepidiaceae,Guignard et al. 1998[START_REF] Yang | Leaf cuticle ultrastructure of Pseudofrenelopsis dalatzensis (Chow et Tsao) Cao ex Zhou (Cheirolepidiaceae) from the Lower Cretaceous Dalazi Formation of Jilin, China[END_REF]), and Ginkgoites (Ginkgoales, Del Fueyo et al. 2013). Reproductive material is also relevant in this field (fertile Podocarpaceae,[START_REF] Archangelsky | Sobre una Podocarpacea fertile del Cretacico Inferior de la provincia Santa Cruz, Republica Argentina[END_REF] corystospermaceous ovules, Barale et al. 2009).

  Dominguez et al. 2011a and b, 2017;[START_REF] Martin | There's more than one way to skin a fruit: formation and functions of fruit cuticles[END_REF][START_REF] Khanal | Mechanical properties of cuticles and their primary determinants[END_REF][START_REF] Schuster | The ecophysiology of leaf cuticular transpiration: are cuticular water permeabilities adapted to ecological conditions[END_REF][START_REF] He | Abiotic Stresses: General Defenses of Land Plants and Chances for Engineering Multistress Tolerance Frontiers in Plant Science[END_REF][START_REF] Heredia-Guerrero | Plant cuticle under global change: Biophysical implications[END_REF]; particularly in Arabidopsis,[START_REF] Kosma | The impact of water deficiency on leaf cuticle lipids of Arabidopsis[END_REF] (cited in Heredia-Guerrero et al. 2017); in pear fruit Pyrus

	communis L., Serra et al. 2018; in eggplant (Solanum melongena L.), Valerga et al. 2019; in
	sunflower (Helianthus annuus), Li et al. 2019; in two Leucospermum cultivars (Proteaceae),
	Suarez et al. 2019; also including (Aragón et al. 2017) in their review of many papers,
	external and internal environmental factors like fungi, bacteria, viruses, yeasts and insects
	(Gorb and Gorb 2017) and herbivorous]. From the beginning of TEM fossil cuticle studies,

[START_REF] Guignard | Insights into the leaf cuticle fine structure of Ginkgoites skottsbergii lund. from the Albian of Patagonia and its relationship within Ginkgoaceae[END_REF][START_REF] Del Fueyo | Leaf cuticle anatomy and the ultrastructure of Ginkgoites ticoensis Archang. from the aptian of patagonia[END_REF]

, Suturovagina and Pseudofrenelopsis (Cheirolepidiaceae,

[START_REF] Guignard | Cuticle ultrastructure of Pseudofrenelopsis gansuensis: Further taxonomical implications for Cheirolepidiaceae[END_REF][START_REF] Yang | Suturovagina intermedia (Cheirolepidiaceae) from the upper Lower Cretaceous Dalazi Formation of Wangqing, Northeast China: cuticle ultrastructure and palaeoenvironmental insights[END_REF]

, Baiera and Sphenobaiera of Ginkgoales

[START_REF] Guignard | this issue. Cuticle ultrastructure of Baiera-like leaves: taxonomic and palaeoecological implications[END_REF]

this issue), Brachyphyllum of Araucariaceae affinity

(Carrizo et al. 2019a

, this issue).

3.2 Palaeoenvironmental significances

Cuticles have been increasingly intensively studied for various relations with the environment in living plants [

  The second group of taxa with studied cuticles concerns Ginkgoales, where several species and genera were compared. Ginkgoites ticoensis[START_REF] Del Fueyo | Leaf cuticle anatomy and the ultrastructure of Ginkgoites ticoensis Archang. from the aptian of patagonia[END_REF]

	2009); Suturovagina intermedia (Mairot et al. 2014) and Yang et al. (2018): Pseudofrenelopsis
	gansuensis (Guignard et al. 2017) exhibited a number of xeromorphic characters, not only a
	thick cuticle, the presence of a cutinized hypodermis, and other'morphological characters that

); Pseudofrenelopsis dalatzensis

(Yang et al.

prevent excessive evaporation from the leaves'; but also fine details, including a thick and wavy polylamellate zone (A1 layer) with measurements of opaque and translucent lamellae at the nanometer scale.

