Jorge Mondéjar-Fernández
email: jorge.mondejar-fernandez@mnhn.fr]

François J Meunier
email: francois.meunier@mnhn.fr]

Richard Cloutier
email: richard_cloutier@uqar.ca

Gaël Clément
email: gael.clement@mnhn.fr]

Michel Laurin
email: michel.laurin@mnhn.fr

A microanatomical and histological study of the scales of the Devonian sarcopterygian Miguashaia bureaui and the evolution of the squamation in coelacanths

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

INTRODUCTION

Coelacanths (or actinistians) have frequently been described as displaying a remarkable morphological conservatism following an early evolutionary burst at their emergence in the fossil record at the beginning of the Devonian period (around 420 million years ago) [START_REF] Forey | The Coelacanth as a Living Fossil[END_REF]Cloutier, 1991a;[START_REF] Zhu | Earliest known coelacanth skull extends the range of anatomically modern coelacanths to the Early Devonian[END_REF]. The discovery in 1938 of Latimeria chalumnae [START_REF] Smith | A living fish of Mesozoic type[END_REF] off the coast of South Africa was completed by the identification of a second species, L. menadoensis [START_REF] Erdmann | Indonesian "king of the sea" discovered[END_REF][START_REF] Pouyaud | Une nouvelle espèce de coelacanthe, preuves génétiques et morphologiques[END_REF] from the Sulawesi Island of Indonesia, allowing access to extant material for key morphological comparisons with the relatively rich fossil record of coelacanths (Cloutier & Forey, 1991;[START_REF] Forey | History of the Coelacanth Fishes[END_REF]. Genomic data obtained from these extant representatives suggest that there is indeed low molecular diversity within populations of L. chalumnae [START_REF] Lampert | Population divergence in East African coelacanths[END_REF]) and low substitution rates in the coelacanth lineage that led to the two extant species [START_REF] Amemiya | Complete HOX cluster characterization of the coelacanth provides further evidence for slow evolution of its genome[END_REF][START_REF] Amemiya | The African coelacanth genome provides insights into tetrapod evolution[END_REF][START_REF] Lang | Conservation of shh cis regulatory architecture of the coelacanth is consistent with its ancestral phylogenetic position[END_REF][START_REF] Higasa | Extremely slow rate of evolution in the HOX cluster revealed by comparison between Tanzanian and Indonesian coelacanths[END_REF].

Molecular data have also revealed the existence of a putative third coelacanth species in Indonesia [START_REF] Kadarusman | A thirteenmillion-year divergence between two lineages of Indonesian coelacanths[END_REF]. However, the supposed morphological stasis of coelacanths has been challenged by new fossil discoveries [START_REF] Casane | Why coelacanths are not 'living fossils': a review of molecular and morphological data[END_REF] that highlighted important differences in the skull, axial skeleton, and internal organs throughout their evolutionary history [START_REF] Lund | New genera and species of coelacanths from the Bear Gulch Limestone (lower Carboniferous) of Montana (USA)[END_REF], 1985;[START_REF] Forey | History of the Coelacanth Fishes[END_REF]Friedman & Coates, 2006;[START_REF] Brito | The histological structure of the calcified lung of the fossil coelacanth Axelrodichthys araripensis (Actinistia: Mawsoniidae)[END_REF]Wendruf & Wilson, 2012;[START_REF] Cavin | Heterochronic evolution explains novel body shape in a Triassic coelacanth from Switzerland[END_REF][START_REF] Cupello | Lung anatomy and histology of the extant coelacanth shed light on the loss of air-breathing during deep-water adaptation in actinistians[END_REF].

In the case of the dermal skeleton, the alleged conservatism of coelacanths has pervaded anatomical descriptions and led to a flagrant lack of comparative histological information on fossil coelacanth scales, with Forey (1998, p.220) even stating that 'there is little to suggest that they vary in any significant way from the structure seen in Latimeria'. Indeed, the scales of all known extinct and extant actinistians are rounded and are ornamented with tubercles and/or coarse or thin ridges that may break into small leaf-or spoon-shaped tubercles near the anterior margin of the exposed area in early forms. This ornamentation pattern is uniformly maintained in many coelacanths from the Palaeozoic to Recent (e.g. Gavinia, Miguashaia, Diplocercides, Nesides, Rhabdoderma, Whiteia, Piveteauia, Undina, Macropoma, Coelacanthus, Swenzia, and Latimeria) [START_REF] Stensiö | On the Devonian coelacanthids of Germany with special reference to the dermal skeleton[END_REF][START_REF] Schaeffer | A revision of Coelacanthus newarki and notes on the evolution of the girdles and basal plates of the median fins in the coelacanthini[END_REF][START_REF] Forey | Acanthodian and coelacanth fish from the Dinantian of Foulden, Berwickshire, Scotland[END_REF][START_REF] Cloutier | The primitive actinistian Miguashaia bureaui Schultze (Sarcopterygii)[END_REF][START_REF] Forey | History of the Coelacanth Fishes[END_REF][START_REF] Forey | A new coelacanth from the Middle Devonian of Latvia[END_REF][START_REF] Clément | The actinistian (Sarcopterygii) Piveteauia madagascarensis Lehman from the Lower Triassic of Northeastern Madagascar: a redescription on the basis of new material[END_REF][START_REF] Clément | A new coelacanth (Actinistia, Sarcopterygii) from the Jurassic of France, and the question of the closest relative fossil to Latimeria[END_REF][START_REF] Long | A new genus of fossil coelacanth (Osteichthyes: Coelacanthiformes) from the Middle Devonian of southeastern Australia[END_REF] (Table 1).

The histological nature of the ridges and tubercles of the scales and dermal bones of coelacanths remains unclear in many taxa and might lack the seeming homogeneity implied by external observation. In the extant Latimeria, the ridges and [START_REF] Millot | Anatomie de Latimeria chalumnae. T. 1. Squelette, muscles et formations de soutien[END_REF][START_REF] Millot | Anatomie de Latimeria chalumnae. T. 3. Appareil digestif, téguments, écailles[END_REF] and are of odontogenic nature, as they are made of dentine capped with enamel [START_REF] Bernhauser | Zur Knochen-und Zahnhistologie von Latimeria chalumnae Smith und einiger Fossilformen[END_REF][START_REF] Smith | The structure of the scales of Latimeria chalumnae[END_REF][START_REF] Castanet | Données préliminaires sur les structures histologiques du squelette de Latimeria chalumnae. I -Dents, écailles, rayons de nageoires[END_REF]Meunier et al., 2008;Meunier et al., 2019). In extinct forms, the ridges have been reported to be made of dentine in Diplocercides jaeckeli from the Devonian [START_REF] Gross | Histologische Studien am Aussenskelett fossiler Agnathen und Fische[END_REF][START_REF] Gross | Kleine Schuppenkunde[END_REF], Undina penicillata from the Jurassic [START_REF] Gross | Histologische Studien am Aussenskelett fossiler Agnathen und Fische[END_REF][START_REF] Gross | Kleine Schuppenkunde[END_REF] and Macropoma lewesiensis from the Cretaceous [START_REF] Williamson | XXIII. On the microscopic structure of the scales and dermal teeth of some Ganoid and Placoid Fish[END_REF].

In Diplurus newarki from the Triassic [START_REF] Schaeffer | The Triassic coelacanth Diplurus, with observations on the evolution of the Coelacanthini[END_REF] the scales were tentatively described as being covered by a thin layer of enamel, except on the ridges and tubercles. In the dermal bones of Spermatodus pustulosus from the Permian [START_REF] Westoll | On Spermatodus pustulosus Cope, a coelacanth from the "Permian" of Texas[END_REF], the dermal bones display the tubercles are composed of dentine and enamel (i.e., odontodes) [START_REF] Meinke | A light and scanning electron microscope study on the dermal skeleton of Spermatodus (Pisces: Coelacanthini) and the evolution of the dermal skeleton in coelacanths[END_REF] organized in displaying several layers (i.e. of superimposed odontodes) with t and traces of resorption and redeposition of bone and odontodes, but the scales were not surveyed. Despite this wealth of data and material availability, no comprehensive and comparative histological work has been done on the scales of coelacanths and thus many observations, not properly illustrated by histological preparations, might be considered as unreliable in the absence of a thorough histological survey. Most notably, [START_REF] Gross | Histologische Studien am Aussenskelett fossiler Agnathen und Fische[END_REF] already pointed out that the histology of coelacanths had been poorly surveyed and comparative data from Palaeozoic, and The Devonian fossil record of coelacanths is scarce, represented by taxa such as Eoactinistia foreyi from the Pragian of Australia [START_REF] Johanson | Oldest coelacanth, from the Early Devonian of Australia[END_REF], Euporosteus from the Pragian-Givetian of China (E. yunnanensis) and Germany (E. eifeliensis) [START_REF] Stensiö | On the Devonian coelacanthids of Germany with special reference to the dermal skeleton[END_REF][START_REF] Zhu | Earliest known coelacanth skull extends the range of anatomically modern coelacanths to the Early Devonian[END_REF], Gavinia syntrips from the Givetian of Australia [START_REF] Long | A new genus of fossil coelacanth (Osteichthyes: Coelacanthiformes) from the Middle Devonian of southeastern Australia[END_REF], Holopterygius nudus from the Givetian of Germany [START_REF] Jessen | Weitere Fischreste aus dem Oberen Plattenkalk der Bergische-Gladbach-Paffrather-Mulde (Oberdevon, Rheinisches Schieferbebirge)[END_REF]Friedman & Coates, 2006), Diplocercides from the Frasnian of Germany (D. kayseri, D. heiligenstockiensis, D. jaeckeli) and the Visean (Carboniferous) of Scotland (D. davisi) [START_REF] Stensiö | Über zwei Coelacanthiden aus dem Oberdevon von Wildungen[END_REF][START_REF] Stensiö | On the Devonian coelacanthids of Germany with special reference to the dermal skeleton[END_REF][START_REF] Moy-Thomas | The Carboniferous coelacanth fishes of Great Britain and Ireland[END_REF][START_REF] Jessen | Die Crossopterygier des Oberen Plattenkalkes (Devon) der Bergisch-Gladbach-Paffrather Mulde (Rheinisches Schiefergebirge) unter Berücksichtung von amerikanischem und europaischem Onychodus-Material[END_REF][START_REF] Forey | The coelacanth Rhabdoderma in the Carboniferous of the British Isles[END_REF], Miguashaia from the Frasnian of Québec (Canada) (M. bureaui) and Latvia (M. grossi) [START_REF] Schultze | Crossopterygier mit heterozerker Schwanzflosse aus dem Oberdevon Kanadas, nebst einer Beschreibung von Onychodontida-resten aus dem Mitteldevon Spaniens und aus dem Karbon der USA[END_REF][START_REF] Cloutier | The primitive actinistian Miguashaia bureaui Schultze (Sarcopterygii)[END_REF][START_REF] Forey | A new coelacanth from the Middle Devonian of Latvia[END_REF], Chagrinia enodis and Coelacanthus welleri from the Famennian of eastern USA [START_REF] Eastman | Notice of a new coelacanth fish from the Iowa Kinderhook[END_REF]Schaeffer, 1962), and Serenichthys kowiensis from the Famennian of South Africa (Gess & Coates, 2015). Early coelacanth fossil material is not only rare but also generally poorly preserved, compressed, or simply too precious to allow histological sectioning. Abundantly preserved individuals displaying a good histological quality are even scarcer, dramatically reducing the access to paleohistological data.

One of the few exceptions to this limited source of information is the material from Miguasha (Québec, Canada). The Miguasha fossil site has yielded exquisitely preserved fossil specimens from a wide range of early vertebrates, including the iconic Eusthenopteron foordi [START_REF] Jarvik | On the dermal bones, sensory canals and pitlines of the skull in Eusthenopteron foordi Whiteaves, with some remarks on E. save-soderberghi Jarvik[END_REF][START_REF] Jarvik | On the visceral skeleton in Eusthenopteron with a discussion of the parasphenoid and palatoquadrate in fishes[END_REF][START_REF] Jarvik | Basic Structure and evolution of Vertebrates[END_REF] and Elpistostege watsoni [START_REF] Westoll | Ancestry of the tetrapods[END_REF]Cloutier et al., 2020). Miguashaia bureaui [START_REF] Schultze | Crossopterygier mit heterozerker Schwanzflosse aus dem Oberdevon Kanadas, nebst einer Beschreibung von Onychodontida-resten aus dem Mitteldevon Spaniens und aus dem Karbon der USA[END_REF]Cloutier, 1991aCloutier, ,b, 1996) ) represents the only coelacanth found in Miguasha so far. It was a main predator in the estuarine paleoenvironment of the Escuminac Formation (Chevrinais et al., 2017a), which is dated as middle Frasnian in age [START_REF] Cloutier | Morphology, characters, and the interrelationships of basal sarcopterygians[END_REF]. The vertebrate specimens from Miguasha are numerous and frequently three-dimensionally preserved, which has given an excellent account of their external morphology (Cloutier et al., 2011[START_REF] Cloutier | Great Canadian Lagerstätten 4. The Devonian Miguasha biota (Québec): UNESCO World Heritage Site and a time capsule in the early history of vertebrates[END_REF].

