Gildas Merceron
email: gildas.merceron@univ-poitiers.fr

Marc Colyn

Denis Geraads

Browsing and non-browsing extant and extinct giraffids: evidence from dental microwear textural analysis

Keywords: Tooth, Ruminant, Neogene, Diet, Ecology

Today, the family Giraffidae is restricted to two genera endemic to the African continent, Okapia and Giraffa, but, with over ten genera and dozens of species, it was far more diverse in the Old World during the late Miocene. We attempt to describe here how several species may have shared feeding resources in the Eastern Mediterranean. Dietary preferences were explored by means of Dental Microwear Textural Analysis in combination with estimation of body mass and the maximum height at which the various species were able to browse.

One of our main results concerns the modern okapi, Okapia johnstoni. It is a forest dweller usually regarded as a browser, but we show that it might also forage on tough plants, possibly herbaceous monocots. Such feeding habits including portions of herbaceous monocotyledons were also found for some extinct species, especially the genera Samotherium and Palaeotragus. Palaeogiraffa shows a contrasted pattern: the specimens of P. pamiri from a site in Thrace were leaf-dominant browsers whereas those belonging to P. major and P. macedoniae from the Axios valley in Greece ingested herbaceous monocotyledons.

Helladotherium duvernoyi, the only sivatheriine analyzed here is described as a leaf-dominant browser. The giraffine Bohlinia attica also falls within the leaf-dominant browser category but could browse on higher foliages than H. duvernoyi. On the whole, the reconstructed diets confirm the relationship between more grazing habits and smaller premolars, but not with higher dental crown height.

Introduction

Today, giraffids are poorly diverse with only two genera, Okapia and Giraffa (Agaba et al., 2016), both endemic to the African continent. The okapi Okapia johnstoni is endemic to the north east of the Democratic Republic of Congo in the Congo Basin. There, the okapi is confined to dense equatorial forest, which explains why its anatomical description in a scientific journal dates back only to the early 20th century. Okapia johnstoni is unknown as a fossil, but its discovery has generated several hypotheses regarding its relationships with the giraffe and the many species of extinct giraffids [START_REF] Colbert | The relationships of the okapi[END_REF][START_REF] Thenius | Das Okapi (Mammalia, Artiodactyla) von Zaire -"lebendes Fossil" oder sekundärer Urwaldbewohner?[END_REF]. The modern giraffe has a wide range of distribution in subsaharan Africa. The genus dates back to the early Pliocene. A large part of the evolutionary history of the Giraffidae did not take place in Africa but in Eurasia during the Neogene [START_REF] Solounias | Dietary interpretation and paleoecology of herbivores from Pikermi and Samos (late Miocene of Greece)[END_REF]. With over 10 genera and dozens of named species that can be conveniently grouped in three subfamilies (Sivatheriinae, Giraffinae, Palaeotraginae), the family reached its highest diversity during the late Miocene.

They share with modern taxa a small set of synapomorphies such as large body size, bilobate canine, and a molariform fourth premolar, but their skin-covered cranial appendages are not exclusive to the family [START_REF] Geraads | Remarques sur la systématique et la phylogénie des Giraffidae (Artiodactyla, Mammalia)[END_REF]. A phylogenetic analysis of giraffids based upon new material from the late Miocene of Spain confirmed the monophylies of the two subfamilies Sivatheriinae and Giraffinae. However, the Sivatheriinae appear as the sister-group of the genus Samotherium and are thus rooted within the Palaeotraginae, whose paraphyletic status is thus emphasized [START_REF] Ríos | A new giraffid (Mammalia, Ruminantia, Pecora) from the late Miocene of Spain, and the evolution of the sivathere-samothere lineage[END_REF][START_REF] Ríos | Comparative anatomy, phylogeny, and systematics of the Miocene giraffid Decennatherium pachecoi Crusafont, 1952 (Mammalia, Ruminantia, Pecora): State of the art[END_REF].

The present study aims at determining the feeding ecology of the extinct giraffids from the late Miocene of Eastern Mediterranean in regard to their differences in morphology mirroring the phylogeny. We use dental microwear textural analysis (DMTA, hereafter) to specify the dietary habits. First, we discuss the relations between dietary habits and dental microwear textural parameters on extant species of ruminants. This allows us to interpret and discuss the dental microwear textural parameters of species with unknown feeding habits.

Before applying these parameters to extinct species of giraffids to identify their feeding type, we question the dietary habits of the present-day okapi, a species assumed to be a browser on the basis of a single field ecological survey [START_REF] Hart | Ranging and feeding behaviour of okapi (Okapia johnstoni) in the Ituri Forest of Zaire: food limitation in a rain-forest herbivore[END_REF]. Second, we discuss the dietary reconstruction of these extinct species in relation to the morphology, and notably the body mass estimates and the length of the metacarpal bone, the latter feature being a proxy to assess the maximum height at which a given species can access the food resources. We may expect that smaller species with shorter forelimbs are more prone to include herbaceous vegetation than large species of giraffids. This permits us to evaluate the ecological niche partitioning when species co-occurred in the fossil record, suggesting an overlap of their home-range, such as at Pikermi (Greece) and Hadjidimovo (Bulgaria). Besides, considering that none of these giraffids displayed post canine hypsodont cheekteeth, we may expect that none of these giraffids included herbaceous vegetation in its diet. However, an early pioneer study on dental abrasion (Solounias et al., 1988) challenged the hypothesis of browsing habits among all extinct giraffids. These authors depicted Samotherium boissieri, a late Miocene giraffid from Samos Island (Greece) as a mixed feeding species incorporating silica-bearing herbaceous monocotyledons in its diet. Previous dental wear analysis on material from historical collections of Samos and Pikermi also found out differences between giraffids species [START_REF] Danowitz | A combinedmesowear analysis of late Miocene giraffids from North Chinese and Greek localities of the Pikermian Biome[END_REF][START_REF] Solounias | Evidence from horn morphology on the phylogenetic relationship of the Pronghorn (Antilocapra americana)[END_REF][START_REF] Solounias | The paleodiet of the Giraffidae[END_REF]. We here go further in significantly increasing the sample with new material from northern Greece, Bulgaria and Turkey.

Material and Methods

Material

The dental microwear textures of extinct giraffids are compared with those of presentday species of ruminants with known differences in diet (housed in the following museum and Natural History of Sofia, Bulgaria). Instead of selecting a large dataset covering the whole spectrum of feeding habits, we chose here to focus on four taxa representing four different dietary poles (Table 1). These taxa are the hartebeest (Alcelaphus buselaphus, grazer; [START_REF] Estes | Behavior Guide to African mammals[END_REF][START_REF] Gagnon | Dietary preferences in extant African Bovidae[END_REF], the red deer (Cervus elaphus, generalist; [START_REF] Gebert | Variations of diet composition of red deer (Cervus elaphus L.) in Europe[END_REF], the giraffe (Giraffa sp., leaf-dominant browser; [START_REF] Estes | Behavior Guide to African mammals[END_REF][START_REF] Leuthold | Ecology of the giraffe in Tsavo East National Park, Kenya[END_REF][START_REF] Parker | The diet of a small group of extralimital giraffe[END_REF] and the yellow-backed duiker (Cephalophus silvicultor; fruit-dominant browser; [START_REF] Estes | Behavior Guide to African mammals[END_REF][START_REF] Gagnon | Dietary preferences in extant African Bovidae[END_REF][START_REF] Gauthier-Hion | A comparison of the diets of three major groups of primary consumers of Gabon (Primates, Squirrels and Ruminants)[END_REF][START_REF] Lumpkin | Cephalophus sylvicultor[END_REF]. These ruminants share a common occlusal molar pattern. A fifth modern species is included in this study: Okapia johnstoni, the okapi, assumed to be a browser [START_REF] Hart | Ranging and feeding behaviour of okapi (Okapia johnstoni) in the Ituri Forest of Zaire: food limitation in a rain-forest herbivore[END_REF].

The taxonomy of the modern giraffe is currently under debate among zoologists.