  Cupressaceae and other families including fossils,[START_REF] Leslie | An overview of extant conifer evolution from the perspective of the fossil record[END_REF], but none of these approaches used fine details of the cuticle ultrastructure. Although fossils Coniferales are up to now only represented by four Families in TEM (Araucariaceae, Cheirolepidiaceae,

	Miroviaceae, Podocarpaceae; Figs. 2

in phylogeny through leaf anatomical characters and chloroplast genes (Taxaceae,

[START_REF] Elpe | Diversity and evolution of leaf anatomical characters in Taxaceae s.l. Fluorescence microscopy reveals new delimitating characters[END_REF]

Pinaceae including fossils,

[START_REF] Gernandt | Phylogenetics of extant and fossil Pinaceae: methods for increasing topological stability[END_REF] and 2018a[START_REF] Saladin | Fossils matter: improved estimates of divergence times in Pinus reveal older diversification[END_REF]

  1-2. A1 straight polylamellate layer. Note the A2 granular layer below ,and extracuticular material over the A1 layer. 1. rare case in Gingkoites ticoensis, OEC. 2. rare case in Pseudofrenelopsis gansuensis, OEC. 3-5.A1 wavy polylamellate layer. Note the A2 granular layer below, and extracuticular material over the A1 layer. 3. Pseudofrenelopsis gansuensis, OEC. 4. Suturovagina intermedia, OEC. 5. Pseudofrenelopsis gansuensis, OEC. 6-7. A1U and A1L polylamellate sublayers. Note that the A1 layer is divided in two parts, A1U ( U = upper) sublayer (or zone) with straight polylamellae and A1L (L = lower) sublayer (or zone), with fewer lamellae oriented more or less oblique or even rather transversal to the outermost part of Gingkoites ticoensis, OEC. 12-14. B2 granular layer. Note the granules of this thin layer in the inner most part of cuticles, directly connected with B1 fibrilous layer just above, above blackish cell remnants as in photo 13. 12-14.Suturovagina intermedia, OEC. For other photos of these cuticle taxa, see Del[START_REF] Del Fueyo | Leaf cuticle anatomy and the ultrastructure of Ginkgoites ticoensis Archang. from the aptian of patagonia[END_REF]

	Gingkoites ticoensis), Guignard et al. 2017 (Pseudofrenelopsis gansuensis), Nosova et al. 2016 (Mirovia
	sibirica), Mairot et al. 2014 and Yang et al. 2018 (Suturovagina intermedia), Carrizo et al. 2019 this issue
	(Brachyphyllum garciarum).

the cuticle. 6. Gingkoites ticoensis, OEC. 7. Gingkoites ticoensis, OEC. 8-9. A2 granular layer, homogeneous or heterogeneous. Note the granules of this layer, very homogeneous for A2U (U = upper) or much more heterogeneous for A2L (L = lower). 8. Pseudofrenelopsis gansuensis, OEC. 9. Mirovia sibirica, OEC, Courtesy Nosova. 10-11. B1 layer, fibrilous or spongy. Note that the fibrils are more or less mixed with delicate granules, they can make a fibrilous scheme or spongy if the concentrations of fibrils is variable and then more or less visible. 10. Brachyphyllum garciarum, GC. 11.

  Table 1 selected criteria for various methods Note. Percentages and duration values are indicated when they are provided in the relevant papers.