Likewise, histological features are represented either as hard tissues (e.g. enamel, dentine, cellular bone, cartilage) or as cell spaces (e.g. osteocyte and chondrocyte lacunae), sufficiently well-preserved to allow genome size inference [START_REF] Thomson | An attempt to reconstruct evolutionary changes in the cellular DNA content of lungfish[END_REF][START_REF] Organ | Macroevolution of genome size in sarcopterygians during the water-land transition[END_REF], as well as soft tissues preservation [START_REF] Cloutier | Great Canadian Lagerstätten 4. The Devonian Miguasha biota (Québec): UNESCO World Heritage Site and a time capsule in the early history of vertebrates[END_REF]. Previous microstructural and histological studies have yielded excellent results on several taxa from the Escuminac Formation, such as the anapsid Euphanerops longaevus [START_REF] Janvier | Calcification of early vertebrate cartilage[END_REF][START_REF] Chevrinais | Evolution of vertebrate postcranial complexity: axial skeleton regionalization and paired appendages in a Devonian jawless fish[END_REF], the osteostracan Escuminaspis laticeps [START_REF] Janvier | Calcified cartilage in the paired fins of the osteostracan Escuminaspis laticeps (Traquair 1880), from the Late Devonian of Miguasha (Québec, Canada), with a consideration of the early evolution of the pectoral fin endoskeleton in vertebrates[END_REF], the 'placoderm' Bothriolepis canadensis [START_REF] Burrow | Histological structure of the cancellous bone layer in Bothriolepis canadensis (Antiarchi, Placodermi)[END_REF][START_REF] Downs | Skeletal histology of Bothriolepis canadensis (Placodermi, Antiarchi) and evolution of the skeleton at the origin of jawed vertebrates[END_REF], the 'acanthodian' Triazeugacanthus affinis (Chevrinais et al., 2017b) [START_REF] Schultze | Die Faltenzähne der Rhipidistiden Crossopterygier, der Tetrapoden und der Actinopterygier-Gattung Lepisosteus nebst einer Beschreibung der Zahnstruktur von Onychodus (Struniiformer Crossopterygier)[END_REF][START_REF] Laurin | A microanatomical and histological study of the paired fin skeleton of the Devonian sarcopterygian Eusthenopteron foordi[END_REF][START_REF] Zylberberg | A microanatomical and histological study of the postcranial dermal skeleton in the Devonian sarcopterygian Eusthenopteron foordi[END_REF]Meunier & Laurin, 2012) and Scaumenacia curta [START_REF] Thomson | An attempt to reconstruct evolutionary changes in the cellular DNA content of lungfish[END_REF][START_REF] Smith | The relationships of Uronemus: a Carboniferous dipnoan with highly modified tooth plates[END_REF], and the actinopterygian Cheirolepis canadensis [START_REF] Zylberberg | A microanatomical and histological study of the postcranial dermal skeleton of the Devonian actinopterygian Cheirolepis canadensis[END_REF][START_REF] Meunier | Histological study of the jaw teeth in the Devonian actinopterygian † Cheirolepis canadensis (Whiteaves)[END_REF].

Here, we study the histological morphological, and microanatomical, and histological organization of the scales of the early coelacanth Miguashaia bureaui from the middle Latimeria menadoensis. Nine scales were sampled from the second known Indonesian extant coelacanth specimen (MZB10003) (CCC 175; adult female 29,2 kg, 124 cm TL) captured off Manado Tua, Sulawesi Island [START_REF] Nulens | An updated inventory of all known specimens of the coelacanth Latimeria spp[END_REF]. The scales were originally fixed in formalin, and then stored in 70º ethanol.

MATERIAL AND METHODS

Material

After measurements of the odontodes, some scales were studied with SEM and others were used for ground sections. The

Methods

Miguashaia

Ground sections were performed on both fossil specimens (MHNM 06-1238 and MHNM 06-1495) at the MNHN. The fossil material was embedded in polyester resin (GBS 1; Brot) and sectioned for study. The various sections were ground and polished to a thickness of 60-80 µm and observed under transmitted natural light with an Olympus BX51 microscope. Pictures were taken with a digital camera Olympus Camedia C-5060 and finalized in Adobe Photoshop.

Latimeria

For the SEM observations, the scales were steeped in 6 or 12% sodium hypochloride solution at room temperature (in order to strip the surface of the scales and to destroy the

DESCRIPTION

Morphology

The external and internal surfaces of the scales match reasonably the previous description by [START_REF] Cloutier | The primitive actinistian Miguashaia bureaui Schultze (Sarcopterygii)[END_REF] (Fig. 2). All scales of Miguashaia bureaui are almost rounded, slightly broader than long, and both sides of the scales at mid-length are linear and parallel.

The ornamented area (posterior field) occupies approximately two fifths of the area of the scale; it is approximately twice as broad as long. The ornamentation pattern of M. bureaui varies according to the position of the scales on the body as well as during ontogeny (Fig. 2). Five types of ornamentation could be observed on the outer ornamented field: (1) coarse tubercles (co.tu), (2) coarse and short longitudinal ridges (co.ri), (3) fine leaf-or spoonshaped tubercles (sp.tu), (4) fine, parallel, longitudinal, low-profile ridges (f.ri), and (5) smooth, thinly and regularly pitted enamel surface (pi.e).

The coarse tubercles (co.tu) vary in shape from circular to ovoid. Occasional coarse tubercles located near the anterior-most limit of their distribution share the leaf-shape of the fine tubercles (Fig. 2C). The coarse tubercles are also found on the pectoral girdle and lateral gulars of M. bureaui.

Although we consider that the coarse tubercles are a distinct category of ornamentation from the coarse ridges, these two types of ornamentation form a continuum. The coarse and short longitudinal ridges share similarities with the coarse tubercles. The coarse ridges (co.ri) are either linear or slightly curved, with the posterior extremity forming an apex.

No bifurcation has been observed with the coarse ridges. In contrast to the fine low-profile ridges, the space between coarse ridges is broader than with the fine ridges. The longest ridges are found in the median region of the ornamented field. The median patch of coarse ridges is surrounded laterally and posteriorly by coarse tubercles. The coarse ornamentation has an extremely restricted distribution on the body (Fig. 2B-C).

The fine leaf-or spoon-shaped tubercles (sp.tu) occur only at the limit between the overlapped area (anterior field) and the ornamented area (posterior field). The tubercles are lanceolate to ovate and medially grooved with the apex pointing posteriorly. The size of these tubercles varies within a scale (Fig. 2C-E). Similar tubercles located in a similar position are also found in Miguashaia grossi (Forey et al., 2000, fig. 6B: most likely the two insets of figure 6B have been inverted).

The fine ridges (fi.r) are mostly linear and without bifurcation (Fig. 2D1,E). There are five (e.g. MHNM 06-2414) to six (e.g. MHNM 06-1233) ridges per mm. Individual ridges are approximately 0,16 mm in width. Most ridges are linear; however, marginal ridges may be slightly curvilinear in the largest specimens. On each scale, there are few ridges that bifurcate anteriorly or posteriorly. Occasionally, adjacent ridges merge along their length, thus keeping their respective width. Along the merged sections, spaces are left unmerged forming small pores (0.16-0.18 mm in length and 0.03 mm in width) that result in a pitted surface. In contrast to M. bureaui, the longitudinal ridges of M. grossi radiate from the medial axis of the scale (Forey et al., 2000, fig. 6B).

The pitted enamel surface (pi.e) covers from the posterior margin of the posterior field up to the complete ornamented area (Fig. 2E). The pores are ovoid to fusiform; the origin of the pores comes from the merging of adjacent ridges. The pores are aligned longitudinally in the prolongation of the interridge space as well as across the width of the scale. Along a longitudinal alignment, there is a spacing of approximately 0.25-0.27 mm. During growth, this condition originates at the posterior margin of the scales and progresses anteriorly up to the condition in which the complete ornamented area is covered by a pitted surface. The pitted enamel surface was referred as the "almost continuous enamel covering" by Forey (1998, p. 329).

In agreement with [START_REF] Cloutier | The primitive actinistian Miguashaia bureaui Schultze (Sarcopterygii)[END_REF], the ornamentation of the posterior field of a scale differs in relation with its position on the body. Few scales from the anteroventral region of the body bear coarse tubercles and coarse ridges (MHNM 06-41, MHNM 06-641, MHNM 06-1809) (Fig. 2C). Flank scales from the anterior region of the trunk present a narrow area of small leaf-or spoon-shaped tubercles (anteriorly) and fine longitudinal ridges and a pitted enamel surface (posteriorly). The inner surface of the scales is flat and characterized by small circular bumps (ci.b) perforated in their centre (Fig. 2D2); these structures had already been reported and illustrated by Cloutier (1996, fig. 17D). Although small, these bumps vary slightly in diameter. They do not seem to be aligned. These bumps are located at the level of the outer ornamented area (posterior field). As far as we know, Miguashaia bureaui is the only sarcopterygian species with this feature. The posterior part of the inner surface shows fine concentric circuli (growth lines) with some variations in terms of width bands and inter-band spacings.

Sensory lateral line scales had been reported by [START_REF] Schultze | Crossopterygier mit heterozerker Schwanzflosse aus dem Oberdevon Kanadas, nebst einer Beschreibung von Onychodontida-resten aus dem Mitteldevon Spaniens und aus dem Karbon der USA[END_REF] and [START_REF] Cloutier | The primitive actinistian Miguashaia bureaui Schultze (Sarcopterygii)[END_REF] on a juvenile specimen (holotype ULQ 120). While Schultze (1973, fig. 1a) illustrated scales with one central elongated canaliculus per scale forming a horizontal trajectory starting at the posteroventral limit of

Histology

In both specimens used for the histology study (MHNM 06-1238 and MHNM 06-1495) (Fig. 1), the scales are imbricate and preserved in articulation. The inner surface of scales is best preserved in specimen MHNM 06-1238 showing the complete shape of the scales (average diameter of 1.5 cm) (sc, Fig. 1A). The inner surface is generally smooth and displays a few weakly marked growth rings and scattered bumps in the central and posterior regions (Fig. 2D2). In addition, there is a patch of imbricated scales showing the outer field with the fine longitudinal ridges. The scales from MHNM 06-1495 (average diameter of 1.7 cm) are visible on their external surface (sc, show large tubercles as well as leaf-shaped tubercles and fine longitudinal ridges.

The exposed area (posterior field)

The scales can be microstructurally divided into three well defined superimposed portions (Fig. 3 Dentinal tubules (oc, odontoblastic canaliculi) (Fig. 4B) are roughly perpendicular to the enamel layer; these radiate from small odontoblastic cavities (pc, pulp cavities) located at the base of the dentine layer (Fig. 3A). The dentine of Miguashaia bureaui can thus be described as orthodentine (Francillon-Vieillot et al., 1990). At least two generations of superimposed odontodes occur in some sections (Fig. 3A, 4D-E), with partial resorption of the older generation by the surrounding cellular bone. Cosmine is confidently considered absent as there is no evidence of a pore-canal system developed between the enamel, dentine and vascular bone. Isopedine. The basal plate is made of a fully mineralized, lamellar bone with a plywood-like structure with a variable orientation of the collagen plies (lb, Figs 3C,7B). The isopedine layer accounts for one third to half of the scale total thickness (approximately between 100-150 µm). The plies of the basal plate are made of thick collagenous fibres, organized in bundles that appear roughly quadrangular in cross section (Fig. 3C). There is only one layer of fibres per ply with each ply being about 10 to 20 µm thick. Horizontal sections of the basal plate confirm that collagenous fibres are parallel to each other in a given ply. The direction of the fibres rotates from a given ply to the next (Figs 3C, 7B).

We have observed the presence of five fibrillary directions (Fig. 8A), indicating that a repetitive pattern occurs every five layers (Figs 3C, 7B) and that each fibre rotates from one layer to the next with an angle of approximately 36°, thus constituting a twisted plywood. But contrary to the basal plate of the scales of Latimeria (Fig. 8B,C), no arciform patterns, characteristic of a double-twisted plywood (Giraud et al., 1978a,b;[START_REF] Meunier | Twisted plywood' structure and mineralization in the scales of a primitive living fish Amia calva[END_REF][START_REF] Mondéjar Fernández | New histological information on Holoptychius Agassiz, 1839 (Sarcopterygii, Porolepiformes) provides insights into the palaeoecological implications and evolution of the basal plate of the scales of osteichthyans[END_REF], occur in transversal section, thus indicating that the isopedine of the scales in Miguashaia bureaui is composed of a simple twisted plywood. Several small osteocytic lacunae and canaliculi are visible between the plies, showing that the Meunier, 1984;Francillon-Vieillot et al., 1990) with their cytoplasmic processes extending along the collagen fibres (Figs 4C-D, 8A). Many vertical vascular canals orthogonally cross the isopedine layer and several open through the internal surface forming a bulge (Fig. 7B), corresponding to the perforate small circular bumps of the internal surface (Fig. 2C). In some sections, the irregular contact of the collagen plies at the internal (or visceral) margin of the scales (Figs 4A, 5A, 7B) may indicate the occurrence of Mandl's corpuscules attesting to a possible delay between collagenous synthesis and its mineralization (see Schönborner et al., 1981 for teleostean elasmoid scales) in successive mineralization events across the basal layer (Fig. 4A).

The overlapped area (anterior field)

The upper part of the superficial layer (sl) of the area overlapped by the adjoining scales is generally deprived of odontodes, yet it shows a superficial ornamentation composed of a layer of compact acellular bone (Fig. 5A,C) that recalls the external layer of the teleostean elasmoid scales (Francillon-Vieillot et al., 1990;[START_REF] Zylberberg | Biomineralization in the integumental skeleton of the living lower Vertebrates[END_REF]. This external acellular ornamented layer overlays a vascularised layer of cellular bone (Fig. 5B). The vascular canals appear to be more uniformly distributed than in the 7A). The basal plate has a slightly greater relative thickness than in the posterior field but displays the same organization and a similar number of stacked layers of collagenous fibres (Fig. 5A).