Several subspecies of Giraffa camelopardalis, differing in their horns and coat ornamentations had been identified [START_REF] Brown | Extensive population genetic structure in the giraffe[END_REF][START_REF] Groves | Ungulate taxonomy[END_REF][START_REF] Hassanin | Mitochondrial DNA variability in Giraffa camelopardalis: consequences for taxonomy, phylogeography and conservation of giraffes in West and central Africa[END_REF][START_REF] Kingdon | Mammals of Africa[END_REF][START_REF] Wilson | Mammal species of the world: a taxonomic and geographic reference[END_REF] but a recent study based on multi-locus DNA analysis recognizes the existence of four species [START_REF] Fennessy | Multi-locus Analyses Reveal Four Giraffe Species Instead of One[END_REF]: G. giraffa (southern giraffe), G. tippelskirchi (Masai giraffe), G. reticulata (reticulated giraffe) and G. camelopardalis (northern giraffe). The geographical distribution of the giraffe and its genetic diversity were even higher until the Holocene since subfossil remains and glyph representations in caves and rock shelters attest to its presence 4,000 years ago in northwestern Africa and along the Nile Valley [START_REF] Le Quellec | Répartition de la grande faune sauvage dans le nord de l'Afrique durant l'Holocène[END_REF]. Because of these taxonomic uncertainties, we made the choice to group all modern giraffes under Giraffa sp. Our purpose here is to have an ecologically homogeneous milestone to represent the leaf-dominant browsing ecospace. None of the specimens of modern species used in this study were captive before death, they were shot in the wild decades ago.

The fossil specimens belong to six genera: Bohlinia, Helladotherium, Palaeogiraffa, Palaeotragus and Samotherium. The material comes from a dozen upper Miocene (Vallesian and Turolian) localities in Greece, Bulgaria, Turkey, Afghanistan, Iran and Tunisia (Table 1, Fig. 1). Helladotherium duvernoyi comes from the turolian faunas of Southern Bulgaria [START_REF] Geraads | Giraffidae (Artiodactyla, Mammalia) from the Late Miocene of Kalimantsi and Hadjidimovo, Southwestern Bulgaria[END_REF][START_REF] Spassov | The Turolian Megafauna of West Bulgaria and the character of the Late Miocene "Pikermian biome[END_REF], continental Greece [START_REF] Kostopoulos | The giraffids from the Late Miocene mammalian localities of Nikiti (Macedonia, Greece)[END_REF][START_REF] Solounias | Mammalian fossils of Samos and Pikermi. Part 2. Resurrection of a classic turolian fauna[END_REF], and southern Tunisia (but the identification there is less secure). The specimens attributed to Bohlinia attica (Giraffinae) come from Nikiti-1 in Greece dated to close to the Vallesian-Turolian transition [START_REF] Kostopoulos | Palaeontology of the upper Miocene vertebrate localities of Nikiti (Chalkidiki Peninsula, Macedonia, Greece): Artiodactyla[END_REF] and from Kalimantsi (K), Bulgaria [START_REF] Geraads | Giraffidae (Artiodactyla, Mammalia) from the Late Miocene of Kalimantsi and Hadjidimovo, Southwestern Bulgaria[END_REF]. Among the Palaeotraginae, the specimens assigned to Samotherium major come from turolian sites: Mytilini-A, Mytilini-B (MTLA and MTLB; two sites from the Mytilini ravines in Samos Island, Greece; [START_REF] Kostopoulos | The Late Miocene mammal faunas of the Mytilinii Basin, Samos Island, Greece: new collection. 13. Giraffidae[END_REF], Vathylakkos-3 (VAT) in the Axios Valley [START_REF] Geraads | Les Paleotraginae (Giraffidae, Mammalia) du Miocène supérieur de la région de Thessalonique (Grèce)[END_REF], Salihpasalar (MYS) and Şerefköy-1 (MYSe-1) in the Muğla Yatağan Basin of Turkey [START_REF] Kaya | Şerefköy-2, a new late Miocene mammal locality from the Yatağan Formation, Muğla, SW Turkey[END_REF], and from the Turolian of Mahmutgazi (MA) in Turkey [START_REF] Sickenberg | Die Gliederung des höheren Jungtertiärs und Altquartärs-in der Türkei nach Vertebraten und ihre Bedeutung für die-internationale Neogen-Gliederung[END_REF]. The specimens of S. boissieri do not come from recent excavations but belong to historical collections from Samos for which site provenance is not guaranteed (see [START_REF] Koufos | The Late Miocene mammal faunas of the Mytilinii Basin, Samos Island, Greece: new collection. 1. History of the Samos Fossil Mammals[END_REF] for a historical review of the paleontological surveys and studies in Samos). A single specimen attributed to Samotherium neumayri comes from Maragha (MAR), Iran (Solounias and Danowitz, 2016, as Alcicephalus neumayri). The genus Palaeotragus is represented by two taxa. A single specimen of Palaeotragus rouenii from the vallesian site of Ravin de la Pluie (RPl; [START_REF] Geraads | Les Paleotraginae (Giraffidae, Mammalia) du Miocène supérieur de la région de Thessalonique (Grèce)[END_REF][START_REF] Koufos | The Neogene mammal localities of Greece: faunas, chronology and biostratigraphy[END_REF] has been analyzed but the species is widespread and abundant in the turolian sites of Hadjidimovo (HD; [START_REF] Geraads | Giraffidae (Artiodactyla, Mammalia) from the Late Miocene of Kalimantsi and Hadjidimovo, Southwestern Bulgaria[END_REF], Dytiko-3 (DIT), Pikermi (PIK; [START_REF] Koufos | The Neogene mammal localities of Greece: faunas, chronology and biostratigraphy[END_REF], Mytilini-B (MTLB; [START_REF] Kostopoulos | The Late Miocene mammal faunas of the Mytilinii Basin, Samos Island, Greece: new collection. 13. Giraffidae[END_REF], Şerefköy-1 and Şerefköy-2 (MYSe-1 and MYSe-2; [START_REF] Kaya | Şerefköy-2, a new late Miocene mammal locality from the Yatağan Formation, Muğla, SW Turkey[END_REF] and Molayan (MOL; Afghanistan; [START_REF] Brunet | Interprétation paléoécologique et relations biogéographiques de la faune de vertébrés du Miocène supérieur d'Injana, Irak[END_REF][START_REF] Sen | The age of the Molayan mammals locality, Afghanistan[END_REF]. Specimens that belong to Palaeotragus coelophrys or to a closely related species come from the vallesian sites of Ravin de la Pluie (RPl) and Pentalophos (PNT) in Greece [START_REF] Koufos | The Neogene mammal localities of Greece: faunas, chronology and biostratigraphy[END_REF] and from Maragha (MAR) in Iran [START_REF] Mecquenem | Contribution à l'étude des fossiles de Maragha[END_REF][START_REF] Solounias | The Giraffidae of Maragheh and the identification of a new species of Honanotherium[END_REF]. Palaeogiraffa pamiri comes from the vallesian sites of Küçükçekmece (KUC) in Turkish Thrace [START_REF] Kostopoulos | Suidae, Tragulidae, Giraffidae, and Bovidae[END_REF]. The only specimen of Palaeogiraffa major comes from the vallesian site of Ravin de la Pluie [START_REF] Koufos | The Neogene mammal localities of Greece: faunas, chronology and biostratigraphy[END_REF]. A third species of Palaeogiraffa, P. macedoniae is known from the vallesian site of Pentalophos (PNT), also in the Axios Valley [START_REF] Koufos | The Neogene mammal localities of Greece: faunas, chronology and biostratigraphy[END_REF]. [START_REF] Ríos | Comparative anatomy, phylogeny, and systematics of the Miocene giraffid Decennatherium pachecoi Crusafont, 1952 (Mammalia, Ruminantia, Pecora): State of the art[END_REF][START_REF] Ríos | A new giraffid (Mammalia, Ruminantia, Pecora) from the late Miocene of Spain, and the evolution of the sivathere-samothere lineage[END_REF] synonymized Palaeogiraffa and Decennatherium (a genus assumed to be the sister group of a clade composed notably by sivatherines and Samotherium major and S. boissieri), a view which is not shared by [START_REF] Bonis | Nouveaux Giraffidae du Miocène supérieur de Macédoine (Grèce)[END_REF]. Pending agreement on this taxonomic issue, we keep using Palaeogiraffa as genus name for specimens from the vallesian sites from Axios, in northern Greece and from Yulaflı in Turkish Thrace.

For some of the species, the number of specimens is large enough (N=>10) to generate robust interpretations regarding their dietary habits. However, for several taxa, the number is moderate (5=<N<10) and even low (5<N). Taking into account that dental microwear textures reflect the dietary bolus from the last few weeks, one might fear that the signal from such small samples is meaningless, but [START_REF] Purnell | Quantitative three-dimensional microtextural analyses of tooth wear as a tool for dietary discrimination in fishes[END_REF] have shown that significant differences in textural parameters can be detected even for small samples (N<5) of fishes (see also [START_REF] Purnell | Within-guild dietary discrimination from 3-D textural analysis of tooth microwear in insectivorous mammals[END_REF]. Besides, the analysis of several scans per individual, as proposed by [START_REF] Purnell | 3D tooth microwear texture analysis in fishes as a test of dietary hypotheses of durophagy[END_REF] and as done in the present study, mitigates the effects of the small sample size..