	Figure 1. Synthetic construction of a fossil cuticle.
	Note. This scheme is based on Archangelsky et al. (1986) following Holloway (1982), and
	taking account of new data from Guignard and Zhou (2005) and Nosova et al. (2016).
	Figure 2. Potential ultrastructural dichotomous key for fossil cuticles.
	To appear in colour
	Note. a. cuticles with A1, A2 /or not, B1 /or not, B2 /or not
	b. cuticles with no A1, A2, B1 /or not, B2 /or not
	Figure 3. HCA made with cuticles formulae
	To appear in colour

  Miroviaceae (Mirovia kazachtanica UC and LC; Mirovia macrophylla UC and LC; Mirovia sibirica UC and LC); Araucariaceae affinity (Brachyphyllum garciarum UC and LC).Gingkoales type I are represented in light green colour by: Sphenobaiera huangii UC and LC; Baiera furcata UC and LC. Gingkoales type II are represented in dark green colour by: Ginkgo biloba male UC and LC, female UC and LC; Ginkgo yimaensis UC and LC; Ginkgoites ticoensis LC; Ginkgoites skottsbergii LC; Pseudotorellia asiatica UC and LC; Pseudotorellia samylinae UC and LC. Pteridospermales type I are represented in red colour by Komlopteris nordenskioeldii UC and LC sun leaf, Komlopteris nordenskioeldii UC and LC shade leaf; Dichopteris visianica UC and LC. Pteridospermale type II are represented in red colour by; Pachypteris gradinarui UC and LC. Czekanowskiales are represented in dark blue colour by: Arctobaiera renbaoi UC and LC; Phoenicopsis eutyphylla UC and LC. Data are from appendix B of data in brief article

intermedia more xerothermic LC, less xerothermic thick cuticle LC, less xerothermic thin cuticle LC; Pseudofrenelopsis dalatzensis LC; Pseudofrenelopsis gansuensis LC);

Brachyphyllum garciarum LC Brachyphyllum garciarum UC Notopehuen brevis UC Notopehuen brevis LC Dichopteris visianica LC Dichopteris visianica UC Komlopteris nordenskioeldii shade LC Komlopteris nordenskioeldii shade UC Komlopteris nordenskioeldii sun UC Komlopteris nordenskioeldii sun LC Ticoa harrisii UC Pseudoctenis ornata LC Pachypteris gradinarui LC Pseudotorellia asiatica UC Mirovia kazachtanica UC Mirovia kazachtanica LC Phoenicopsis eutyphylla LC Arctobaiera renbaoi LC Ticoa harrisii LC Baiera furcata LC Baiera furcata UC Sphenobaiera huangii UC Sphenobaiera huangii LC Phoenicopsis eutyphylla UC Ruflorinia orlandoi UC Arctobaiera renbaoi UC Pachypteris gradinarui UC Pseudotorellia samylinae LC Pseudotorellia samylinae UC Pseudotorellia asiatica LC Ginkgoites skottbergii LC Ginkgoites ticoensis LC Ginkgo yimaensis LC Ginkgo yimaensis UC Ginkgo biloba female LC Ginkgo biloba female UC Ginkgo biloba male UC Ginkgo biloba male LC Mirovia macrophylla UC Mirovia macrophylla LC Mirovia sibirica UC Mirovia sibirica LC Pseudofrenelopsis gansuensis LC Pseudofrenelopsis dalatzensis LC Suturovagina intermedia more xerothermic LC Hirmeriella muensteri UC Hirmeriella muensteri LC Ruflorinia orlandoi LC 0 100 (cuticular membrane) cuticle proper A 2, 3 A mean and A % 4, 5 wavy A1U mean and % 6, 7 straight A1U mean and % 8, 9 A1L mean and % 10 OL opaque lamellae of A1 mean in nm 11 TL translucent lamellae of A1 mean in nm 12, 13 A2U mean and % 14, 15 A2L mean and % cuticular layer B 16, 17 B mean and % 18, 19 spongy B1 mean and % 20, 21 fibrilous B1 mean and % 22 , 23 B2 mean and %

	b. cuticles with no A1, A2,
	B1 /or not, B2 /or not
			Coniferales	Ginkgoales
			Araucariaceae	type I = A2,
			= A2, spongy B1	fibrilous B1
		Arctobaiera Zhou Guignard	Nothopehuen Del Fueyo 1991	Sphenobaiera Wang et al. 2005	Dichopteris Thévenard et al.
	Ruflorinia Carrizo et al. 2014 upper and lower	1998 Phoenicopsis Zhou Guignard 1998	Araucaria Del Fueyo Archangelsky 2002 Brachyphyllum Carrizo et al 2019	Baiera Guignard et al. 2019	Komlopteris 2005 Guignard et al. 2001
	differences
	(A2)
		A2	A2	A2	A2
			spongy	fibrilous
	B1	B1	B1	B1
		B2
	B2