DISCUSSION

Histology of Miguashaia and comparisons

Despite their rich fossil record, comprising more than 60 genera and 130 species (Cloutier & Forey, 1991;[START_REF] Forey | History of the Coelacanth Fishes[END_REF], few surveys of the dermal skeleton of coelacanths have been undertaken. Gross (1966:42) first noted that the scales of coelacanths had barely changed their histological and microstructural organisation during their long evolutionary history, a statement that was later reprised in subsequent reviews of coelacanth evolution (e.g. [START_REF] Janvier | Early Vertebrates[END_REF][START_REF] Forey | History of the Coelacanth Fishes[END_REF]. To our knowledge, few detailed accounts on the palaeohistology of the scales and dermal bones of extinct coelacanths have been published or properly illustrated. These include most notably histological reports on Macropoma lewesiensis [START_REF] Williamson | XXIII. On the microscopic structure of the scales and dermal teeth of some Ganoid and Placoid Fish[END_REF], Coelacanthus granulatus [START_REF] Goodrich | On the scales of fish, living and extinct, and their importance in classification[END_REF], Undina penicillata [START_REF] Gross | Histologische Studien am Aussenskelett fossiler Agnathen und Fische[END_REF][START_REF] Gross | Kleine Schuppenkunde[END_REF], Diplurus newarki and Chagrinia enodis [START_REF] Schaeffer | The Triassic coelacanth Diplurus, with observations on the evolution of the Coelacanthini[END_REF](Schaeffer, , 1962)), and Spermatodus pustulosus [START_REF] Meinke | A light and scanning electron microscope study on the dermal skeleton of Spermatodus (Pisces: Coelacanthini) and the evolution of the dermal skeleton in coelacanths[END_REF]. Indeed, many sections (e.g. [START_REF] Schaeffer | The Triassic coelacanth Diplurus, with observations on the evolution of the Coelacanthini[END_REF][START_REF] Forey | History of the Coelacanth Fishes[END_REF]. As such, the reliability of many descriptions is difficult to assess due to the lack of proper illustrations since because no description can consistently capture all the information contained in a histological section.

The thin, imbricate round scales of all histologically studied coelacanths, as evidenced here in Miguashaia (Figs 3, 4A, 7A), display in the posterior field three well-defined portions, differing in their microstructure: 1) a superficial layer of odontogenic components (odontodes generally made of enamel and dentine), 2) a middle layer composed of vascular bone (spongiosa) and 3) a basal plate made of lamellar bone (isopedine). The combination of this microstructural arrangement with a rounded outline allows to ascribe the scales of coelacanths to the elasmoid type [START_REF] Bertin | Modifications proposées dans la nomenclature des écailles et des nageoires[END_REF][START_REF] Smith | The structure of the scales of Latimeria chalumnae[END_REF][START_REF] Castanet | Données préliminaires sur les structures histologiques du squelette de Latimeria chalumnae. I -Dents, écailles, rayons de nageoires[END_REF][START_REF] Miller | Observations on the structure of mineralized tissue of the coelacanth, including scales and their associate odontodes[END_REF]Meunier & Zylberberg, 1999;[START_REF] Hadiaty | Morphological study of the scales of Latimeria menadoensis Pouyaud et al[END_REF]Meunier et al., 2008), a scale morpho-and histotype that is widely spread and convergently acquired in osteichthyans (e.g. Francillon-Vieillot et al., 1990;[START_REF] Schultze | Ausgangsform und Entwicklung der rhombischen Schuppen der Osteichthyes (Pisces)[END_REF][START_REF] Schultze | Scales, enamel, cosmine, ganoine, and early osteichthyans[END_REF][START_REF] Schultze | Hard tissues in fish evolution: history and current issues[END_REF][START_REF] Mondéjar Fernández | New histological information on Holoptychius Agassiz, 1839 (Sarcopterygii, Porolepiformes) provides insights into the palaeoecological implications and evolution of the basal plate of the scales of osteichthyans[END_REF]. The general structure of the elasmoid scales displayed in Miguashaia has been maintained across coelacanth evolution until the Recent as evidenced in Latimeria [START_REF] Castanet | Données préliminaires sur les structures histologiques du squelette de Latimeria chalumnae. I -Dents, écailles, rayons de nageoires[END_REF][START_REF] Meunier | Histological study of the jaw teeth in the Devonian actinopterygian † Cheirolepis canadensis (Whiteaves)[END_REF], The superficial layer comprises the outer ornamentation of the exposed area of the scales, which is composed of pointed to rounded tubercles and/or ridges, variable in width and number within as well as between taxa (e.g. Miguashaia, Diplocercides, Spermatodus) (Table 1). The presence of ridges has been noticed in numerous extinct coelacanths (e.g. Diplurus, Rhabdoderma, Axelrodichthys, Holophagus) [START_REF] Forey | History of the Coelacanth Fishes[END_REF]. In many instances, their shiny appearance is suggestive of the odontogenic nature of these ridges, composed either solely of dentine or more likely covered by enamel (e.g.

Diplocercides, Rhabdoderma, Axelrodichthys, Holophagus).

However, few histological studies have been performed on the scales to confirm the putative universal occurrence of dentine, let alone the co-occurrence of enamel (e.g. [START_REF] Gross | Histologische Studien am Aussenskelett fossiler Agnathen und Fische[END_REF]. For instance, the scales of Diplocercides [START_REF] Stensiö | On the Devonian coelacanthids of Germany with special reference to the dermal skeleton[END_REF] have not been histologically surveyed although Gross (1935:44) stated that in D. jaeckeli the small ridges were "certainly made of dentine". The scales of Macropoma are ornamented with elongate tubercles, with the largest ones occupying the central position in the exposed area (Forey, 1998, fig. 11.12). Most remarkably, the scales from the posterior half of the body display large pointed tubercles resembling "hollow cylinders" (Forey, 1998, fig. 11.12B). Williamson (1849, pl. XLIII 27,28) illustrated these tubercles in M. lewesiensis, confirming their odontogenic nature. The scales of Diplurus [START_REF] Schaeffer | A study of Diplurus longicaudatus with notes on the body form and locomotion of the coelacanthini[END_REF][START_REF] Schaeffer | The Triassic coelacanth Diplurus, with observations on the evolution of the Coelacanthini[END_REF] do not possess tubercles, only large ridges, probably odontogenic. Schaeffer (1952:52) stated that in D. newarki "the bones, and apparently also the scales, are covered by a thin layer of enamel", but this is difficult to confirm given that thin sections were not illustrated. Coelacanthus presents "elongated tubercles or shiny ridges" (Goodrich, 1907:766) that appeared as "hollow" in the poorly illustrated cross sections of C. granulatus (Goodrich, 1907, pl.XLIV 12). The odontode nature of these structures was not confirmed, but they might represent dentine tubercles with large pulp cavities capped with enamel or large vascular canals of the superior portion of the spongiosa. The odontodes of Undina penicillata (Gross, 1966, fig.7B) are remarkably composed exclusively of dentine and do not present enamel, as opposed to those of Miguashaia, Spermatodus and Macropoma. Noteworthy, Mawsonia appears to lack odontogenic ornamentation as the ridged surface of the dermal bones and scales has been described as being solely made of bone in M. brasiliensis [START_REF] Yabumoto | A new coelacanth from the Early Cretaceous of Brazil (Sarcopterygii, Actinistia)[END_REF], although histological data have not been published. The ridges and tubercles of Chagrinia enodis (Schaeffer, 1962;Cloutier, 1996, fig. 9D) were also described as made of bone, although but according to Forey (1998), the absence of odontogenic ornamentation might be due to a preservation artefact. Other taxa described as displaying a "rugose" ornamentation of the scales (sensu [START_REF] Forey | History of the Coelacanth Fishes[END_REF] such as Chinlea sorenseni [START_REF] Schaeffer | Late Triassic fishes from the western United States[END_REF], are not suited to histological survey as the scales are preserved as impressions.

An ornamentation composed of tubercles and/or ridges is thus fairly common and widespread in the scales and dermal bones of coelacanths (Figs 9,11). However, due to the lack of histological data for many taxa, it is currently difficult to determine whether it is consistently composed of odontogenic tissues and to study its ontogenetic development. In some Mesozoic taxa, the odontodes usually display large but shallow pulp cavities, connected to a horizontal network of vascular canals in the spongiosa (e.g. Undina, Macropoma). This condition, retained in Latimeria chalumnae [START_REF] Roux | The microscopic anatomy of the Latimeria scale[END_REF][START_REF] Ørvig | A survey of odontodes ('dermal teeth') from developmental, structural, functional, and phyletic points of view[END_REF][START_REF] Castanet | Données préliminaires sur les structures histologiques du squelette de Latimeria chalumnae. I -Dents, écailles, rayons de nageoires[END_REF] 4), Spermatodus pustulosus [START_REF] Meinke | A light and scanning electron microscope study on the dermal skeleton of Spermatodus (Pisces: Coelacanthini) and the evolution of the dermal skeleton in coelacanths[END_REF], and Undina penicillata (Gross, 1936[START_REF] Gross | Kleine Schuppenkunde[END_REF], but not in Macropoma lewesiensis [START_REF] Williamson | XXIII. On the microscopic structure of the scales and dermal teeth of some Ganoid and Placoid Fish[END_REF]. For instance, the presence and distribution of odontodes in the dermal bones of Spermatodus is similar to those of Latimeria, but in the former the old generations of odontodes are more robustly developed and less resorbed than in the latter [START_REF] Meinke | A light and scanning electron microscope study on the dermal skeleton of Spermatodus (Pisces: Coelacanthini) and the evolution of the dermal skeleton in coelacanths[END_REF]. This condition, also present in Undina, appears closer to that of early osteichthyans with large partially to non-resorbed odontodes (e.g. Andreolepis, Lophosteus, Psarolepis) [START_REF] Qu | Scales and dermal skeletal histology of an early bony fish Psarolepis romeri and their bearing on the evolution of rhombic scales and hard tissues[END_REF][START_REF] Qu | The origin of novel features by changes in developmental mechanisms: ontogeny and three-dimensional microanatomy of polyodontode scales of two early osteichthyans[END_REF][START_REF] Jerve | Threedimensional paleohistology of the scale and median fin spine of Lophosteus superbus (Pander 1856)[END_REF]. The occurrence of separated and superimposed odontodes represents the primitive pre-cosmine condition in osteichthyans (Mondéjar-Fernández, 2018) since these odontodes (either in the form of tubercles and/or ridges) are found in stem osteichthyans like Andreolepis hedei [START_REF] Gross | Fragliche Actinopterygier-schuppen aus dem Silur Gotlands[END_REF][START_REF] Qu | The origin of novel features by changes in developmental mechanisms: ontogeny and three-dimensional microanatomy of polyodontode scales of two early osteichthyans[END_REF], Lophosteus superbus [START_REF] Gross | Lophosteus superbus Pander, ein Teleostome aus dem Silur Oesels[END_REF][START_REF] Jerve | Threedimensional paleohistology of the scale and median fin spine of Lophosteus superbus (Pander 1856)[END_REF], Ligulalepis toombsi and Orvikuina sp. [START_REF] Schultze | Palaeoniscoidea-schuppen aus dem Unterdevon Australiens und Kansas und aus dem Mitteldevon Spitzbergens[END_REF], Dialipina (D. salgueiroensis and D. markae) [START_REF] Schultze | Palaeoniscoidea-schuppen aus dem Unterdevon Australiens und Kansas und aus dem Mitteldevon Spitzbergens[END_REF][START_REF] Schultze | Ausgangsform und Entwicklung der rhombischen Schuppen der Osteichthyes (Pisces)[END_REF], and Naxilepis gracilis [START_REF] Wang | Discovery of Late Silurian microfossils of Agnatha and fishes from Yunnan, China[END_REF]. However, the condition of coelacanths may represent a return to the odontode-ornamented condition of certain stem osteichthyans (e.g. Andreolepis), probably derived from a cosmine-covered condition in more crownward stem osteichthyans (e.g. Psarolepis) and sarcopterygians (e.g. [START_REF] Smith | The microstructure of the dentition and dermal ornament of three dipnoans from the Devonian of Western Australia: a contribution towards dipnoan interrelations. and morphogenesis. growth and adaptation of the skeletal tissues[END_REF][START_REF] Smith | Scanning electron microscopy of odontodes in the scales of coelacanth embryo, Latimeria chalumnae Smith[END_REF][START_REF] Schultze | Ausgangsform und Entwicklung der rhombischen Schuppen der Osteichthyes (Pisces)[END_REF].