Methods

DMTA is a method quantifying tooth abrasion (for detailed reviews of intra-and interobserver errors, see [START_REF] Desantis | Direct Comparisons of 2D and 3D Dental Microwear Proxies in Extant Herbivorous and Carnivorous Mammals[END_REF][START_REF] Galbany | Error rates in buccal-dental microwear quantification using scanning electron microscopy[END_REF][START_REF] Grine | Error rates in dental microwear quantification using scanning electron microscopy[END_REF]Mihlbachler et al., 2012;[START_REF] Calandra | Dental microwear texture analysis in mammalian ecology: DMTA in ecology[END_REF]Mihlbachler and Beatty, 2012;[START_REF] Scott | Dental microwear texture analysis of extant African Bovidae[END_REF][START_REF] Scott | Dental microwear texture analysis: technical considerations[END_REF] . Tooth wear reflects individual senescence and physical properties of foods; thus, it is correlated with the availabilities of food resources and can be used to explore niche partitioning among sympatric species of mammals [START_REF] Calandra | Dental microwear texture analysis in mammalian ecology: DMTA in ecology[END_REF].

From the scale of a whole tooth to the micrometric scars on dental facets, differences in dietary preferences are mirrored by tooth wear. DMTA has proved to be a particularly useful methodology free of (inter-and intra-)observer measurement errors at least at the analytic step (for detailed reviews of intra-and inter-observer errors, see [START_REF] Desantis | Direct Comparisons of 2D and 3D Dental Microwear Proxies in Extant Herbivorous and Carnivorous Mammals[END_REF][START_REF] Galbany | Error rates in buccal-dental microwear quantification using scanning electron microscopy[END_REF][START_REF] Grine | Error rates in dental microwear quantification using scanning electron microscopy[END_REF]Mihlbachler et al., 2012;Mihlbachler and Beatty, 2012) in assessing diets of fossil as well as modern taxa [START_REF] Calandra | Dental microwear texture analysis in mammalian ecology: DMTA in ecology[END_REF].

The analysis was performed preferentially on second upper or lower molars (Figs. 2 and3). However, third or first molars were considered when the second molars are weathered, too much worn or too recently erupted. Following standard procedures, replicas of dental facet were produced with a silicone (medium consistency) polyvinylsiloxane (Coltène Whaledent, President Regular Body, ISO 4823). Scans (320 x 280 µm) were produced on replicas using a surface profilometer confocal DCM8 Leica Microsystems with a 100× lens (Leica Microsystems; NA = 0.90; working distance = 0.9 mm) at the Palevoprim, CNRS and University of Poitiers, France. Lateral resolution is 0.129 µm and vertical spacing is 0.002 µm. Up to four surfaces (140 × 100 µm; 1088 × 776 pixels) were generated from the original scans and saved as .Plµ files (Figs. 2 and3; details on surface preparation are given in [START_REF] Merceron | Untangling the environmental from the dietary: dust does not matter[END_REF].

The DMTA was performed using the Scale-Sensitive Fractal Analysis using Toothfrax and Sfrax software (Surfract, http://www.surfract.com) following [START_REF] Scott | Dental microwear texture analysis: technical considerations[END_REF]. The individual values of surface parameters for extinct and extant species are given in Appendix 1.

Three variables were extracted from the surface: complexity (Asfc), anisotropy (epLsar), and heterogeneity of complexity (HAsfc calculated with a 9-cell mesh; Table 1). [START_REF] Scott | Dental microwear texture analysis: technical considerations[END_REF] detailed the variables used here. Complexity (Asfc or Area-scale fractal complexity) is a measure of the roughness at a given scale. Anisotropy (epLsar or exact proportion of lengthscale anisotropy of relief) measures the orientation concentration of surface roughness.

Heterogeneity of complexity (HAsfc or heterogeneity of area-scale fractal complexity), quantifies the variation of complexity within scan. All of these three textural parameters are dimensionless (see [START_REF] Scott | Dental microwear texture analysis: technical considerations[END_REF]. Scanned surfaces display dental microwear but also, for some of the teeth, structural reliefs such as growth lines or perikemata reflecting the intersection between Retzius lines with the enamel surface. One may argue that such enamel structural relief influences the parameter values, and notably the anisotropy of the dental microwear textures. Although the perikemata are more or less preferentially orientated, they do not interfere with the anisotropy calculation, because the height amplitudes of these perikemata are lower than topographic variations due to the microwear on enamel surfaces (Fig. 4).

As the distribution of textural parameters violates conditions for parametric tests, variables were rank-transformed before analysis [START_REF] Conover | Rank transformations as a bridge between parametric and nonparametric statistics[END_REF]Sokal and Rohlf, 1969). One-way factorial ANOVAs with post-hoc tests for each parameter were used to determine the sources of significant variation (Tables 2 and3). Any potential difference was then highlighted using the combination of the conservative HSD test (Tukey's Honest Significant Differences) together with the less conservative LSD test (Fisher's Least Significant Differences).

Body mass (calculated according [START_REF] Scott | Postcranial dimensions of ungulates as predictors of body mass[END_REF]) and height at the withers are two body traits that can be used in combination with DMTA to explore the partitioning of food resources between sympatric species. Here we use metacarpal length as a proxy of height at the withers and the height at which a species can gather its resources. This certainly suffers exceptions, but the length of the metacarpals is a good proxy for the height at which the animal can browse (for a given metacarpal length, if the animal is heavier, height at the withers will be higher, but neck will be shorter). We also noted classic parameters of dental morphology, notably the premolar/molar ratio that is distinctly lower for hyper-grazing bovids (Table 4).

Results and Discussion

Dental microwear textures and diet

The four modern species used as milestones differ from each other in all three surface textural parameters (Tables 123). The hartebeest (Alcelaphus buselaphus), a grazing alcelaphine, has the lowest enamel surface complexity (Asfc) and the highest anisotropy (epLsar) of dental microwear textures (Tables 1 and3; Figs. 2 and4). The yellow-backed duiker (Cephalophus silvicultor) displays the highest complexity (Asfc) and low values of anisotropy (epLsar) whereas the giraffe (Giraffa sp.) has the lowest anisotropy (epLsar) and an intermediate complexity (Asfc) between the hartebeest and the yellow-backed duiker (Tables 1 and3; Figs. 2 and5). The differences in dental microwear textures between the two browsing species reflect the ingestion of fruits and seeds by the duiker [START_REF] Ramdarshan | Seeds, browse, and tooth wear: a sheep perspective[END_REF]. The dental microwear textures (intermediate complexity and anisotropy) of the red deer (Cervus elaphus) reflect its mixed feeding habits that involve both browsing and grazing (Tables 1 and3; Figs. 2 and5). The third variable, heterogeneity of complexity (HAsfc), provides a significant complement. The giraffe and the hartebeest foraging mostly on a homogeneous source of vegetation (tree leaves and herbaceous monocotyledons, respectively; Estes, 1991, Gagnon and[START_REF] Gagnon | Dietary preferences in extant African Bovidae[END_REF][START_REF] Leuthold | Ecology of the giraffe in Tsavo East National Park, Kenya[END_REF][START_REF] Parker | The diet of a small group of extralimital giraffe[END_REF] over long time span (over a year) have lower values than the red deer and the yellow-backed duiker whose diets are mechanically diverse and variable from a day to another or from a season to another (Tables 1 and3; Figs. 2 and 5; [START_REF] Estes | Behavior Guide to African mammals[END_REF][START_REF] Gebert | Variations of diet composition of red deer (Cervus elaphus L.) in Europe[END_REF][START_REF] Gagnon | Dietary preferences in extant African Bovidae[END_REF][START_REF] Gauthier-Hion | A comparison of the diets of three major groups of primary consumers of Gabon (Primates, Squirrels and Ruminants)[END_REF][START_REF] Lumpkin | Cephalophus sylvicultor[END_REF].

The modern okapi (Okapia johnstoni) has a wide distribution across the dental microwear textural ecospace. It differs from the giraffe in having higher anisotropy (epLsar) and from the yellow-backed duiker in having lower complexity (Asfc; Tables 1 and3; Figs. 3 and5). The okapi also differs from the hartebeest and the giraffe in having higher heterogeneity of complexity (HAsfc). It does not differ from the red deer when the two most discriminating variables (Asfc and epLsar) are considered (Tables 1 and3; Figs. 3 and5).