  Table 1 selected criteria for various methods

	(Archangelsky et al. 1995) (Guignard et al. 2019)
	Cycadales Ginkgoales	glutaraldehyde 4%	osmium osmium	Spurr Epon	uranyl acetate and lead citrate uranyl acetate and lead citrate
	(Bajpai 2000) (Wang et al. 2005) Cycas Sphenobaiera	fixation	postfixation resin tetraoxide 2% tetraoxide	staining
	Cycadales Mesosingeria, Ticoa		osmium tetraoxide	Spurr type	potassium permanganate 5-10 min and uranyl acetate 30 s
	Coniferales Cheirolepidiaceae (Guignard et al. 1998) (Passalia et al. 2010) Hirmeriella (Villar de Seoane 2005) Cycadales Restrepophyllum	paraformaldehyde glutaraldehyde	osmium tetraoxide 1% osmium tetraoxide 2%	Epon Spurr	uranyl acetate 7% 20 min and lead citrate 1min and uranyl lead citrate 20 min acetate 10 min
	Coniferales Cheirolepidiaceae Pteridospermales Pachypteris (Baldoni and Barale 1996) Pseudofrenelopsis (Yang et al. 2009) Coniferales Cheirolepidiaceae Corystospermaceous taxa Dicroidium (Maheshwari and Bajpai 1996a) Suturovagina Pteridospermales Pachypteris, (Mairot et al. 2014) Coniferales Cheirolepidiaceae Komlopteris (Bajpai and Maheshwari 2000) Pteridospermales Pseudofrenelopsis (Guignard et al. 2017) Coniferales Cheirolepidiaceae Komlopteris (Guignard et al. 2001) Corystospermales Pachypteris Suturovagina (Guignard et al. 2004) (Yang et al. 2018) Coniferales Glenrosa Zhou et al. 2000 Corystospermales Zuberia (Martinez et al. 2019) Coniferales Podocarpaceae Squamastrobus (Archangelsky and Del Fueyo Bennettitales Zamites, dictyozamites, Ptilophyllum, Otozamites, 1989) Coniferales Podocarpaceae Coniferales Podocarpaceae Squamastrobus (Barale et al. 1992) Coniferales Taxodiaceae Coniferales Cheirolepidiaceae Tomaxellia Coniferales Miroviaceae (Nosova et al. 2016) (Carrizo et al. 2019a) Mirovia Ptilophyllum Bennettitales (Villar de Seoane 2003) (Villar de Seoane 1998) Williamsonia Pterophyllum, Cycadolepis, Zamites, Ptilophyllum, (Villar de Seoane 1998) Dictyozamites, Otozamites, Athrotaxis Bennettitales (Villar de Seoane 2001) Ptilophyllum, Cycadolepis Otozamites, Pterophyllum, Bennettitales (Villar de Seoane 1999) (Del Fueyo et al. 1990) Otozamites Morenoa Bennettitales (Barale and Baldoni 1993) Cycadolepis	paraformaldehyde paraformaldehyde glutaraldehyde 4% glutaraldehyde 4% paraformaldehyde paraformaldehyde paraformaldehyde 4% paraformaldehyde Archangelsky 2005 Del Fueyo and technique of	osmium tetraoxide 1% osmium tetraoxide 1% osmium tetraoxide 2% osmium osmium tetraoxide 1% tetraoxide 4% osmium osmium tetraoxide 1% tetraoxide 1% osmium tetraoxide osmium tetraoxide 2% osmium tetraoxide 1% 100 potassium permanganate 1% 0 osmium tetraoxide 2% tetraoxide 2% Osmium tetraoxide osmium osmium tetraoxide 1% 0	Epon Spurr Epon Spurr Spurr Epon Epon Epon Epon Spurr Spurr and Epon Spurr Spurr Spurr Epon-araldite Spurr Spurr Spurr Spurr Spurr Spurr Spurr 100	uranyl acetate 7% 15 min and potassium permanganate 1% lead citrate 20 min 10 min, uranyl acetate 2% 5 min, lead citrate 2-3 min uranyl acetate 15 min and