Styloichthys

The middle layer of vascular bone is variably developed in the scales of coelacanths. In the case of taxa with superimposed odontodes, the older generations are surrounded by vascular bone and may show traces of resorption as seen in Miguashaia bureaui , Undina penicillata [START_REF] Gross | Kleine Schuppenkunde[END_REF], and Spermatodus pustulosus [START_REF] Meinke | A light and scanning electron microscope study on the dermal skeleton of Spermatodus (Pisces: Coelacanthini) and the evolution of the dermal skeleton in coelacanths[END_REF]. The vascular bone layer is made of a cellular woven-fibered bone, as evidenced by the occurrence of numerous osteocyte lacunae concentrically arranged around the embedded odontodes and vascular canals (e.g. Miguashaia, Spermatodus, Undina) (Fig. 6C). In Miguashaia bureaui (Figs 4, 7A), Macropoma lewesiensis [START_REF] Williamson | XXIII. On the microscopic structure of the scales and dermal teeth of some Ganoid and Placoid Fish[END_REF] and Undina penicillata [START_REF] Gross | Histologische Studien am Aussenskelett fossiler Agnathen und Fische[END_REF][START_REF] Gross | Kleine Schuppenkunde[END_REF], the vascular canals of the spongiosa open to the surface through pores, and, when present, some of them are connected with the pulp cavities of the dentine. In Latimeria, the middle layer is extremely reduced but horizontally-connected vascular canals occur at the base of the odontodes [START_REF] Smith | The structure of the scales of Latimeria chalumnae[END_REF][START_REF] Castanet | Données préliminaires sur les structures histologiques du squelette de Latimeria chalumnae. I -Dents, écailles, rayons de nageoires[END_REF] (Fig. 10B). In Undina penicillata, [START_REF] Gross | Histologische Studien am Aussenskelett fossiler Agnathen und Fische[END_REF] The basal plate of the scales and dermal bones of actinistians is composed of a layer of lamellar bone with a plywood-like structure (i.e., isopedine). The basal plate is well ossified in the scales of all extinct coelacanths (e.g. Miguashaia, Coelacanthus, Undina, Macropoma), but it is not in the extant Latimeria [START_REF] Castanet | Données préliminaires sur les structures histologiques du squelette de Latimeria chalumnae. I -Dents, écailles, rayons de nageoires[END_REF]Meunier & Zylberberg, 1999;Meunier et al., 2008). Generally, the basal plate is about as thick as the superficial and middle layers combined. However, in Macropoma lewesiensis [START_REF] Williamson | XXIII. On the microscopic structure of the scales and dermal teeth of some Ganoid and Placoid Fish[END_REF], the basal plate was described as being thinner than these layers, a condition that appears to be variably developed within taxa. On the contrary, in Latimeria, the unmineralized basal plate is greatly developed whereas the middle and superficial layers are reduced [START_REF] Castanet | Données préliminaires sur les structures histologiques du squelette de Latimeria chalumnae. I -Dents, écailles, rayons de nageoires[END_REF]Meunier et al., 2008) (Fig. 10). In Miguashaia bureaui, the thickness of the isopedine varies depending on the location of the thin section across the scales: the basal plate appears relatively thicker in the anterior portion (overlapped area) than in the posterior portion (exposed area) of the scales (Figs 3C, 4, 5A, 7A).

The isopedine layer consists of a cellular lamellar bone that can present enclosed osteocytes, as in Miguashaia bureaui (Figs 4C,D, 7B, 8A), Spermatodus pustulosus [START_REF] Meinke | A light and scanning electron microscope study on the dermal skeleton of Spermatodus (Pisces: Coelacanthini) and the evolution of the dermal skeleton in coelacanths[END_REF] and Undina penicillata [START_REF] Gross | Histologische Studien am Aussenskelett fossiler Agnathen und Fische[END_REF][START_REF] Gross | Kleine Schuppenkunde[END_REF]. In S. pustulosus, the [START_REF] Meinke | A light and scanning electron microscope study on the dermal skeleton of Spermatodus (Pisces: Coelacanthini) and the evolution of the dermal skeleton in coelacanths[END_REF], as in the lateral line scales of Latimeria chalumnae (Meunier 1980, figs. 14, 15). Vertical vascular canals can also occur in the scales of Macropoma lewesiensis [START_REF] Williamson | XXIII. On the microscopic structure of the scales and dermal teeth of some Ganoid and Placoid Fish[END_REF] and in Miguashaia bureaui (Fig. 7B). The isopedine of Miguashaia has a twisted, plywood-like structure, similar to that of Latimeria and possibly other extinct coelacanths with well-developed lamellated basal plates (Fig. 8). However, it differs from the isopedine of Latimeria in being fully mineralized and in the lamellar bone being simply twisted, as opposed to the double-twisted arrangement in Latimeria (Giraud et al., 1978a,b;Francillon-Vieillot et al., 1990). A double-twisted plywood is also known to occur convergently in other extinct sarcopterygians with rounded scales, like in the tetrapodomorph Eusthenopteron foordi [START_REF] Zylberberg | A microanatomical and histological study of the postcranial dermal skeleton in the Devonian sarcopterygian Eusthenopteron foordi[END_REF] and the dipnomorph Holoptychius sp. [START_REF] Mondéjar Fernández | New histological information on Holoptychius Agassiz, 1839 (Sarcopterygii, Porolepiformes) provides insights into the palaeoecological implications and evolution of the basal plate of the scales of osteichthyans[END_REF], as well as in the living dipnoans Neoceratodus forsteri, Lepidosiren paradoxa and Protopterus (P. annectens and P. aethiopicus) (Meunier & François, 1980). The twisted condition is thus commonly found in round-scaled sarcopterygians and appears to be derived from a primitive orthogonal arrangement of the isopedine present in stem osteichthyans (e.g. Psarolepis) [START_REF] Qu | Scales and dermal skeletal histology of an early bony fish Psarolepis romeri and their bearing on the evolution of rhombic scales and hard tissues[END_REF]) and many sarcopterygians with rhombic scales (e.g. Porolepis, Osteolepis) [START_REF] Mondéjar Fernández | New histological information on Holoptychius Agassiz, 1839 (Sarcopterygii, Porolepiformes) provides insights into the palaeoecological implications and evolution of the basal plate of the scales of osteichthyans[END_REF]. However, given the current availability of

Scale development and ontogeny

Information on the development and patterning of the squamation in coelacanths is available through the study of the extant Latimeria. [START_REF] Smith | Scanning electron microscopy of odontodes in the scales of coelacanth embryo, Latimeria chalumnae Smith[END_REF] showed that in the scales of Latimeria the odontodes develop separately from the basal plate (isopedine) and later fuse to it. In embryos of Latimeria, the odontodes are firmly associated with the vascular bone of the spongiosa and these develop synchronously after differentiation of the layers of the basal plate. During growth and development of the scales, supplementary odontodes and more spongy bone are added centrifugally and some occur superimposed onto the earlier generation (Fig. 10A; [START_REF] Roux | The microscopic anatomy of the Latimeria scale[END_REF][START_REF] Smith | The structure of the scales of Latimeria chalumnae[END_REF][START_REF] Ørvig | A survey of odontodes ('dermal teeth') from developmental, structural, functional, and phyletic points of view[END_REF][START_REF] Smith | Scanning electron microscopy of odontodes in the scales of coelacanth embryo, Latimeria chalumnae Smith[END_REF]. Developmental data on Latimeria thus matches developmental [START_REF] Sire | Formation of dermal skeletal and dental tissues in fish: a comparative and evolutionary approach[END_REF] and evolutionary [START_REF] Sire | Origin and evolution of the integumentary skeleton in non-tetrapod vertebrates[END_REF] scenarios of the squamation in osteichthyans as the development and evolution of the squamation appear to occur on two levels, matching the two main portions of the scales: 1) the odontogenic component (neural crest-derived) comprising the combination of the superficial odontode-bearing layer and associated vascular bone from the middle layer, and 2) the osteogenic component (mesodermallly-derived) represented by the basal plate. The superficial layer and basal plate of the scales of osteichthyans show independent patterning and evolutionary trends, consistent with separate developmental origins (Gillis et al., 2009), a pattern that can be identified from the first manifestations of the dermoskeleton in early vertebrates [START_REF] Donoghue | Early evolution of vertebrate skeletal tissues and cellular interactions, and the canalization of skeletal development[END_REF]. Microanatomical and histological changes of tissues (e.g. loss of cosmine, modification of odontode shape and pattern of distribution) can occur in the superficial layer of the scales without affecting the basal plate, and vice-versa (e.g. loss of mineralization of the isopedine), revealing the independence of the cell lineages responsible for the odontogenic and osteogenic components of the dermal skeleton [START_REF] Schaeffer | The dermal skeleton in fishes[END_REF].

Unfortunately, little information is available on the development of the squamation in extinct actinistians.

Ontogenetic series are fairly rare and embryonic or early [START_REF] Schultze | Crossopterygier mit heterozerker Schwanzflosse aus dem Oberdevon Kanadas, nebst einer Beschreibung von Onychodontida-resten aus dem Mitteldevon Spaniens und aus dem Karbon der USA[END_REF][START_REF] Cloutier | The primitive actinistian Miguashaia bureaui Schultze (Sarcopterygii)[END_REF]Cloutier et al., 2009), Serenichthys kowiensis from the Devonian of South Africa (Gess and Coates, 2015), Rhabdoderma exiguum from the Carboniferous of the USA [START_REF] Schultze | Early growth stages in coelacanth fishes[END_REF][START_REF] Schultze | Eier legende und lebend gebärende Quastenflosser[END_REF][START_REF] Cloutier | The fossil record of fish ontogenies: insights into developmental patterns and processes[END_REF], Caridosuctor populosum, Hadronector donbairdi and Polyosteorhynchus simplex from the Carboniferous of the USA [START_REF] Lund | Coelacanths from the Bear Gulch Limestone (Namurian) of Montana and the evolution of the Coelacanthiformes[END_REF], Luopingcoelacanthus eurylacrimalis from the Triassic of China [START_REF] Wen | Coelacanths from the Middle Triassic Luoping Biota, Yunnan, South China, with the earliest evidence of ovoviviparity[END_REF], Undina penicillata from the Jurassic of Germany [START_REF] Watson | The reproduction of the coelacanth fish, Undina[END_REF][START_REF] Clack | Otoliths in fossil coelacanths[END_REF], Axelrodichthys sp. from the Cretaceous of Brazil [START_REF] Brito | Discovery of a juvenile coelacanth in the Lower Cretaceous, Crato Formation, northeastern Brazil[END_REF], and a juvenile coelacanth from the Carboniferous of Germany (Witzmann et al., 2010). The most diagnostic character to recognize juvenile coelacanths is the distinctly longer supplementary lobe of the caudal fin relative to that of adults. Miguashaia [START_REF] Cloutier | The primitive actinistian Miguashaia bureaui Schultze (Sarcopterygii)[END_REF] and Gavinia [START_REF] Long | A new genus of fossil coelacanth (Osteichthyes: Coelacanthiformes) from the Middle Devonian of southeastern Australia[END_REF] are considered morphologically primitive coelacanths by the possession of an heterocercal caudal fin lacking a supplementary lobe as opposed to more derived and anatomically modern coelacanths with a characteristic trilobed diphycercal caudal fin [START_REF] Zhu | Earliest known coelacanth skull extends the range of anatomically modern coelacanths to the Early Devonian[END_REF]. However, in Miguashaia, the notochordal lobe is proportionately much longer in juveniles (Cloutier et al., 2009).

Developmental studies on extant osteichthyans (e.g. Danio)

have shown that the squamation develops gradually across the body, usually starting posteriorly, at the level of the caudal [START_REF] Sire | Scale development in fish: a review, with description of sonic hedgehog (shh) expression in the zebrafish (Danio rerio)[END_REF], a pattern that can be confidently transposed to fossil ontogenies of extinct taxa, with minor differences [START_REF] Cloutier | The fossil record of fish ontogenies: insights into developmental patterns and processes[END_REF]. The date of appearance of the first scales is still unknown in Latimeria, but embryos found in the oviducts of the females of L. chalumnae can be up to 13 months old before birth, and display two lines of arrested growth [START_REF] Hureau | Détermination de l'âge et croissance du coelacanthe Latimeria chalumnae Smith, 1939 (Poisson, Crossoptérygien, Coelacanthidé)[END_REF]. However, the squamation is rarely preserved in juvenile coelacanth fossil specimens as scales are among the latest elements to ossify during the ontogeny, and this usually prevents fossilization. One noteworthy exception is Rhabdoderma exiguum for which small yolk-sac embryonic specimens (e.g. YPM 2024, 24.2 mm in standard length; FMNH PF 5521) display extremely thin scales present throughout the complete body and are ornamented with small longitudinal ridges (R.C., pers. observ.). The case of Miguashaia bureaui is also remarkable since juvenile as well as adult specimens are known in which the squamation is fully developed and preserved [START_REF] Schultze | Crossopterygier mit heterozerker Schwanzflosse aus dem Oberdevon Kanadas, nebst einer Beschreibung von Onychodontida-resten aus dem Mitteldevon Spaniens und aus dem Karbon der USA[END_REF][START_REF] Cloutier | The primitive actinistian Miguashaia bureaui Schultze (Sarcopterygii)[END_REF].

The variation in the ornamentation of the gular plates and scales has been listed among the main ontogenetic changes occurring in Miguashaia [START_REF] Cloutier | The primitive actinistian Miguashaia bureaui Schultze (Sarcopterygii)[END_REF]. In the case of the scales, the extent of the pitted enamel surface increases during growth, from juveniles with incompletely covered exposed areas in the posterior portion of the trunk to adults displaying a fully sheeted ornamented area (Fig. 2E). It has while coarse tubercles and larger ridges are mainly found in larger individuals (MHNM 06-641). In the small juveniles, the internal surface of the scales lacks the small circular bumps which become visible only in adult specimens (Fig. 2D2).

Ornamentation variation across the ontogeny is also known in Rhabdoderma exiguum in which the ornamentation of posterior scales is more pronounced than in the anterior part of the body (e.g. FMNH PF 5760) and the number of longitudinal ridges increases from embryonic to juvenile specimens.