These results for the present-day okapi assumed to be a browser can be discussed in light of the ecological data. Hitherto, only one single study dealt with the feeding ecology of the okapi in the wild [START_REF] Hart | Ranging and feeding behaviour of okapi (Okapia johnstoni) in the Ituri Forest of Zaire: food limitation in a rain-forest herbivore[END_REF]. It was conducted at Epulu, Ituri forest, a lowland forest located at the far north eastern of the Congo basin. The authors tracked eight collared individuals for several months and recorded from direct observations browse signs on vegetation but only those ranging from 0.5 to 2.0 meters in height because "this is the forest layer most frequently browsed by okapi" (Hart and Hart, 1989: 35). There is indeed no record regarding lower vegetal layers. [START_REF] Hart | Ranging and feeding behaviour of okapi (Okapia johnstoni) in the Ituri Forest of Zaire: food limitation in a rain-forest herbivore[END_REF] identified shade-and light-tolerant plant species, all of them being dicotyledonous trees or bushes, constituting the staple food spectrum of the okapi. However, one of these two authors from the pioneer study mentions in Kingdon et al. (2013: in vol. VI, p. 110-115) that herbaceous monocotyledons are part of the okapi diet but does not provide details regarding their abundance. [START_REF] Hart | Ranging and feeding behaviour of okapi (Okapia johnstoni) in the Ituri Forest of Zaire: food limitation in a rain-forest herbivore[END_REF] had actually made mention that the okapi favors tree-fall gaps where it leaves the highest density of browse signs. These tree-fall gaps are also hotspots of biodiversity in the Central African forest where the herbaceous vegetation (mostly monocotyledons) reaches its highest diversity and density. These plant resources are known to be critical in dense forested habitats for other mammals such as buffaloes [START_REF] Blake | Forest buffalo prefer clearings to closed-canopy forest in the primary forest of northern Congo[END_REF], gorillas, and pygmy chimpanzees [START_REF] Blake | Swamp gorillas in northern Congo[END_REF][START_REF] Malenky | A quantitative comparison of terrestrial herbaceous food consumption by Pan paniscus in the Lamako forest, Zaire, and Pan troglodytes in the Kibale forest, Uganda[END_REF][START_REF] Williamson | Composition of the diet of lowland gorillas at Lopé in Gabon[END_REF]. For instance, these apes feed on herbaceous stems (sometimes only the soft inner pith) of monocotyledonous plants such as Marantacea and Zingiberacea [START_REF] Malenky | A quantitative comparison of terrestrial herbaceous food consumption by Pan paniscus in the Lamako forest, Zaire, and Pan troglodytes in the Kibale forest, Uganda[END_REF][START_REF] Williamson | Composition of the diet of lowland gorillas at Lopé in Gabon[END_REF][START_REF] Wrangham | The significance of fibrous foods for Kibale forest chimpanzees[END_REF]. Monocotyledonous plants have generally higher silica content than dicotyledonous plants [START_REF] Hodson | Phylogenetic variation in the silicon composition of plants[END_REF]. Besides, the herbaceous stems that are eaten are tougher than most fruits [START_REF] Elgart-Berry | Fracture toughness of mountain gorilla (Gorilla gorilla beringei) food plants[END_REF]. Therefore, on the basis of the dental microwear textures of the okapis analyzed in the present study and the information provided by [START_REF] Hart | Ranging and feeding behaviour of okapi (Okapia johnstoni) in the Ituri Forest of Zaire: food limitation in a rain-forest herbivore[END_REF] and Hart in [START_REF] Kingdon | Mammals of Africa[END_REF], we hypothesize that the okapi does include some terrestrial herbaceous vegetation in its diet, especially when its home range overlaps tree fall gaps. Our hypothesis needs to be tested in the future through alternative direct or non direct dietary proxies and new material. It is worth mentioning that [START_REF] Clauss | The allocation of a ruminant feeding type to the okapi (Okapia johnstoni) on the basis of morphological parameters[END_REF] had described the digestive tract of two captive okapis as similar to that of modern selective browsers with the exception that the parotid glands were found to be small, a feature shared with ruminants feeding on monocotyledons.

All fossil giraffids but Samotherium boissieri (and the only specimen of Palaeogiraffa major, see Table 1) significantly differ from the grazing hartebeest in having either higher complexity (Asfc) or lower anisotropy (epLsar) or the combination of the two conditions (Tables 2 and3; individual values are provided in table S1 in supplementary material). Like the modern giraffe, Helladotherium duvernoyi has lower anisotropy (epLsar) than the red deer and the hartebeest, and lower complexity (Asfc) than the yellow-backed duiker; it also has lower values in heterogeneity of complexity (HAsfc) than the red deer and the duiker, supporting a monotypic diet for this sivatheriine (Tables 1 and3; Figs. 3 and5; table S1).

There is little doubt that Helladotherium (N= 15) was a leaf-dominant browser. The five specimens of Bohlinia attica all have low values of both anisotropy (epLsar) and complexity (Asfc) suggesting that these individuals fed on soft browse (Tables 1 and3; Figs. 3 and5; table S1). The specimens of Palaeogiraffa pamiri from Turkish Thrace show low values in all three textural parameters (Tables 1 and3; Figs. 3 and5; table S1), showing that they were undoubtedly leaf-dominated browsers. By contrast, Palaeogiraffa macedoniae and the specimen of P. major from the Axios Valley sites in northern Greece display a sharply different pattern (Tables 1 and3; Fig. 3; table S1). They have higher anisotropy (epLsar) than the sample from Thrace, the modern giraffe and the yellow-backed duiker. When complexity (Asfc) is also considered, such dental microwear textures suggest mixed feeding habits for Palaeogiraffa from the lower Axios Valley (Tables 1 and3; Fig. 5; table S1). One may argue that differences in windblown dust deposit on vegetation could be the key factor controlling differences in dental microwear textures. However the only study that actually tested these hypotheses on living captive domesticated animals concludes that differences in (dust-free) diet generates significant differences in dental microwear textures [START_REF] Merceron | Untangling the environmental from the dietary: dust does not matter[END_REF].

Moreover, the effects on foods of dust simulating the Western Africa Harmattan windblown dust are not significant enough to hide the dietary signal [START_REF] Merceron | Untangling the environmental from the dietary: dust does not matter[END_REF]. Samotherium also displays a wide range of values (Tables 1 and3; Figs. 3 and5; table S1). Samotherium major differs from the modern giraffe in having higher anisotropy (epLsar), from the hartebeest in having a higher complexity (Asfc) and from the red deer, the yellow-backed duiker, and the okapi in its lower heterogeneity of complexity (HAsfc; Tables 1 and3; Figs. 3 and5; table S1). Samotherium boissieri also has higher anisotropy (epLsar) than the giraffe and the yellow-backed duiker. It is worth noting that there is no significant difference between S. boissieri and the grazing and mixed feeding species. The sample of Palaeotragus rouenii displays higher anisotropy (epLsar) than the modern giraffe and lower complexity (Asfc) than the yellow-backed duiker. This species covers the whole spectrum. cf. P. coelophrys shares the same pattern. On the whole, mixed feeding habits seem to be prevalent for all palaeotragines.

Morphology, diet, and niche partitioning

Modern giraffids constitute a relic of a diverse group that became impoverished during Pliocene and Pleistocene times, but during the late Miocene, their species and ecomorphological diversities were far greater than today. Body mass and heights at which they could gather food resources differed from one species to another (Table 4, Fig. 6).

Helladotherium duvernoyi was likely the heaviest giraffe in Europe, weighting perhaps as much as two tons, whereas Palaeotragus rouenii was not larger than the modern okapi, at about 500 kg. The length of their metacarpals, approximating height at the withers, and the height at which each species could reach its food, covered the whole range of the modern forms, from the okapi to the giraffe. The relative proportions of the premolar / molar rows are not very variable in the Giraffidae, which suggests that there were probably no huge differences in diets. On the basis of the index (Table 4, Fig. 6), and given low sample size, most fossil giraffes do not significantly differ from their modern relatives, except the paleotragines that have smaller premolars. This is in agreement with the dental microwear textural analysis that suggests more versatile feeding habits for paleotragines than other extinct giraffids.

Bohlinia attica was a giraffine weighting about a ton; it was likely able to reach foliage as high as the modern giraffe, from 4 to 6 m above ground (Table 4, Fig. 6;[START_REF] Leuthold | Ecology of the giraffe in Tsavo East National Park, Kenya[END_REF][START_REF] O'connor | Foraging ecologies of giraffe (Giraffa camelopardalis reticulata) and camels (Camelus dromedarius) in northern Kenya: effects of habitat structure and possibilities for competition?[END_REF]. The dental microwear textures attest that B. attica fed mostly on soft foliages, as modern giraffes do (Figs. 5 and6). The body traits found in Bohlinia and Giraffa and their similarities in tooth wear both suggest that the ecological niche of leafdominated browsers targeting the highest tree foliages was shared by the common ancestor.