lead uranyl acetate 7% 15 min and citrate 5 min lead citrate 20 min uranyl acetate and lead citrate uranyl acetate 7% 15 min and lead citrate 20 min uranyl acetate 7% 15 or 20 min and lead citrate 20 min uranyl acetate 7% 15 min and lead citrate 20 min potassium permanganate 5-10 min, uranyl acetate 30 sec potassium permanganate 5-10 potassium permanganate 1% 5 min and uranyl acetate 30 s min, uranyl acetate 5% 10 min and lead citrate 2 min uranyl acetate 10 min lead citrate 1 min, lead citrate 5 min min and uranyl acetate 30 s potassium permanganate 5-10 min and uranyl acetate 30 s potassium permanganate 5-10 min and uranyl acetate 30 s potassium permanganate 5-10 5 min and lead citrate 2-3 min min and uranyl acetate 30 s 15 min, uranyl acetate 2% potassium permanganate 5-10 potassium permanganate 1,5% min and uranyl acetate 30 s potassium permanganate 5-10 min and uranyl acetate 30 s potassium permanganate 5-10 200	20 10 0	0	OL mean -TL mean 20 40	60
	Coniferales Araucariaceae (Del Fueyo and Archangelsky (Villar de Seoane 1997) Araucaria Ginkgoites Ginkgoales		osmium tetraoxide 2%	Spurr Spurr	lead citrate 1 min and uranyl acetate 10 min
	2002) Ginkgoales	paraformaldehyde	osmium	Epon	uranyl acetate 7% 15 min and
	Coniferales Araucariaceae affinity (Del Fueyo et al. 2013) Ginkgoites	paraformaldehyde	osmium tetraoxide 1% tetraoxide 1%	Epon	lead citrate 20 min uranyl acetate 7% 15 min and lead citrate 20 min
	Brachyphyllum Ginkgoales	paraformaldehyde	osmium	Epon	uranyl acetate 7% 15 min and
	(Carrizo et al. 2019) Ginkgoites		tetraoxide 1%		lead citrate 20 min
	(Guignard et al. 2016)
	Cycadales Ticoa (Archangelsky et al. 1986) (Nosova and Yakovleva 2019) Pseudotorellia Ginkgoales	glutaraldehyde paraformaldehyde -	osmium tetraoxide 1% tetraoxide 2% osmium	Spurr araldite Epon-	5-10min or lead citrate 1-5min 10-30 min, uranyl acetate 2% potassium permanganate 5% lead citrate 5 min
	Cycadales Pseudoctenis Ginkgoales Baiera	paraformaldehyde	osmium tetraoxide 1%	Spurr Epon	uranyl acetate 7% 15 min and potassium permanganate 5-10 min and uranyl acetate 30 s lead citrate 20 min

CM mean -A mean

Acknowledgements

I wish to thank here all researchers -colleagues who with I worked on fossil cuticles approach by transmission elctron microscopy, also all other members of laboratories involved in this kind of research. Special thanks are for Georgina Del Fueyo and her careful reading of the manuscript before submission, also for two anonymous reviewers.

and 3 = S. intermedia less xerothermic thick cuticle LC and thin cuticle LC. Pseudofrenelopsis 1 = P. dalatzensis LC. Pseudofrenelopsis 2 = P. gansuensis LC. Mirovia 1and 2 = M. kazachstanica UC and LC. Mirovia 3 and 4 = M. macrophylla UC and LC. Mirovia 5 and 6 = M. sibirica UC and LC. For original values and % data of each cuticle, see appendix C.

 Carrizo, M.A., Lafuente Diaz, M.A., Del Fueyo, G.M., Guignard, G., 2019b ----------------------------------------------------------------------------------------------------------------