Remodeling of bone, which produces structures such as the secondary vascular canals in Miguashaia bureaui (Fig. 4E,6), is also evidence of advanced ontogenetic growth [START_REF] Dias | On the squamation of Australerpeton cosgriffi Barberena, a temnospondyl amphibian from theUpper Permian of Brazil[END_REF] and may be related to sexual maturity in these specimens.

The development of larger tubercules and ridges has been associated with the function of tubercles as a reinforcement of the adhesion properties between the scales and the overlying epithelium [START_REF] Burdak | Funktsionai'naya morfologiya cheshuynogo pokrova ryb[END_REF]. The shape and localization of these tubercules across the body are also related to the control of the water flow during swimming throughout ontogeny, as evidenced in extant osteichthyans (e.g. Mugil) [START_REF] Burdak | Funktsionai'naya morfologiya cheshuynogo pokrova ryb[END_REF]. The ornamentation in Latimeria does not vary significantly during ontogeny, with the only exception of the number of tubercles, which are less numerous Growth rings (annuli) are frequently encountered in the scales of extant and extinct osteichthyans and they occur in the overlapped area of the scales of many early coelacanths (Fig. 11) and of Latimeria. These annuli have been counted in L. chalumnae (Hureau & Ousouf, 1977;[START_REF] Millot | Anatomie de Latimeria chalumnae. T. 3. Appareil digestif, téguments, écailles[END_REF], where juvenile specimens have fewer annuli than adult ones, and assuming that each ring represents a seasonal or annual line of arrested growth, a putative age can be assigned to specimens based on isolated scales. In L. chalumnae, two episodes of arrested growth (January-February and August-September) occur each year during the dry season [START_REF] Hureau | Détermination de l'âge et croissance du coelacanthe Latimeria chalumnae Smith, 1939 (Poisson, Crossoptérygien, Coelacanthidé)[END_REF], allowing to assign a relatively accurate age to specimens caught in the wild. In Miguashaia bureaui equally spaced annuli are present in large adult specimens (e.g. MHNM 06-641, 06-1232, 06-1236, 06-1239, 06-1809) and are more numerous than in smaller individuals. P; peripheral growth rings are more closely spaced towards the periphery of large scales, congruent with the acquisition of sexual maturity and a slower growth rate in adults. Approximative ages could also be given to fossil specimens but these estimations can be less reliable given the tentative reconstruction of the paleoenvironnements in which these forms lived and the uncertainty regarding the metabolism, food supply, competitive

Evolutionary trends of the squamation in coelacanths

Among other synapomorphies, coelacanths are characterized by the possession of rounded scales [START_REF] Janvier | Early Vertebrates[END_REF][START_REF] Forey | History of the Coelacanth Fishes[END_REF].

However, the squamation of actinistians is rarely composed of perfectly circular scales. The scales can either be roughly circular (e.g. in early forms like Miguashaia and Diplocercides) or subcircular (oblong to oval), with a slightly quadrangular anterior margin (e.g.

Guizhoucoelacanthus, Latimeria).

Other taxa possess ovate scales, with a pronounced posterior edge that may display a rounded (e.g. Sassenia, Whiteia) or pointed margin (e.g. Lualabea, Diplurus, Rhaboderma). Given the morphological plasticity of the squamation, the body size variation, and the relative environmental cosmopolitanism of coelacanths across their evolutionary history, no distinctive trend can be depicted for the evolution of the scale outline; however, the earliest taxa (e.g. Miguashaia, Diplocercides) display the most circular contour whereas in more recent forms the scales become more antero-posteriorly elongated and the posterior margin displays a more variable profile, with different degrees of acuteness (Fig. 11).

The size, extension, and combination pattern of tubercles and ridges that occur on the exposed area is also extremely [START_REF] Forey | History of the Coelacanth Fishes[END_REF]. In many genera, the size and distribution of the ridges and tubercles are homogeneous (e.g. Rhabdoderma, Caridosuctor, Holophagus and Latimeria) but some taxa display a differentiated ornament in which large ridges usually located in the central portion of the exposed area are associated with smaller ridges that are distributed laterally (e.g. Axelrodichthys, Chinlea, Diplurus, Heptanema, Lualabea, Mawsonia) [START_REF] Forey | History of the Coelacanth Fishes[END_REF][START_REF] Yabumoto | A new Mesozoic coelacanth from Brazil (Sarcopterygii, Actinistia)[END_REF][START_REF] Renesto | First record of a coelacanth fish from the Middle Triassic Meride Limestone of Monte San Giorgio (Canton Ticino, Switzerland)[END_REF]Fragoso et al., 2019). This condition appears to be especially developed in the Mawsoniidae (Fig. 11) and may constitute a shared derived feature of these taxa but further phylogenetic inquiry is needed to confirm it.

VOrnamental variations of the ornamentation have been used

to discriminate taxa and thus have a taxonomic value at generic or species levels (e.g. various species of Rhabdoderma and Whiteia) (Table 1). In completely preserved specimens, ornamentation variation has been observed to occur over various regions of the body (e.g. Miguashaia, Macropoma, Luopingcoelacanthus) [START_REF] Cloutier | The primitive actinistian Miguashaia bureaui Schultze (Sarcopterygii)[END_REF][START_REF] Forey | History of the Coelacanth Fishes[END_REF][START_REF] Wen | Coelacanths from the Middle Triassic Luoping Biota, Yunnan, South China, with the earliest evidence of ovoviviparity[END_REF] (Fig. 2) and thus taxonomic identification must be cautious when describing new taxa based solely on isolated scale remains. A general evolutionary pattern nevertheless emerges, in which the earliest coelacanths (e.g. Miguashaia, Diplocercides) tend to show numerous tubercles or ridges, whereas younger taxa (e.g. Rhabdoderma, Diplurus, Latimeria) display fewer but larger tubercles and ridges (Figs 2C,11). These small odontodes are also characteristic of other Devonian sarcopterygians (e.g. onychodonts, porolepiforms, early dipnoans), where they may occur associated with a welldeveloped cosmine sheet (e.g. the porolepiforms Porolepis, Heimenia, and the early dipnoan Uranolophus) [START_REF] Ørvig | Remarks on the vertebrate fauna of the lower Upper Devonian of Escuminac Bay, P.Q. Canada, with special reference to the porolepiform crossopterygians[END_REF], 1969;[START_REF] Denison | Early Devonian lungfishes from Wyoming, Utah, and Idaho[END_REF][START_REF] Mondéjar-Fernández | Squamation and scale microstructure evolution in the Porolepiformes (Sarcopterygii, Dipnomorpha) based on Heimenia ensis from the Devonian of Spitsbergen[END_REF] or not (e.g. the porolepiforms Holoptychius, Glyptolepis, and Laccognathus, and the onychodonts Onychodus and Selenodus) [START_REF] Ørvig | Remarks on the vertebrate fauna of the lower Upper Devonian of Escuminac Bay, P.Q. Canada, with special reference to the porolepiform crossopterygians[END_REF]Cloutier & Schultze, 1996;[START_REF] Andrews | The structure of the sarcopterygian Onychodus jandemarrai n. sp. from Gogo, Western Australia: with a functional interpretation of the skeleton[END_REF][START_REF] Mondéjar-Fernández | A new onychodont (Osteichthyes; Sarcopterygii) from the Middle Devonian of Morocco and its bearing on early osteichthyan evolution[END_REF]. Such small odontodes thus represent a primitive shared feature of sarcopterygians. Past the Carboniferous, coelacanth scales lost this type of tubercles, yielding a relatively smooth overlapped area and fewer and larger tubercles and ridges in the exposed area (e.g.

Caridosuctor, Rhabdoderma, Axelrodichthys) (Fig. 11).

The overlapped area generally occupies between a half and two thirds of the external surface of the scales of coelacanths. However, the isolated scale material is scarce in many post-Devonian taxa and thus neither the smoothness nor the precise contour of the overlapped area can be properly described or illustrated in these taxa (see [START_REF] Forey | History of the Coelacanth Fishes[END_REF]. In the overlapped area with a roughly 50/50 ratio (Fig. 2B,E), most probably representing the primitive condition in actinistians. Other Devonian taxa such as Diplocercides also display an important exposed area relative to the overlapped area (Fig. 11). However, the anterior portion of the overlapped area is slightly more developed and tend to widen relative to a narrower posterior portion of the exposed area, a pattern that is accentuated in post-Devonian forms (e.g.

Diplurus, Axelrodichthys, Guizoucoelacanthus, Latimeria).

The internal surface of the scales is poorly-known in extinct coelacanths as few specimens preserve the scales in internal view. To our knowledge, the internal surface has solely been described among Palaeozoic coelacanths in Miguashaia bureaui, where small numerous bony bumps occur in the posterior region (Fig. 2D2). In Mesozoic forms, the internal surface of the scales is preserved in the Triassic Luopingcoelacanthus eurylacrimalis [START_REF] Wen | Coelacanths from the Middle Triassic Luoping Biota, Yunnan, South China, with the earliest evidence of ovoviviparity[END_REF], the Jurassic Indocoelacanthus robustus [START_REF] Jain | Indocoelacanthus robustus n. gen., n. sp. (Coelacanthidae, Lower Jurassic), the first fossil coelacanth from India[END_REF], and the Cretaceous Megalocoelacanthus dobiei [START_REF] Dutel | The giant Cretaceous coelacanth (Actinistia, Sarcopterygii) Megalocoelacanthus dobiei Schwimmer, Stewart & Williams, 1994, and its bearing on Latimerioidei interrelationships[END_REF] where it is smooth but concentric growth rings are well developed. The scales of the extant Latimeria have a smooth, slightly concave but otherwise featureless, internal surface.

Foreyia maxkuhni from the Triassic [START_REF] Cavin | Heterochronic evolution explains novel body shape in a Triassic coelacanth from Switzerland[END_REF]) is an interesting case as numerous scales from the trunk and base of the fins are known. The internal surface is generally smooth but in some portions of the body a drop-shaped boss is clearly visible (Cavin et al., 2017, fig. 2.4,5,8). This feature is reminiscent of the distinctive drop-shaped boss of certain Devonian sarcopterygians like rhizodonts and tristichopterids [START_REF] Jarvik | Basic Structure and evolution of Vertebrates[END_REF]. The phylogenetic history and significance of this character is difficult to retrace. Its occurrence in Foreyia may represent the result of a simplification of the pattern seen in Miguashaia with numerous bumps or an independently acquired convergent feature with Devonian tetrapodomorphs.

The early evolution of the coelacanth scales

Two enigmas remain in the early evolution of coelacanth's the squamation in coelacanths: 1) did coelacanths lose cosmine (as in many other sarcopterygian groups during the Devonian), or was cosmine never present in their ancestors? and 2) was the primitive scale morphotype of coelacanths rhombic or rounded? Indeed, Miguashaia and Gavinia, the currently considered basal-most undisputed coelacanths [START_REF] Zhu | Earliest known coelacanth skull extends the range of anatomically modern coelacanths to the Early Devonian[END_REF]Gess & Coates, 2015) 44 key to answer these questions, however both issues are still unsettled.

Styloichthys changeae from the Lochkovian (Lower Devonian) of China was initially described as the sister-group of rhipidistians (defined as the largest clade that includes all taxa more closely related to tetrapods and diponans than to actinistians) [START_REF] Zhu | A primitive fish close to the common ancestor of tetrapods and lungfish[END_REF] and this position has been subsequently confirmed in other analyses (e.g. [START_REF] Zhu | 441, 77-80. oldest articulated osteichthyan reveals mosaic gnathostome characters[END_REF]Zhu et al., , 2009;;Lu et al., 2016a,b). However, after Friedman's (2007) new interpretation, its status as a putative early coelacanth has started to gain acceptance (e.g. Gess & Coates, 2015; [START_REF] Lu | A new stem sarcopterygian illuminates patterns of character evolution in early bony fishes[END_REF]. Styloichthys displays shared derived characters of the lower jaw, braincase and dermal skull with coelacanths (Friedman, 2007), however, as opposed to other actinistians, Styloichthys presents rhombic cosmoid scales [START_REF] Lu | An Early Devonian (Pragian) sarcopterygian from Zhaotong, Yunnan, China[END_REF]Cui et al., 2019), similar in shape to those of early dipnomorphs (e.g. Youngolepis) [START_REF] Chang | Is Youngolepis a porolepiform?[END_REF] and tetrapodomorphs (e.g. Kenichthys) [START_REF] Chang | A new Middle Devonian osteolepidid from Qujing, Yunnan[END_REF]. Cross sections performed ion the dermal shoulder girdle (cleithrum), but not in the scales, show that Styloichthys possesses a peculiar type of cosmine in which large odontodes are associated with a pore-canal system that opens to the surface through wider pores than in rhipidistians (i.e., dipnomorphs and tetrapodomorphs) but similar to other early osteichthyans (e.g. Psarolepis and Meemannia) [START_REF] Zhu | 441, 77-80. oldest articulated osteichthyan reveals mosaic gnathostome characters[END_REF][START_REF] Qu | Scales and dermal skeletal histology of an early bony fish Psarolepis romeri and their bearing on the evolution of rhombic scales and hard tissues[END_REF][START_REF] Mondéjar-Fernández | On cosmine: its origins, biology and implications for sarcopterygian interrelationships[END_REF]. In Styloichthys, the pore- canal system is embedded in several layers of enamel and dentine, organized as superimposed odontodes [START_REF] Zhu | 441, 77-80. oldest articulated osteichthyan reveals mosaic gnathostome characters[END_REF][START_REF] Zhu | Meemannia eos, a basal sarcopterygian fish from the Lower Devonian of Chinaexpanded description and significance[END_REF]. This condition differs from the classical "true" cosmine (eucosmine) of rhipidistians where a single layer of enamel overlays the dentine [START_REF] Sire | Origin and evolution of the integumentary skeleton in non-tetrapod vertebrates[END_REF][START_REF] Mondéjar-Fernández | On cosmine: its origins, biology and implications for sarcopterygian interrelationships[END_REF] and highlights that Styloichthys exhibits a primitive type of cosmine, similar to that of stem osteichthyans like Psarolepis and Achoania, and early actinopterygians like Meemannia [START_REF] Zhu | 441, 77-80. oldest articulated osteichthyan reveals mosaic gnathostome characters[END_REF][START_REF] Zhu | Meemannia eos, a basal sarcopterygian fish from the Lower Devonian of Chinaexpanded description and significance[END_REF][START_REF] Qu | Scales and dermal skeletal histology of an early bony fish Psarolepis romeri and their bearing on the evolution of rhombic scales and hard tissues[END_REF]Qu et al., , 2016;;Lu et al., 2016b).