Among large giraffids, the sivatheriine Helladotherium duvernoyi weighted as much as two tons; the low values for the three textural parameters depict it as a likely leaf-dominant browser (Fig. 5). Thus, H. duvernoyi shared similar feeding habits with B. attica, in spite of its distinctly higher molar crowns (Table 4), but could not reach foliages as high as this species, which was less stoutly built, but taller (Table 4, Fig. 6). Palaeogiraffa is represented in this study by three species. They were less tall than B. attica, and thus had reduced competition with it to exploit tree foliages (Fig. 6). Palaeogiraffa pamiri from Thrace was likely a leaf-dominant browser while the contemporaneous P. macedoniae and P. major from the lower Axios valley in Greece may have incorporated herbaceous monocotyledons in their diet. Such contrasted differences between species of the same genus (Figs. 5 and6) might reflect differences in food resources between the two regions (woody landscapes in the Thracian site and floodplain grasslands along the Axios).

In the late Miocene of the Mediterranean region, palaeotragines were more diverse than sivatheriines and giraffines, but the height range at which they could browse is far smaller than their species diversity and body mass range suggest. The palaeotragine Samotherium major has a body mass similar to that of the sivatheriine H. duvernoyi (Table 4, Fig. 6). These two species could reach the same foliage heights. Based on tooth morphology, we would have expected more grazing habits for the large palaeotragine compared to the sivatheriine, because small premolars compared to molars are usually taken as indicating more grazing habits in ruminants [START_REF] Solounias | Dietary adaptations and paleoecology of the late Miocene ruminants from Pikermi and Samos in Greece[END_REF], but no significant difference in the present study seems to distinguish the species with the larger premolars (H. duvernoyi) from the similar-sized species with the smaller premolars (S. major; Table 4, Figs. 5 and6). However, using dental microwear analysis on a larger sample, Solounias et al.

(2010) regarded S. major as a mixed feeder. Based on a dental mesowear scoring approach, [START_REF] Danowitz | A combinedmesowear analysis of late Miocene giraffids from North Chinese and Greek localities of the Pikermian Biome[END_REF] depicted it as a browser or a mixed feeder. To sum up, although the present study fails to discriminate S. major from H. duvernoyi, previous studies supported the view that S. major included herbaceous monocotyledons in its diet, as suggested by its dental morphology.

Samotherium boissieri differs from S. major in its lighter body mass and in being less tall at the withers. Besides, the slight differences in the morphology of the premaxilla (but not in the teeth) between these co-generic species (Fig. 7) suggest that S. boissieri might have ingested more herbaceous monocotyledons than S. major, although according to our own observation, the specimen NHMUK M 4215 has a distinctly less squarish premaxilla than typical grazers. Although our study fails to detect any significant differences between these two species of Samotherium, it is worth mentioning that, in contrast to S. major, S. boissieri has significantly higher values of anisotropy than modern browsing species (yellow-backed duiker and giraffe) and the three most presumably leaf browsing extinct giraffids (B. attica,H. duvernoyi,P. pamiri;Table 4,Figs. 5 and 6). Solounias et al. (1988) had also shown that S. boissieri includes herbaceous monocotyledons in its diet.

The genus Palaeotragus is represented by two species. Palaeotragus rouenii has a body mass similar to that of the modern okapi but could reach vegetal layers as high as those that H. duvernoyi and Samotherium spp. browsed (Table 4, Fig. 6). A more versatile diet than that of H. duvernoyi could explain their coexistence at some sites, and thus the probable overlap of their home ranges. Palaeotragus coelophrys was stouter than P. rouenii, but based on their metacarpal lengths, they could reach the same foliages. A fifth species of palaeotragine, Samotherium neumayri, is represented by a single individual whose body mass was similar to that of the modern giraffe but with a height at the withers similar to that of other palaeotragines; its dental microwear texture suggests that this specimen fed on soft foliages the few weeks before its death.

Dietary adaptation and Phylogeny

A recent study based on new material challenges the phylogeny of the giraffids [START_REF] Ríos | A new giraffid (Mammalia, Ruminantia, Pecora) from the late Miocene of Spain, and the evolution of the sivathere-samothere lineage[END_REF]. Giraffines are represented by the modern giraffes and the species of the genus Bohlinia. The elongation of the cervical vertebrae and the extreme elongation of the metacarpal allowing them to forage on the highest arboreal stratum may be seen as an autapomorphy of the Giraffinae [START_REF] Ríos | A new giraffid (Mammalia, Ruminantia, Pecora) from the late Miocene of Spain, and the evolution of the sivathere-samothere lineage[END_REF]. Our data strongly support such dietary adaptations. Besides, the monophyly of the paleotragines is challenged. According to their study, the genus Samotherium, previously thought to be close to Palaeotragus, actually shares a set of derived features with the Sivatherinae. The species of the genus Palaeotragus compose the sister group of a monophyletic group including the modern okapi, Decennatherium, Palaeogiraffa (assumed to be close to the former genus according to these authors) and Samotherium in which the sivatherines (including Helladotherium) are rooted.

More versatile feeding habits might have been the ancestral conditions for this second clade.

In view of this phylogeny proposed by [START_REF] Ríos | A new giraffid (Mammalia, Ruminantia, Pecora) from the late Miocene of Spain, and the evolution of the sivathere-samothere lineage[END_REF], we may consider that either browsing or mixed feeding habits were the ancestral ecological conditions for giraffids. The adaptation to leaf browsing occurred twice, first among Giraffines and then at least in Helladotherium within the Sivatherines. It is worth mentioning that the African sivatheres show a shortening of the forelimbs in the course of the Pliocene, a morphological trend correlated with the incorporation of a greater amount of C 4 plants, i.e. herbaceous monocots, in their diet.

Conclusions

The present study explores the ecological diversity of the diverse radiation of giraffids that took place during the late Miocene of the Eastern Mediterranean. In combination with body traits such as wither heights, and thus estimation of the height at which these extinct ruminants may have foraged, DMTA detects differences in feeding preferences. As expected, several of these extinct giraffids were leaf-dominant browsers but foraging at different heights. The modern giraffe can be consider as an appropriate model for understanding the ecology of Bohlinia. Helladotherium duvernoyi browsed at lower heights. Our study supports previous views that regarded the species of Samotherium as engaged in both browsing and grazing. Palaeogiraffa from the Axios valley in Greece undoubtedly included high amounts of tough plants, most likely tall herbaceous monocots, in its diet. The present study enlarges our knowledge of the ecology of this diversified group and emphasizes the importance of considering megaherbivores to faithfully depict past ecosystems and available resources.

Indeed, together with proboscideans, rhinocerotids and chalicotheriids, giraffids compose a guild of megaherbivores with no analog in the modern ecosystems.

In addition, the present study provides surprising results regarding the okapi. Its dental microwear textures significantly differ from those of the leaf-eating giraffes as well as from those of the fruit-eating yellow-backed duikers. They show similarities with those of the red deer, a species eating both monocots and dicots. Here, we hypothesize that the okapi is not a browser sensu stricto. This giraffid feeds on a wider dietary spectrum than previously thought and likely forages on tough plants, possibly tall herbaceous monocotyledons when exploiting resources in tree fall gaps. Variables were rank-transformed before analysis. When both LSD and HSD detect significant differences, the variable is underlined and in bold. Metacarpal length (in mm), body mass (in kg) based upon humerus distal articular width (in mm) following the equation of [START_REF] Scott | Postcranial dimensions of ungulates as predictors of body mass[END_REF] for Ruminants (body mass = 10 (2.5518*log(Humerus W)+0.4093) ), indexes of upper and lower premolar/molar rows of modern and extinct giraffids. Most data are our own, with a few additions from Bohlin (1926 (1) ), Kostopoulos (2009 (2) , 2016 (3) ) and Senyürek (1954 (4) ). averages of body mass (in kg) and length of metacarpal bone (in mm), which approximates the height at the withers. Note that specimens used for body traits and dental microwear analysis are not the same (see Table 4).

  institutions: AMNHS Aegean Museum of Natural Histry of Samos, Greece; NHMEU Natural History Museum of the Ege University of Izmir, Turkey; NHML Natural History Museum of London, UK; NHMB Naturhistorisches Museum Basel, Switzerland; SMNS Staatliches Museum für Naturkunde Stuttgart, Germany; SMNK Staatliches Museum für Naturkunde Karlsruhe, Germany; SNG Senckenberg Museum, Franckfort, Germany; UP Palevoprim lab, University of Poitiers, France; KNM National Museums of Kenya, Nairobi, Kenya; MNHN Muséum National d'Histoire Naturelle, Paris, France; MRAC Central African Royal Museum of Tervuren, Belgium; FSL Collection de Géologie de la Faculté des Sciences de l'Université de Lyon, France; AM-NHNMS Assenovgrad Museum; a division of the National Museum of

Figure captions Figure 1 .

 captions1 Figure captions

Figure 2 .