The case of onychodonts is also of interest when dealing with the primitive condition of the squamation in actinistians. A close relationship between onychodonts and coelacanths has been proposed in numerous recent studies of sarcopterygian interrelationships (e.g. [START_REF] Zhu | A primitive fossil fish sheds light on the origin of bony fishes[END_REF][START_REF] Zhu | A primitive sarcopterygian fish with an eyestalk[END_REF][START_REF] Zhu | 441, 77-80. oldest articulated osteichthyan reveals mosaic gnathostome characters[END_REF]Zhu et al., , 2009;;[START_REF] Botella | Jaws and teeth of the earliest bony fishes[END_REF][START_REF] Lu | An onychodont fish (Osteichthyes, Sarcopterygii) from the Early Devonian of China, and the evolution of the Onychodontiformes[END_REF]Lu et al., 2016b[START_REF] Lu | A new stem sarcopterygian illuminates patterns of character evolution in early bony fishes[END_REF][START_REF] Clement | Neurocranial anatomy of an enigmatic Early Devonian fish sheds light on early osteichthyan evolution[END_REF] and it is currently suspected that onychodonts might represent a paraphyletic group of stem actinistians (Lu et al., 2016a;[START_REF] Mondéjar-Fernández | A new onychodont (Osteichthyes; Sarcopterygii) from the Middle Devonian of Morocco and its bearing on early osteichthyan evolution[END_REF]. However, this result complicates the reconstruction of the primitive scale morpho-and histotype in coelacanths. The scales of all known onychodonts are rounded, except in Qingmenodus yui from the Pragian (Lower Devonian) of China, for which the scale shape is still unknown [START_REF] Lu | An onychodont fish (Osteichthyes, Sarcopterygii) from the Early Devonian of China, and the evolution of the Onychodontiformes[END_REF]Lu et al., 2016a). The exposed area of the scales is classically ornamented with characteristic pointed tubercles, [START_REF] Mondéjar-Fernández | A new onychodont (Osteichthyes; Sarcopterygii) from the Middle Devonian of Morocco and its bearing on early osteichthyan evolution[END_REF]. In the earliest undisputed onychodont Bukkanodus jesseni from the Pragian of Australia [START_REF] Johanson | New onychodontiform (Osteichthyes; Sarcopterygii) from the Lower Devonian of Victoria, Australia[END_REF], the exposed area is ornamented with wide dentine ridges separated by grooves containing pores. The odontodes of onychodonts are histologically identical to those of coelacanths like Latimeria [START_REF] Castanet | Données préliminaires sur les structures histologiques du squelette de Latimeria chalumnae. I -Dents, écailles, rayons de nageoires[END_REF]Meunier et al., 2008), and now also confirmed in Miguashaia (Figs 3,4), which probably represents a primitive shared condition of onychodonts and actinistians. Absence of cosmine is also a diagnostic feature for of onychodonts [START_REF] Andrews | The structure of the sarcopterygian Onychodus jandemarrai n. sp. from Gogo, Western Australia: with a functional interpretation of the skeleton[END_REF]. However, putative onychodont remains from the Lower and Middle Devonian of China are covered with cosmine [START_REF] Zhu | Lower jaw character transitions among major sarcopteygian groups-a survey based on new materials from Yunnan, China[END_REF][START_REF] Zhu | An onychodont-like sarcopterygian fish from the Middle Devonian of Yunnan, southwestern China[END_REF][START_REF] Lu | An onychodont fish (Osteichthyes, Sarcopterygii) from the Early Devonian of China, and the evolution of the Onychodontiformes[END_REF]. The dermal skull bones of Qingmenodus display a shiny coating, probably made of enamel, and closely spaced tiny pores, but no histological information is available to confirm or refute the occurrence of cosmine [START_REF] Lu | An onychodont fish (Osteichthyes, Sarcopterygii) from the Early Devonian of China, and the evolution of the Onychodontiformes[END_REF]Lu et al., 2016a). It would thus not be surprising that new material from early onychodonts present cosmine, which would suggest that the cosmine was lost in younger onychodonts and actinistians.

The current phylogenetic picture of coelacanth interrelationships within sarcopterygians evidences two problems that complicate the unravelling of the origin of the coelacanth squamation (Fig. 12A). If Styloichthys is considered as the oldest coelacanth and the sister-group of all other actinistians (Friedman, 2007;Gess & Coates, 2015), then 1) a rhombic and cosmoid morpho-and histotype may be primitive for coelacanths, which implies that 2) the acquisition of a rounded shape and the loss of cosmine probably occurred during the Lower-Middle Devonian, as displayed in Miguashaia, Gavinia and Diplocercides (Fig. 11).

However, this scenario is at odds with the supposed sistergroup relationship of onychodonts and coelacanths, especially if onychodonts are no longer considered a monophyletic group but a subset of stem actinistians [START_REF] Lu | An onychodont fish (Osteichthyes, Sarcopterygii) from the Early Devonian of China, and the evolution of the Onychodontiformes[END_REF]Lu et al., 2016aLu et al., ,b, 2017;;[START_REF] Mondéjar-Fernández | A new onychodont (Osteichthyes; Sarcopterygii) from the Middle Devonian of Morocco and its bearing on early osteichthyan evolution[END_REF]. In this latter case, the evolution of a cosmoid rhombic morphotype of Styloichthys from the rounded and cosmine-lacking scales of onychodontids implies a reversal (Fig. 12B).

A series of plausible scenarios could reconcile the phylogenetic position of Styloichthys with the close relationship of onychodonts and coelacanths and parsimoniously account for the evolutionary origin of the actinistian squamation. The position of Styloichthys has been reconstructed as shifting between the sister-group of the onychodont-actinistian clade (Lu et al., 2016b) (Fig. 12C) and the sister-group of a more inclusive taxon that includes the onychodont-actinistian clade and rhipidistians [START_REF] Lu | A new stem sarcopterygian illuminates patterns of character evolution in early bony fishes[END_REF] (Fig. 12D). If either of these hypotheses are further The last possible scenario is to consider onychodonts as stem sarcopterygians, not closely related to actinistians, but rather as the sister-group of a clade comprising coelacanths (including Styloichthys) and rhipidistians (e.g. [START_REF] Cloutier | Morphology, characters, and the interrelationships of basal sarcopterygians[END_REF]Friedman, 2007) (Fig. 12E). In this case, the acquisition of a rounded and tuberculated squamation is convergent in onychodonts and actinistians, and both originated from a rhombic ancestral morphotype covered in cosmine, similar that of stem osteichthyans like Psarolepis [START_REF] Qu | Scales and dermal skeletal histology of an early bony fish Psarolepis romeri and their bearing on the evolution of rhombic scales and hard tissues[END_REF]. In the case of onychodonts, primitive cosmoid forms may be represented by the putative cosminecovered onychodonts from China [START_REF] Zhu | Lower jaw character transitions among major sarcopteygian groups-a survey based on new materials from Yunnan, China[END_REF][START_REF] Zhu | An onychodont-like sarcopterygian fish from the Middle Devonian of Yunnan, southwestern China[END_REF][START_REF] Lu | An onychodont fish (Osteichthyes, Sarcopterygii) from the Early Devonian of China, and the evolution of the Onychodontiformes[END_REF], whereas for coelacanths, the primitive condition may be exemplified by the rhombic scales of Styloichthys [START_REF] Zhu | A primitive fish close to the common ancestor of tetrapods and lungfish[END_REF][START_REF] Zhu | 441, 77-80. oldest articulated osteichthyan reveals mosaic gnathostome characters[END_REF][START_REF] Zhu | Meemannia eos, a basal sarcopterygian fish from the Lower Devonian of Chinaexpanded description and significance[END_REF]Friedman, 2007). Overall, the combination of traits described in Miguashaia do not represent a drastic departure from the histological structure of the scales of other coelacanths like Macropoma [START_REF] Williamson | XXIII. On the microscopic structure of the scales and dermal teeth of some Ganoid and Placoid Fish[END_REF], Coelacanthus [START_REF] Goodrich | On the scales of fish, living and extinct, and their importance in classification[END_REF], or Undina [START_REF] Gross | Histologische Studien am Aussenskelett fossiler Agnathen und Fische[END_REF][START_REF] Gross | Kleine Schuppenkunde[END_REF]. Coelacanths are considered to have experienced an early burst of diversification during the Palaeozoic, followed by remarkable morphological conservatism with limited variations until Recent (e.g. Cloutier, 1991a). Longitudinal section of the exposed area in microradiography (anterior to the right). The ornamented superficial layer displays four odontodes (white arrowheads). Some vascular canals from the reduced spongiosa can be seen at the base of the odontodes. The whole basal plate is unmineralized and thus does not appear in the image, its approximative contour is represented with a white dashed line. Scale bar equals 1 mm (from Meunier et al. 2008). Abbreviations: bp, basal plate; pc, pulp cavity; vc, vascular canal. Jorge Mondéjar-Fernández (1)* , François J. Meunier (2) , Richard Cloutier (3) , Gaël Clément (1) and Michel Laurin (1) ( [francois.meunier@mnhn.fr] (3) Université du Québec à Rimouski, 300 allée des Ursulines, Rimouski, Québec, G5L 3A1, Canada.

The scales of

[richard_cloutier@uqar.ca]

HIGHLIGHTS

 The microstructure of the scales of the early coelacanth Miguashaia bureaui from the Devonian of Canada is revealed for the first time.  The rounded scales display superimposed tubercles of dentine capped with enamel (odontodes) and a fully ossified basal plate.  Miguashaia illustrates the primitive condition for coelacanths and reveals that the general histological arrangement of the coelacanth dermoskeleton has not significantly varied in the last 400 million years, contrary to the scale morphology and ornamentation, which has been more variable.  Data on the squamation have important implications for the phylogenetic reconstruction of the evolutionary history of coelacanths.

like

  rue Cuvier, CP38, Paris F-75005. Email : jorge.mondejar-fernandez@mnhn.fr Running title: Scales of the early coelacanth Miguashaia been described as morphologically conservative throughout their long evolutionary history, which that spans more than 400 million years. After an initial burst during the Devonian, a morphological stasis was long thought to have prevailed since the Carboniferous, as shown by the extant Latimeria. New fossil discoveries have challenged this view, with punctual and sometimes unusual departures from the general coelacanth Bauplan. The dermal skeleton is considered to represent one, if not the main, example of morphological stasis in coelacanth evolution and as a consequence, has remained poorly surveyed. The flagrant lack of paleohistological data on the dermoskeleton has resulted in a poor understanding of the early establishment and evolution of the coelacanth squamation. Here we describe the scales of Miguashaia bureaui from the Upper Devonian of Miguasha, Québec (Canada), revealing histological data for a Palaeozoic coelacanth in great detail and for the first time, and adding to our knowledge on the dermal skeleton of sarcopterygians. Miguashaia displays rounded scales ornamented by tubercules and narrow ridges made of dentine and capped with enamel. At least two generations of superimposed odontodes occur, which is reminiscent of the primitive condition of stem osteichthyans like Andreolepis or Lophosteus, and onychodonts Selenodus. The middle vascular layer is well developed and shows traces of osteonal remodelling. The basal plate consists of a fully mineralized lamellar bone with a repetitive rotation pattern every five layers indicating a twisted plywood-like arrangement of the collagen plies. Comparisons with the extant Latimeria and other extinct taxa show that these features are consistently conserved across coelacanth evolution with only minute changes in certain taxa. The morphological and histological features displayed in the scales of Miguashaia enable us to draw a comprehensive picture of the onset of the coelacanth squamation and to propose and discuss evolutionary scenarios for the coelacanth dermoskeleton.Keywords: coelacanth, dermoskeleton, histology, odontodes,

  were crucially lacking for comparisons with more recent taxa.

  Frasnian of eastern Québec (Canada). It constitutes a the first thorough investigation of the dermal scales of a Palaeozoic coelacanth and provides much needed comparative information on the early establishment of the histological features of the actinistian squamation. Comparison with other extinct coelacanths and with the extant Latimeria will provide a more complete picture of the histological diversity in actinistians and better document the evolution of the dermal skeleton in sarcopterygians and early osteichthyans. With these detailed descriptions and holistic approach, we hope to invigorate the study of the paleohistological diversity of the dermal skeleton of actinistians, which has been largely overlooked in most analyses of coelacanth evolution.

  Original and newly discovered material of Miguashaia bureaui has been studied, focusing on the external morphology of the scales: MHNM 06-41ab, MHNM 06-494 (mean scale diameter: 1.87 cm; total length of specimen: 37.5 cm), MHNM 06-641, MHNM 06-1232, MHNM 06-1233, MHNM 06-1234, MHNM 06-1236ab, MHNM 06-1237; MHNM 06-1239; MHNM 06-1318ab; MHNM 06-1633 (mean scale diameter: 0.16 cm; total length of specimen: 6.98 cm), MHNM 06-1809ab (mean scale diameter: 1.28 cm; total length of specimen: ca. 43 cm); MHNM 06-2414, ULQ 120ab (holotype).Fossil material used for the histological investigation consists of two separated blocks (MHNM 06-1238; MHNM 06-1495) containing numerous articulated trunk scales. Specimen MHNM 06-1238 (Fig. 1A) displays disarticulated axial and appendicular elements including at least two neural arches (n.ar) and probably haemal spines, caudal fin radials (ra), and a few fine lepidotrichia (lp) most likely belonging to the second dorsal fin. Specimen MHNM 06-1495 (Fig. 1B) shows disarticulated fragments of the opercular series (op.s) and pectoral girdle elements (cleithrum, cl), most of them preserved as imprints of the smooth inner surface of the dermal bones, and some segments of pectoral lepidotrichia (lp) with the characteristic interlocking system. Scales (sc) of specimen MHNM 06-1495 are most likely from the anterior part of the trunk as well as some anteroventral scales. Scales (sc) of specimen MHNM 06-1238 are most likely from the posterior part of the flanks, posteroventral to the second dorsal fin.

  from the middle Frasnian Escuminac Formation, Miguasha, Québec, Canada. All the fossil material is housed at the Musée d'Histoire naturelle de Miguasha (MHNM), parc national de Miguasha, Québec (Canada) with the exception of the holotype that is housed at the Université Laval (ULQ), Québec City, Québec (Canada). Latimeria Latimeria chalumnae. The morphology (SEM) and the microstructure (ground sections) of the scales have been examined predominantly from two main specimens: (a) the scales overlying the basis of the pelvic fin of MNHN-ZA-AC-2012-26 (CCC79, adult female, 78 kg, 163 cm TL) and (b) several scattered scales of specimen MNHN-ZA-AC-2012-29 (CCC117, adult female, 19 kg, 109,5 cm TL), both captured off the Comoro Islands (Nulens et al. 2011). The scales were fixed in neutral formalin and kept in 70º ethanol. All specimens are housed at the Muséum national d'Histoire naturelle (MNHN) of Paris (France).

  at the Museum Zoologicum Bogoriense (MZB) (Indonesian National Zoological Museum) in Bogor (Indonesia). Institutional abbreviations: FMNH, Field Museum of Natural History (Chicago, USA); MHNM, Musée d'Histoire naturelle de Miguasha (Miguasha, Canada); MNHN, Muséum national d'Histoire naturelle (Paris, France); MZB, Museum Zoologicum Bogoriense (Bogor, Indonesia); ULQ, Université Laval (Québec City, Canada); YPM, Peabody Museum, Yale University (New Haven, USA).

  , thus cleaning the mineralized front of the basal plate). The scales were then washed in distilled water, dehydrated in absolute ethanol, air-dried, and coated with evaporated gold. The external and internal surfaces of the scales were examined in a JEOL-SEM-35 scanning electron microscope. Thin sections of the scales (30 to 100 µm in thickness) were observed in polarized light with a Zeiss Axiovert 35 microscope. Microradiographies were performed on non-decalcified sections (focal point-object distance 10 cm, power 10 to 15 Kv at 7 mA). Histological sections after decalcification with EDTA (ethylenediaminetetraacetic acid) and inclusion in paraffin (sections 5 to 10 µm in thickness) were treated with the following stains: hemalun-picro-indigocarmin, APS, toluidine blue, Prenant's trichrome.

Flank

  scales from the posterior region of the trunk are either covered by (1) fine leaf-or spoon-shaped tubercles and fine longitudinal ridges (Fig. 2D1) (MHNM 06-1633, MHNM 06-1236), (2) fine leaf-or spoon-shaped tubercles, fine longitudinal ridges and pitted enamel surface (MHNM 06-1234), (3) fine longitudinal ridges and pitted enamel surface (Fig. 2E) (MHNM 06-494, MHNM 06-1265, MHNM 06-1809), or (4) solely pitted MHNM 06-1237, MHNM 06-1265). The extent of the pitted enamel surface increases during growth in order to cover the complete ornamented area. Smaller scales from the caudal lobe only bear fine longitudinal ridges in both small and large individuals (MHNM 06-1318, MHNM 06-1809). Although the ornamentation is not preserved on the complete specimens, the smallest individuals appear to only have fine longitudinal ridges (ULQ 120, MHNM 06-1633). The coarse tubercles and ridges seem to develop only in larger individuals (MHNM 06-641).

  Cloutier (1996, fig. 1a) illustrated scales with one central elongated canaliculus associated with two pores above and below forming also a horizontal trajectory on most of the length of the trunk but slightly curving dorsally along the anocleithrum. Seven to eight scales belonging to the anterior section of the main lateral line canal are preserved on specimen MHNM 06-1236 starting above the dorsal part of anocleithrum. Each scale has the main elongated canaliculus flanked dorsally and ventrally by three or four slightly curving canals.

Fig

  Fig. 1B). Most scales are preserved as imprints showing either

  -preserved scales (most likely anteroventral scales)

  Figs 3,4) and is variable in thickness (between 50-100 µm).

  Figs 3A-B, 4B-E, 5). As the dentine layer, the spongiosa is

  sl, Fig.7A) and open regularly onto the surface between the small striae made of woven-fibered bone(Figs 5,

  features are either solely based on external observation, drawings, or on unfigured thin

  although the ornamentation patterns, ossification rate, and relative thickness of the various layers differ across taxa. Nevertheless, several key differences can be recognized between certain taxa, revealing a significant histological disparity.

  (Figs 9, 10A), differs from that of Miguashaia bureaui, in which pulp cavities are reduced(Figs 3A, 4). A possible evolutionary trend can be proposed, from small and numerous odontodes with small pulp cavities in early Palaeozoic coelacanths (e.g. Miguashaia) to fewer but larger odontodes with wider pulp cavities in younger taxa from the Mesozoic (e.g. Undina, Macropoma) to the Recent (Latimeria).Odontodes are rarely found isolated, either on the scales or on the dermal bones. Indeed, superimposed generations of the spongiosa are known in Miguashaia bureaui(Figs 3,

  described the presence of Sharpey's fibres in the spongiosa, whereas Williamson (1849) described similar structures in Macropoma lewesiensis but identified them as dentinal tubules (odontoblastic canaliculi). No distinctive Sharpey's fibres have been confidently identified layer nor in the basal plate of the scales of Miguashaia bureaui.

  dermal bones shows traces of secondary ossification

  fossil coelacanth material, it is unclear whether a double-twisted plywood is an autapomorphy of Latimeria or if it was inherited from closely-related taxa. Finally, it is now well established that the irregularly mineralized basal plate of the scales of Latimeria derives from the fully mineralized basal plate of closely-related extinct coelacanths and is topologically and structurally homologous to the isopedine layer from other early sarcopterygians (e.g. early actinistians like Miguashaia, dipnomorphs like Porolepis, Holoptychius and Dipterus, and tetrapodomorphs like Megalichthys and Eusthenopteron) (Mondéjar-Fernández & Meunier, 2020).

  solely known Miguashaia bureaui

  progresses anteriorly following the lateral line

  that small individuals mainly display fine longitudinal ridges across the body (ULQ 120, MHNM 06-1633),

  due to the small size of the scales, and more abundant in adult specimens with larger scales.

  and duration of the seasons, among other possible factors.

  the overlapped area of the scales is not ornamented, excepted in some Devonian taxa like Miguashaia, Gavinia, and Diplocercides, which display a reduced field of small leaf-or spoon-shaped tubercles spanning the margin between the overlapped and exposed areas 11).

  the posterior part of the trunk of Miguashaia bureaui, the ornamented exposed area is almost as developed as

  display rounded scales without cosmine and ornamented with the classical odontogenic combination of tubercles and ridges. The position of the putative actinistian Styloichthys and the likely sister-group relationship between coelacanths and onychodonts among early sarcopterygians are

  odontodes made of dentine capped with a thin enamel layer in Selenodus aquesbiae from the Eifelian (Middle Devonian) of Morocco

  fossil and phylogenetic data, then it will corroborate that the combination of a rounded scale morphotype and the ridged/tuberculated ornamentation of onychodonts and coelacanths is a shared feature. Rounded scales devoid of cosmine might have evolved from primitive cosmoid scales similar to those of early sarcopterygians like Styloichthys (Fig.12C) or from early cosmine-covered onychodonts (Fig.12D) before the establishment of the rounded morphotype in the common ancestor of more recent onychodonts (like Bukkanodus, Selenodus, Onychodus or Strunius) and actinistians.

  the early coelacanth Miguashaia bureaui from the Frasnian (Upper Devonian) of Miguasha, Québec (Canada), have been histologically investigated for the first time, revealing a combination of primitive features for actinistians that allow us to reconstruct the primitive condition of the squamation of coelacanths. The rounded scales devoid of cosmine of Miguashaia can be described as elasmoid scales, in which the superficial layer is ornamented by pointed tubercules and narrow ridges made of dentine capped with enamel (odontodes). At least two generations of superimposed odontodes occur, reminiscent of the primitive condition of stem osteichthyans like Andreolepis or Lophosteus. The middle vascular layer (spongiosa) is well developed and tightly linked to the odontodes. The basal plate (isopedine) consists of a fully mineralized layer of lamellar bone with a twisted plywood-like arrangement of the collagen plies, which display a repetitive rotation pattern every five layers. The presumably primitive condition of the scales of Miguashaia allows testing evolutionary scenarios for the squamation in coelacanths by comparing Miguashaia with other extinct (e.g. Diplocercides, Diplurus) and extant (Latimeria) actinistians. Despite an important lack of paleohistological comparative material from the scales of extinct coelacanths, can be inferred from Palaeozoic taxa to Recent: 1) a reduction in number but increase in size of the tubercles and ridges ornamenting the exposed area, 2) an antero-posterior elongation of the scales, 3) the development of larger pulp cavities in the dentine forming the odontodes, and 4) an increase in the relative thickness of the basal plate, coupled with the reduction of the mineralization rate of the isopedine in Latimeria.

  Miguashaia already possess the more characteristic features of the actinistian squamation, consistent with this general evolutionary scenario. However, the current uncertainties regarding the position of the putative coelacanth Styloichthys and the role of onychodonts as the sister-group of actinistians complicates the reconstruction of the onset of the coelacanth scales prior to the emergence of early coelacanths forms like Miguashaia or Gavinia. Additional paleohistological data are thus needed from other extinct actinistians and early sarcopterygians.

  could then be incorporated into future phylogenetic analyses, which would contribute to the unravelling of actinistian interrelationships and continue shedding more light on the evolutionary history of the squamation of coelacanths.

  . & Forey, P. L. 1991. Diversity of extinct and living actinistian fishes (Sarcopterygii). Environmental Biology of Fishes, 32: 59-74 Cloutier, R., & Schultze, H. P. 1996. Porolepiform fishes (Sarcopterygii). pp: 248-270 in H.-P. Schultze & R. Cloutier (eds.), Devonian Fishes and Plants of Miguasha, Quebec, Canada. Verlag Dr. Friedrich Pfeil, München, Germany. Cloutier, R., Béchard, I., Charest, F., & Matton, O. 2009. La contribution des poissons fossiles du parc national de Miguasha à la biologie évolutive du développement. Le naturaliste canadien, 133(3): 84-95. Cloutier, R., Loboziak, S., Candilier, A. M., & Blieck, A. 1996. Biostratigraphy of the Upper Devonian Escuminac Formation, eastern Québec, Canada: A comparative study based on miospores and fishes. Review of Palaeobotany and Palynology, 93(1-4): 191-215. Cloutier, R., Proust, J. N., & Tessier, B. 2011. The Miguasha Fossil-Fish-Lagerstätte: a consequence of the Devonian land-sea interactions. Palaeobiodiversity and Palaeoenvironments, 91(4): 293-323. Cloutier, R., Clement, A. M., Lee, M. S., Noël, R., Béchard, I., Roy, V., & Long, J. A. 2020. Elpistostege and the origin of the vertebrate hand. Nature, 579(7800): 549-554. Cui, X., Qiao, T., & Zhu, M. 2019. Scale morphology and squamation pattern of Guiyu oneiros provide new insights .J. 1984. Structure et minéralisation des écailles de quelques Osteoglossidae (Ostéichthyens, Téléostéens). Ann Sc Nat, Zool. 6: 111-124. 13ème Série. Meunier, F.J., François, Y. 1980. L'organisation spatiale des fibres de collagène et la minéralisation des écailles des Dipneustes actuels. Bull Soc Zool Fr. 105: 215-226. Meunier, F. J., & Laurin, M. 2012. A microanatomical and histological study of the fin long bones of the Devonian sarcopterygian Eusthenopteron foordi. Acta Zoologica, 93(1): 88-97. Meunier, F. J. & Zylberberg, L. 1999. The structure of the external layer and of the odontodes of scales in Latimeria chalumnae (Sarcopterygii, Actinistia, Coelacanthidae) revisited using scanning and transmission electron microscopy. In: Séret, B. and Sire, J.-Y. (eds.), Proc. 5th Indo-Pacific Fish Conf., Nouméa, 1997, Soc. Fr. Ichtyol., Paris, pp. 109-116. Meunier, F. J., Cupello, C. & Clément, G. 2019. The skeleton and the mineralized tissues of the living coelacanths. Bulletin of Kitakyushu Museum of Natural History and Human History, 17, 37-48. Meunier, F. J., Erdmann, M. V., Fermon, Y. & Caldwell, R. L. 2008. Can the comparative study of the morphology and histology of the scales of Latimeria menadoensis and L. chalumnae (Sarcopterygii: Actinistia, Coelacanthidae) bring new insight on the taxonomy and the biogeography of recent

FIGURE 2 .FIGURE 3 .