 2 Figure 2. Occlusal view of left upper molar of a modern giraffe, Giraffa sp. (A; UP-M10-5-001-A) and 3D surface simulation on shearing dental facet (B; NHMB-Z553). The photosimulations C-H were generated through the LeicaMap 7.0: Giraffa sp. (C: NHMB-Z553), Okapia johnstoni (D: NHMB-Z245; E: NHMB-Z1085), Cephalophus silvicultor (F: NHMB-1611), Cervus elaphus (G: UP-Chat-9-4168) and Alcelaphus buselaphus (H: UP-ART8-21).

Figure 3 .

 3 Figure 3. The photosimulations A-D were generated through the LeicaMap 7.4: Helladotherium duvernoyi from Kalimansti, Bulgaria (A; K5159), Palaeogiraffa pamiri from Küçükçekmeçe, Turkey (B; MNHN-TRQ-430), Palaeotragus rouenii from Pikermi, Greece (C; MNHN-PIK-1672), and Samotherium major from Salihpasalar, Turkey (D; MTA-MYS-843).

Figure 4 .

 4 Figure 4. Photosimulations of dental microwear textures of modern ruminants with values in complexity (Asfc) and anisotropy (epLsar) and the corresponding rosette plot of relative lengths taken at 36 different orientations. The two first surfaces (Cervus elaphus and Alcelaphus buselaphus) display low relative length correlated with the main microwear textural orientation (here the main directionality is highlighted with the black arrows). The last three surfaces (Cephalophus silvicultor, Giraffa camelopardalis and Okapia johnstoni)display a main orientation of dental microwear textures disconnected from the perikemata direction, meaning that anisotropy does not reflect the perikemata directionality, but the dental microwear texture one.

Figure 5 .

 5 Figure 5. Bar plots (mean and standard error of the mean) of dental microwear textural parameters Asfc: complexity; epLsar: anisotropy calculated at the 1.8 µm scale; HAsfc: Heterogeneity of complexity (calculated with a 9-cell mesh) for modern ruminants (in gray) and extinct giraffids (in black).

Figure 6 .

 6 Figure 6. 3D plot of the average of anisotropy (epLsar) of the dental microwear textures vs.

Figure 7 .

 7 Figure 7. Outlines of the premaxilla in Samotherium. Square outline denotes grazers. S & D-S: Solounias and Dawson-Saunders (1988); outline of S. major based upon the specimen from Vathylakkos 3[START_REF] Geraads | Les Paleotraginae (Giraffidae, Mammalia) du Miocène supérieur de la région de Thessalonique (Grèce)[END_REF]; outlines of S. boissieri based upon NHMUK M4215.

Table 1 .

 1 Descriptive statistics of dental microwear textural parameters of modern ruminants 632 and extinct species of giraffids.

	633
					Asfc*	epLsar (×10 -3 )	HAsfc
			Taxa	N	m	sd	m	sd	m	sd
			Cephalophus silvicultor	25	4.03	3.25	3.07	1.58	0.54	0.30
			Giraffa sp.	12	2.51	1.48	2.10	1.57	0.35	0.12
		Modern species	Cervus elaphus	29	2.12	0.80	4.59	1.91	0.55	0.25
			Okapia johnstoni	25	2.46	1.46	4.07	2.07	0.52	0.27
			Alcelaphus buselaphus	28	1.60	0.77	5.80	1.64	0.39	0.12
			Bohlinia attica	5	1.47	0.73	2.47	0.94	0.49	0.11
			Helladotherium duvernoyi	15	2.46	1.23	2.99	1.37	0.43	0.52
			Palaeogiraffa macedoniae	6	2.61	0.65	5.13	2.63	0.36	0.14
			Palaeogiraffa major	1	2.87	-	6.41	-	0.38	-
			Palaeogiraffa paimiri	6	1.68	0.93	1.92	0.97	0.51	0.29
		Extinct species
			Samotherium neumayri	1	0.66	-	1.99	-	0.15	-
			Palaeotragus rouenii	21	2.30	1.06	3.80	1.63	0.35	0.11
			cf. P. coelophrys	3	1.84	0.93	3.60	2.43	0.34	0.07
			Samotherium boissieri	5	2.43	1.33	4.97	2.10	0.37	0.14
			Samotherium major	9	2.89	1.63	3.70	1.63	0.37	0.25
	634
	635
	636	complexity; epLsar: anisotropy (calculated at the 1.8 µm scale); HAsfc: Heterogeneity of complexity (calculated
		with a 9-cell mesh).

* all of these three parameters are dimensionless (see

[START_REF] Scott | Dental microwear texture analysis: technical considerations[END_REF]

; m: mean; sd: standard deviation; Asfc:

Table 2 .

 2 Analysis of variance on rank-transformed variables.

		df	SS	MS	F	p
	Asfc	14	115848.3 8274.9 3.1334 0.0002
		176	464791.7 2640.9
		df	SS	MS	F	P
	epLsar	14	176760.3 12625.7 5.5020 0.0000
		176	403878.7 2294.8
		df	SS	MS	F	P
	HAsfc	14	109086.9 7791.9 2.9082 0.0005
		176	471552.1 2679.3

Asfc: complexity; epLsar: anisotropy (calculated at the 1.8 µm scale); HAsfc: Heterogeneity of complexity (calculated with a 9-cell mesh). ; SS: sum of squares; MS: mean of squares; F: F statistic; p: p-value

Table 3 .

 3 Post-hoc test of comparisons (the Fisher Least Significant Differences and the conservative Tukey Honest Significant Differences test).

	C. silvicultor	C. elaphus	Giraffa sp.	O. johnstoni	A. buselaphus	B. attica	H. duvernoyi	P. paimiri	P. macedoniae	P. major	S. neumayri	P. rouenii	cf. P. coelophrys	S. boissieri

Table 4 .

 4 Body anatomical traits of modern and extinct giraffids.

		Metacarpal		Body mass estimate	Pm/M index	Pm/M index
			length				(upper teeth)	(lower teeth)
	Species N	L	N Humerus	body
					W	mass
	Giraffa sp.	33	701	26	116	1362	75 (mean)	68 (mean)
	Okapia johnstoni	2	307	2	75	441	74, 75	60, 65, 70
	Bohlinia attica	6	704	7	107	1118	73, 76, 78	-
	Helladotherium duvernoyi	20	435	15	141	2201	77, 79, 81	63, 65, 67 (3)
	Palaeogiraffa macedoniae	1	465	2	115	1324	72, 73, 75	65
	Palaeogiraffa pamiri	4	469	1	115	1324	76 ?	68
	Palaeotragus coelophrys	4	370	-	-	-	70, 71, 73, 74	68
	Palaeotragus rouenii	5	409	6 (2)	75	439	70, 74	63, 67, 68
	Samotherium boissieri	14 (2)	357	4 (2)	104	1022	67.5 (mean of 7) (1) 57, 59, 60, 60
	Samotherium major	32 (2)	417	7 (2)	141	2185	65, 67, 68 (4)	63, 66
	Samotherium neumayri	11	382	4	116	1341	65	58, 67

Acknowledgments

The authors thank C. Sagne and P. Tassy (Muséum National d'Histoire Naturelle, Paris, France), E. Robert (Geological Collections, UMR CNRS 5276 Laboratoire de Géologie de Lyon -CERESE, University Lyon 1), N. Spassov and the late D. Kovatchev (National Museum of Natural History, Sofia, Bulgaria), D.S. Kostopoulos and G.D. Koufos (Aristotle University of Thessaloniki, Greece), S. Mayda and T. Kaya (Natural History Museum, Ege University, Izmir, Turkey), W. Wendelen (RMCA, Tervuren, Belgium), L. Costeur (Naturhistorisches Museum at Basel, Swizterland), and W. Munk (Staatliches Museum für Naturkunde, Karlsruhe, Germany), for giving access to collections. We are grateful to A. Souron (University of Bordeaux-1) who contributed to the sampling of okapis. A significant part of the fossil material was sampled in various European institutions thanks to the International Research Collaborative Grant "Environmental Dynamics of Western Eurasian Hominids during the Late Miocene" (PIs: R. S. Scott and T. Kaya) funded by the Wenner-Gren Foundation. G.M. thanks his colleagues of the ''Wenner-Gren International Research Collaborative Grant'' group: R.S. Scott (Rutgers University, USA), D.S. Kostopoulos (Aristotle University of Thessaloniki, Greece), T. Kaya and S. Mayda (Ege

University, Izmir, Turkey) as well as G. Reynaud and S. Riffaut (Palevoprim). We also thanks the associate editor I. Montanez and four reviewers including N. Spassov for their helpful comments that greatly improve the manuscript. This study was funded by the Project TRIDENT (ANR-13-JSV7-0008-01, PI: G. Merceron, France; http://anr-trident.prd.fr/).