 23 FIGURE 2. Squamation of Miguashaia bureaui. A. Complete

FIGURE 4 .

 4 FIGURE 4. Ornamentation and microstructural organization of

FIGURE 5 .

 5 FIGURE 5. Ornamentation and microstructural organization of

FIGURE 6 .FIGURE 7 .FIGURE 8 .FIGURE 9 .

 6789 FIGURE 6. Microstructural and histological organization of the

FIGURE 10 .

 10 FIGURE 10. Ornamentation and microstructural organization of

  Squamation of Miguashaia bureaui. A. Complete specimen (MHNM 06-494). B. Reconstruction and body outline of M. bureaui with the scales from three locations of the left flank highlighted. C-E. Reconstruction of different scales from the left flank: C. Scale from the anteroventral portion of the trunk, posterior to the pectoral fin, in external view, D. Scale from the posterior portion of the trunk, in external (1) and internal (2) view. E. Scale from the posterior-most region of the trunk, close to the caudal fin, in external view. Drawings based on MHNM 06-6, MHNM 06-641 (Cloutier 1996; fig. 17), MHNM 06-1238, MHNM 06-1239, and MHNM 06-1495. Abbreviations: ci.b, circular bumps; co.tu, coarse tubercles; co.ri, coarse ridges; f.ri, fine ridges; pi.e, pitted enamel; sp.tu, spoon-shaped tubercles. Arrow points anteriorly. Scale bars equal 10 cm (A) and 10 mm (C-E). General microstructural organization of the scales of Miguashaia bureaui (vertical cross sections of MHNM 06-1495). A. Superimposed odontodes from the superficial layer. B. Vascular bone (spongiosa) from the superficial layer. Arrow heads point to osteocytes embedded in the bone. C. Lamellar bone (isopedine) from the basal plate. Black arrows indicate the repetitive pattern of collagenous plies with the same orientation. Abbreviations: d1-2, dentine layer of two generations of odontodes; e1-2, enamel layer of two generations of odontodes; lb, lamellar bone; pc, pulp cavity; vb, vascular bone. Scale bar equals 50 µm. Ornamentation and microstructural organization of the exposed area of the scales of Miguashaia bureaui (vertical cross sections of MHNM 06-1495). A. General view of a scale showing the distribution of the odontodes in the superficial layer. White arrow heads indicate the irregular front of mineralization of the ventral surface of the basal plate. B. Detail of an odontode composed of a thin layer of enamel and dentine with numerous odontoblastic canaliculi overlying the layer of vascular bone (spongiosa). C. Inset of two odontodes from A. Note the opening of vascular canals from the spongiosa through pores in the surface, as in B, and the large odontoblastic lacunae. Arrow heads indicate the occurrence of osteocyte lacunae in the lamellar bone (isopedine) of the basal plate. D-E. Different distributions and arrangements of two generations of superimposed odontodes. Note the reduced thickness of the spongiosa and the occurrence of vascular canals and vascular bone surrounding the older odontodes. Abbreviations: bp, basal plate; d, dentine; d1-2, dentine layer of two generations of odontodes; e, enamel; e1-2, enamel layer of two generations of odontodes; lb, lamellar bone; ml, middle layer; oc, odontoblastic canaliculi; od1-2, two generations of odontodes; vb, vascular bone; vc, vascular canal;. Scale bars equal 100 µm. Ornamentation and microstructural organization of the overlapped area of the scales of Miguashaia bureaui (vertical and horizontal cross sections of MHNM 06-1238). A. Three scales vertically sectioned at different regions of the overlapped area (from anterior to posterior): 1) Anterior most margin; 2) Central region; 3) Posterior region, close to the contact with the exposed area. Note the increase in thickness of the vascular bone layer (spongiosa) and the lamellar bone layer (isopedine) from anterior to posterior and the changes in the bony ornamentation. Scale bar equals 500 µm. B. Bony ornamentations of the overlapped area from the anterior most margin (1) and the central region (2). Scale bar equals 100 µm. C. Horizontal cross section through the small bony tubercles of the overlapped area from the anterior most margin of a scale (1). Note the concentric patterns of ossification, the radial arrangement of osteocyte lacunae, and the openings of vascular canals between the tubercles. Scale bar equals 50 µm. Abbreviations: lb, lamellar bone; vb, vascular bone.

  Microstructural and histological organization of the vascular bone of the scales of Miguashaia bureaui (horizontal cross sections of MHNM 06-1238). A. General view of the vascular bone layer (spongiosa) from the overlapped area of the scales. Note the uniform diameter and distribution of the vascular canals. Scale bar equals 500 µm. B. Detailed inset from A. Note that each vascular canal is surrounded by a series of concentric layers of woven-fibered bone (light brown) while each row of vascular canals is separated from the other by a denser bone tissue (dark brown). Scale bar equals 100 µm. C. Detailed inset from B. Arrow heads indicate the occurrence of osteocyte lacunae surrounding the vascular canals. Scale bar equals 50 µm. D. General view of the vascular bone and vascular canals from another region of the overlapped area of a scale, probably from a more posterior region than in A. Note the more irregular distribution of the vascular canals and the occurrence of ossification waves. Scale bar equals 100 µm. E. Detailed inset from D. Note the occurrence of large osteocyte lacunae in the woven-fibered bone around the vascular canals. Scale bar equals 50 µm. F. Detailed inset from D. Arrow heads point to embedded osteocyte lacunae with large osteocytic prolongations. Scale bar equals 50 µm. Microstructural and histological organization of the basal plate of the scales of Miguashaia bureaui (vertical cross sections of MHNM 06-1238 and MHNM 06-1495). A. General view of the vascular bone layer (spongiosa) of the superficial layer and lamellar bone (isopedine) of the basal plate from the anterior portion of the overlapped area of the scales (MHNM 06-1238). The horizontal plane corresponds to the approximative cross section plane in Fig. 6D-F. Note the clear differences in bone structure between the spongiosa (made of woven-fibered bone) and the isopedine (made of lamellar bone). Scale bar equals 100 µm. B. Detailed view of the basal plate from the central portion of the exposed area of the scales (MHNM 06-1495). Black arrows indicate the repetitive pattern of collagenous plies with the same orientation. White arrow heads indicate the irregular front of mineralization of the visceral surface of the basal plate. The black arrow head points to a vertical vascular canal piercing the basal plate and opening through the ventral surface of the scale. Note that the thickness of the basal plate is more important around the vascular canal resulting in a bump of the internal surface of the scale at this point. Scale bar equals 100 µm. Abbreviations: bp, basal plate; lb, lamellar bone; sl, superficial layer; vb, vascular bone; vc, vascular canal.

  Basal plate comparison of the scales of Miguashaia and Latimeria in horizontal cross section. A. Miguashaia bureaui (MHNM 06-1238). Note the occurrence of 5 directions of the collagen fibers. B. Latimeria chalumnae (MNHN-ZA-AC-2012-29). C. Latimeria menadoensis (MZB 10003). Note the arciform pattern of the collagen plies evidencing a twisted-plywood arrangement of the layers of the isopedine. Scale bars equal 100 µm. Odontodes from the exposed area of the scales of Latimeria (SEM). A. Latimeria menadoensis (MZB 10003). General view of the odontodes. The posterior margin of the scale is on the upper left of the image. The radial ridges of the superficial layer are clearly seen between the odontodes. Scale bar equals 500 µm. B-C. Latimeria menadoensis (MZB 10003). Detail of the odontodes and ridged surface of the superficial layer between the odontodes. Scale bar equals 100 µm. D. Latimeria chalumnae (MNHN 1989-806). Detail of the odontodes. The posterior margin of the scale is on the upper right of the image. Scale bar equals 100 µm.

  Ornamentation and microstructural organization of the scales of Latimeria. A. Latimeria chalumnae (MNHN-ZA-AC-2012-26). Transversal section of the exposed area of the scale in polarized light. The ornamented superficial layer displays eight odontodes (white arrow heads) with large pulp cavities and overlies the basal plate. Scale bar equals 1 mm (from Castanet et al. 1975). B. Latimeria menadoensis (MZB 10003).

  Phylogenetic hypothesis on early coelacanth evolution with special emphasis on the squamation. A. Phylogenetic interrelationships of selected osteichthyans, highlighting the unresolved position of Styloichthys and onychodonts (represented here by Selenodus) relative to coelacanths (including here Miguashaia and Latimeria) (consensus of phylogenetic hypotheses after Friedman 2007; Lu et al. 2016a,b, 2017; Mondéjar-Fernández 2020, among others). Abbreviations: A: actinopterygians, C: coelacanths D: dipnomorphs, R: rhipidistians, S: sarcopterygians, T: tetrapodomorphs. Original artwork of Miguashaia, Latimeria and Danio and modified drawings of Andreolepis after Gross 1968; Neoceratodus after Brien, 1962; Porolepis, Osteolepis and Eusthenopteron after Jarvik, 1980; Cheirolepis after Pearson 1982; Styloichthys Lu & Zhu 2008; Psarolepis after Qu et al. 2013; Selenodus after Mondéjar-Fernández 2020. Not to scale. B-E. Phylogenetic hypothesis of the possible interrelationships between Styloichthys, onychodonts (irrespective of their monophyly) and coelacanths. B. Onychodonts as stem coelacanths and Styloichthys as the sister group of coelacanths. C. Styloichthys as the sister group of the onychodont-coelacanth clade. D. Styloichthys as the sister group of the onychodont-coelacanth clade and rhipidistians. E. Onychodonts as the sister group of coelacanths (including Styloichthys) and rhipidistians.

  , see below), rather than a retention of a pre-

	cosmoid state as previously suggested
	F
	o r
	P
	e e r
	R e v
	i e w
	O n
	l
	y
	30

Table 1 .

 1 List of extinct coelacanths for which the squamation has been studied and figured.

	F
	o r
	P
	e e r
	R e v
	i e w
	O n
	l
	y

  1) UMR 7207 (MNHN-Sorbonne Université-CNRS), Centre de Recherche en Paléontologie -Paris, Département Origines & Évolution, Muséum national d'Histoire naturelle, 57 rue Cuvier, CP38, Paris F-75005. [jorge.mondejarfernandez@mnhn.fr], [gael.clement@mnhn.fr], [michel.laurin@mnhn.fr] (2) FRE BOREA 2030, (MNHN -Sorbonne Université -Univ. Caen Normandie -Univ. Antilles -CNRS -IRD), Département Adaptations du Vivant, Muséum national d'Histoire naturelle, CP26, 43 rue Cuvier, 75231 Paris cedex 05, France.

Journal of Anatomy

Schaeffer, B. 1962. A coelacanth fish from the Upper Devonian

ACKNOWLEDGMENTS

We would like to warmly thank Severin Morel (MNHN) for the thin-sections, Philippe Loubry (MNHN) for the photographs, and Marc Herbin (MNHN) for granting access to the Latimeria collections under his care. Johanne Kerr (parc national de Miguasha) prepared the loan of specimens and Nathaniel Bertrand Maltais helped with the photographs. Richard Cloutier was funded by NSERC (Discovery Grant).

Fragoso, L. G. C., Brito, P., and Yabumoto, Y. 2019. Maisey, 1986 revisited. Historical Biology, 31(10): 1350-1372.

Axelrodichthys araripensis

Francillon-Vieillot H., Buffrenil V. De, Castanet J., Géraudie J., Meunier Fj., Sire J-Y., Zylberberg L., & Ricqlès A. De. 1990. Microstructure and mineralization of vertebrate skeletal tissues. In: Carter, J. G. (ed.), Skeletal Biomineralization: Patterns, Processes and Evolutionary Trends. Vol. I, Van Nostrand, New York, pp. 471-530. Friedman, M. 2007. Styloichthys as the oldest coelacanth: implications for early osteichthyan interrelationships. Journal of Systematic Palaeontology, 5(3): 289-343. Friedman, M. & Coates, M. I. 2006. A new recognized fossil coelacanth highlights the early morphological diversification of the clade. Proceedings of the Royal Society B 273, 245-250. Geng, B. H., Zhu, M., & Jin, F. 2009. A revision and phylogenetic analysis of Guizhoucoelacanthus (Sarcopterygii, Actinistia) from the Triassic of China. Vertebrata PalAsiatica, 47(3): 165-177. Gess, R. W., & Coates, M. I. 2015. Fossil juvenile coelacanths from the Devonian of South Africa shed light on the order of character acquisition in actinistians. Zoological Journal of the Linnean Society, 175(2): 360-383. Gillis, J. A., Dahn, R. D., & Shubin, N. H. 2009. Shared developmental mechanisms pattern the vertebrate gill arch

AUTHORS CONTRIBUTION

JMF, FJM and GC designed the study. ML provided the fossil material. JMF and FJM acquired the histological cross sections and analysed the data. RC completed the morphological observations. JMF prepared the figures and table, and wrote the text with contributions from FJM and RC. All authors revised the manuscript and approved its final version.