Data availability

The dental microwear texture parameters for each fossil specimen are given in Appendix 1.


Diego Perez-Galacho

Laurent Bramerie

Charles Baudot

Mohamed Chaibi

Sonia Messaoudene

Nathalie Vulliet

Laurent Vivien

Frederic Boeuf

Christophe Peucheret

Delphine Marris-Morini

QPSK Modulation in the O-Band Using a Single Dual-Drive Mach-Zehnder Silicon Modulator

Keywords: Advanced modulation formats, coherent optical communications, integrated optics, optical transmitters, silicon modulators, silicon photonics

come   L'archive ouverte pluridisciplinaire

I. INTRODUCTION

ILICON photonics devices have shown impressive evolution from the first demonstration of active functions to high-performance transmitters and receivers [START_REF] Boeuf | Silicon Photonics R&D and manufacturing on 300-nm wafer platform[END_REF][START_REF] Dong | Silicon photonic devices and integrated circuits[END_REF]. Interestingly, the use of Si photonics for coherent communications in long haul network could pave the way for future large-scale industrialization of high performance and low cost optoelectronic transceivers [START_REF] Dong | Silicon photonic devices and integrated circuits[END_REF]. Indeed, while today standards are based on 100-Gb/s transmission, future standards will require 200 or 400-Gb/s transponders, to meet Manuscript received December XX, 2017; revised January XX, 2018. Published XX, 2018. This work was supported by the European project Plat4m (FP7-2012-318178), the European project Cosmicc (H2020-ICT-27-2015-688516) and the French Industry Ministry Nano2017 program.

L. Vivien and D. Marris-Morini are with the Center for Nanoscience and Nanotechnology, CNRS, Univ. Paris-Sud, Université Paris-Saclay, C2N-Orsay, 91405 Orsay cedex, France (e-mail: laurent.vivien@u-psud.fr; delphine.morini@u-psud.fr).

D. Pérez-Galacho was with the Center for Nanoscience and Nanotechnology, CNRS, Univ. Paris-Sud, Université Paris-Saclay, C2N-Orsay, 91405 Orsay cedex, France. He is now with the ITEAM research institute, Universitat Politècnica de València, Camino de Vera s/n, 46022, Valencia, Spain (e-mail: diepega@upv.es).

L. Bramerie, and C. Peucheret are with Univ Rennes, CNRS, FOTON -UMR 6082, F-22305 Lannion, France (e-mail: laurent.bramerie@enssat.fr; christophe.peucheret@univ-rennes1.fr).

M. Chaibi was with Univ Rennes, CNRS, FOTON -UMR 6082, F-22305 Lannion, France. He is now with Center for Nanoscience and Nanotechnology, CNRS, Univ. Paris-Sud, Université Paris-Saclay, C2N-Orsay, 91405 Orsay cedex, France (e-mail: mohamed.chaibi@c2n.upsaclay.fr).

C. Baudot, S. Messaoudène, N. Vulliet and F. Boeuf are with STMicroelectronics, 850 rue Jean Monnet, 38920 Crolles, France (e-mail: charels.baudot@st.com; sonia.messaoudene-ext@st.com; nathalie.vulliet@st.com; frederic.boeuf@st.com).

with the ever-growing demand triggered by multi-media and cloud-based applications [START_REF] Witzens | Modulators make efficiency leap[END_REF]. State-of-the-art silicon on-off keying (OOK) modulators are currently reaching 50-Gb/s operation, [START_REF] Dong | High-speed low-voltage single-drive push-pull silicon Mach-Zehnder modulators[END_REF][START_REF] Thomson | 50-Gb/s Silicon Optical Modulator[END_REF], while higher modulation format such as n-level pulse amplitude modulation (PAM-n), quadrature phase-shift keying (QPSK), or 16-state quadrature amplitude modulation (16-QAM) allow to increase transmission rate up to 224 Gb/s [START_REF] Dong | 112-Gb/s monolithic PDM-QPSK modulator in silicon[END_REF][START_REF] Dong | 224-Gb/s monolithic PDM-16-QAM modulator and receiver based on silicon photonic integrated circuits[END_REF][START_REF] Ding | Silicon 16-QAM optical modulator driven by four binary electrical signals[END_REF][START_REF] Chagnon | Experimental study of 112-Gb/s short reach transmission employing PAM formats and SiP intensity modulator at 1.3µm[END_REF][START_REF] Samani | Experimental parametric study of 128 Gb/s PAM-4 transmission system using a multielectrode silicon photonic Mach Zehnder modulator[END_REF][START_REF] Milivojevic | Small-Size Silicon Photonic IQ Modulator and Low-Power CMOS Driver for Next Generation Coherent Transceiver[END_REF][START_REF] Shastri | Ultra-Low-Power Single-Polarization QAM-16 Generation Without DAC Using a CMOS Photonics Based Segmented Modulator[END_REF]. As a major step forward, the phase efficiency of the modulators has recently been significantly increased [START_REF] Hiraki | Heterogeneously integrated III-V/Si MOS capacitor Mach-Zehnder modulator[END_REF][START_REF] Han | Efficient low-loss InGaAsP/Si hybrid MOS optical modulator[END_REF]. While most of the reported work has been done in the C-band of telecommunication (around 1550 nm), silicon photonics systems are highly attractive for short-distance intra-data-center communications [START_REF] Pinguet | Monolithically integrated high-speed CMOS photonic transceivers[END_REF], where the O-band (around 1300 nm) is widely used. Despite the lower free-carrier dispersion effect at this wavelength in comparison with 1550 nm, high-performance devices have been demonstrated, showing 50-Gbit/s OOK modulation [START_REF] Streshinsky | Low power 50 Gb/s silicon traveling wave Mach-Zehnder modulator near 1300nm[END_REF] or low-voltage operation [START_REF] Perez-Galacho | Low voltage 25Gbps silicon Mach-Zehnder modulator in the O-band[END_REF]. As the volume of data exchanged inside data centers is continuously increasing, even higher data rates are still required. Developing higher-order modulation formats in the O-band of communications has thus recently received significant attention. 56-Gb/s PAM-4 [START_REF] Xiong | Monolithic 56Gb/s silicon photonic pulse-amplitude modulation transmitter[END_REF], 112-Gb/s PAM-8 [START_REF] Chagnon | Experimental study of 112-Gb/s short reach transmission employing PAM formats and SiP intensity modulator at 1.3µm[END_REF] and 200-Gb/s QAM-16 [START_REF] Doerr | Silicon Photonics Coherent Modulator/Receiver[END_REF] were thus demonstrated. While standard IQ modulators used for QPSK signal generation are based on nested Mach-Zehnder (MZ) modulator structures [START_REF] Dong | 112-Gb/s monolithic PDM-QPSK modulator in silicon[END_REF], a single dual-drive Mach-Zehnder modulator (DD-MZM) has been proposed as a simpler structure, allowing a reduction of the power consumption of the transceiver to generate the QPSK constellation [START_REF] Ho | Generation of Arbitrary Quadrature Signals Using One Dual-Drive Modulator[END_REF]. This scheme has already been demonstrated using a LiNbO3 modulator [START_REF] Krause | Demonstration of 20-Gb/s DQPSK With a Single Dual-Drive Mach-Zehnder Modulator[END_REF]. Interestingly, this structure offers a better power efficiency and scalability in term of modulation format order that are key advantages for short-distance intra-data-center communications. In this context, we report the first demonstration of QPSK signal generation using a single dual-drive Mach-Zehnder silicon modulator operating in the O-band. As a first demonstration, 20-Gb/s operation is shown, with a large potential for increasing the data rate. Preliminary results of this work have been recently reported in [START_REF] Bramerie | 20 Gbps QPSK Signal Generation using a Silicon Dual-Drive Mach-Zehnder Modulator Operating in the O-Band[END_REF]. They are here complemented by a more thorough experimental characterization of QPSK modulation at 10 Gbaud.

QPSK modulation in the O-band using a single dual-drive Mach-Zehnder silicon modulator Diego Pérez-Galacho, Laurent Bramerie, Charles Baudot, Mohamed Chaibi, Sonia Messaoudène, Nathalie Vulliet, Laurent Vivien, Frederic Boeuf, Christophe Peucheret, and Delphine Marris-Morini

S II. MODULATOR DESIGN AND PROPERTIES

The phase-shifter used for optical modulation in silicon is based on carrier depletion, using a PN diode embedded in the core of a silicon-on-insulator (SOI) waveguide. The cross section of the phase shifter and its SEM image are reported in Fig 1 (a) and (b), respectively. The rib waveguide is 300-nm high and 400-nm wide. A 50-nm slab is used and provides electrical connection to the highly doped part of the diode. Free-carrier concentration variation is obtained in the core of the waveguide. The PN junction interface is slightly shifted (25 nm) from the center of the waveguide to take benefit from the larger refractive index variation obtained by varying the concentration of holes in comparison with electrons [START_REF] Soref | Electrooptical effects in silicon[END_REF]. The targeted doping concentrations are P = 5×10 17 cm -3 and N = 1.4×10 18 cm -3 . Previous work based on the same phaseshifter design allowed demonstrating high modulation efficiency with VπLπ products of 0.95 to 1.15 Vcm for reverse bias from 1 to 4 V at 1310 nm wavelength [START_REF] Perez-Galacho | Low voltage 25Gbps silicon Mach-Zehnder modulator in the O-band[END_REF].

The DD-MZM is schematically reported in Fig 1 (c). 2-mm long phase-shifters are embedded in both arms of a symmetric Mach-Zehnder structure. MMI couplers are used as input/output splitter and combiner. Metallic heaters form low-speed/high-efficiency static phase-shifters that are used to bias the interferometer by tuning the DC phase-shift Φ between both arms, as reported in Fig 1 (d). The modulator was fabricated using the 300-mm DAPHNE technological platform from STMicroelectronics [START_REF] Baudot | Daphne silicon photonics technological platform for research and development on wdm applications[END_REF]. The on-chip losses of the modulator are estimated below 3.8 dB, corresponding to 2.6 dB for the 2-mm-long active region, while losses below 0.6 dB are estimated for each MMI couplers. Radio-frequency (RF) signals are applied to each phase-shifter independently, using GSGSG coplanar waveguides.

The device bandwidth has been characterized by applying an RF signal to one arm of the modulator, the obtained electro-optical response is shown in Fig. 2. It can be seen that the 3dB electro-optical bandwidth is 5 GHz, while the 6-dB bandwidth is 15 GHz. In comparison the RC cut-off frequency was estimated to be 21 GHz [START_REF] Perez-Galacho | Low voltage 25Gbps silicon Mach-Zehnder modulator in the O-band[END_REF]. This result indicates that the electrical/optical signal velocity matching and impedance matching could be further improved. Due to this limitation we decided to focus the QPSK operation at 10 Gbaud. However, this device has the potential to operate at higher speeds of 25 Gbaud, either by improving the RF electrode design, or by slightly reducing the phase-shifter length, which would increase the bandwidth, at the expense of a reduction of the phase shift per applied voltage.

III. EXPERIMENTAL SET-UP AND QPSK GENERATION

To generate a QPSK signal with a single DD-MZM [START_REF] Ho | Generation of Arbitrary Quadrature Signals Using One Dual-Drive Modulator[END_REF], two independent binary modulating signals are applied to each of the phase-shifters in order to achieve a peak-to-peak phase modulation of π in each arm. At the same time, the phase-shift introduced by the heater is adjusted to operate the DD-MZM in the quadrature point (Φ=π/2). Compared to conventional QPSK transmitters based on IQ modulator architecture, the DD-MZM only required one constant phase-shift and a π phase modulation, hence an applied voltage of V, for each phase-shifter. On the other hand, the IQ modulator requires three constant phase-shifts, two for biasing the two MZMs to the null transmission point and another to set the phase between the two MZMs to quadrature. Furthermore, the IQ modulator requires peak-to-peak voltage differences of 2V to be applied to each nested MZM in order to implement the required π phase modulation. This in turns requires peak-to-peak voltages of V to be applied to the four phaseshifters for proper push-pull operation. Differential logic (data and inverted data) electrical data signals with proper timing alignment are also required. Finally, it can be noted that in the case of the DD-MZM, an applied peak-to-peak voltage below V will affect the shape of the constellation diagram while in the case of the IQ modulator, a reduced peak-to-peak voltage will be responsible for an increase of the optical loss. Theoretical QPSK eye and constellation diagrams obtained with IQ and DD-MZM modulators are represented in Fig. 3(a) and (b), respectively. It can be seen that the signal generated by the DD-MZM exhibits more intensity ripples than that of the IQ modulator, which are linked to symbol transitions. The areas where the intensity is constant, and which are therefore suited for sampling and phase recovery are represented in grey in the eye diagrams of Fig. 3. It can be seen how the temporal opening of the signal generated by the DD-MZM is reduced compared to the signal generated by the IQ modulator, making the DD-MZM more sensitive to symbol transitions. However, these drawbacks are compensated by a very much simpler hardware architecture and reduced driving voltages.

The experimental set-up used for QPSK modulation characterization is reported in Fig. 4. TE-polarized light from an O-band external cavity laser followed by a fiber polarization controller is coupled to the photonic integrated circuit using a single-polarization grating coupler (GC) with typical transmission around -6 dB. The same GC is used to couple the light out of the photonic circuit. Total fiber-to-fiber losses, including coupling and on-chip losses, are thus estimated around 17 dB. The laser source output power was kept to 14 dBm for all the experiments. Two 2 15 -1 long decorrelated pseudo random binary sequences (PRBSs) at 10 Gb/s are used and amplified to reach a swing voltage Vpp of 7 V with DC bias of 4V. GSGSG probes were used at the input to apply the RF signals and at the output of the coplanar waveguide to connect 50-ohm terminations. DC tips were used to connect the heater and adjust the constant phase shift to bias the interferometer at a quadrature point (Φ=π/2). The modulated optical signal is then sent to a coherent receiver through a variable optical attenuator in order to adjust the received power. For the purpose of this first demonstration, the same laser was used at the transmitter and as local oscillator (LO) at the receiver, thus implementing homodyne detection. A semiconductor optical amplifier (SOA) was used to amplify the LO signal. The I/Q signals of the receiver were fed to a 40-GSps digital sampling oscilloscope (DOS) with 16-GHz electrical bandwidth. Blocks of 10 5 symbols were acquired and then processed offline. An adaptive finite impulse response (FIR) filter with decision-directed least mean square (DD-LMS) tracking was used as digital signal processing. The processed samples were used to obtain the constellation diagrams and to compute the bit-error-rate (BER). The BER was evaluated over five blocks of 10 5 symbols.

IV. RESULTS AND DISCUSSION

High-speed measurements of the DD-MZM silicon modulator were performed using the described setup. In order to benchmark its performance, the same characterizations were also performed with a commercial 28-GHz LiNbO3 DD-MZM as well as a 25-GHz LiNbO3 IQ modulator.

The raw received constellation diagrams of the 20 Gb/s QPSK signals generated with the three modulators are represented on the left column of Fig. 5. The raw constellation diagram of the signal generated by the silicon DD-MZM is shown in Fig. 5 (a), whereas Fig. 5 (b) and (c) correspond to LiNbO3 DD-MZM and IQ modulator, respectively. The constellations were obtained for a receiver input power of -22 dBm from 20000 samples after resampling, decimation to the baud rate, but without digital equalization. Four group of samples can be observed, which correspond to the four phase states of the QPSK signal. The particular shape of the constellations generated by the silicon and LiNbO3 DD-MZMs is due to the inherent trajectories in the complex plane between the four modulation states, which result in intensity ripples between consecutive symbols, in QPSK generation based on DD-MZMs. Therefore, the measured raw constellation diagrams have the expected shape predicted in Fig. 3 (b). Furthermore, the electro-optical bandwidth limitation of the silicon modulator is responsible for further fluctuations since the phase-shifters directly map residual voltage fluctuations to phase fluctuations. Thanks to its less complicated trajectories of symbol transitions, the raw constellation diagram obtained with the IQ modulator is cleaner. After DSP, classical QPSK constellation diagrams were obtained in the three cases, as represented in the rightside column of Fig. 5. The measured error vector magnitude (EVM) was 18% for the LiNbO3 IQ modulator, whereas for the silicon and LiNbO3 DD-MZMs, EVMs of 33% and 28% were obtained, respectively. The small EVM degradation observed in the silicon DD-MZM with respect to its LiNbO3 counterpart is related to intensity ripples induced by its lower electro optical bandwidth (15-GHz) compared to the LiNbO3 modulator (28-GHz) and by the intensity modulation associated to the free-carrier dispersion effect.

The measured BER performance of the three modulators is shown in Fig. 6 as a function of the receiver input power. It can be seen that a penalty of 5 dB occurs between the LiNbO3 IQ modulator and the LiNbO3 DD-MZM for a BER of 10 -3 . This power penalty is linked to the presence of intensity ripples that are inherent to this generation scheme, as explained previously. The silicon DD-MZM presents an outstanding 1.5-dB extra penalty with respect to its LiNbO3 counterpart. This power penalty, as explained previously, is linked to the lower electro optical bandwidth of the silicon DD-MZM with respect to the LiNbO3 and to the small intensity modulation present on the silicon DD-MZM.

V. CONCLUSIONS

We have reported the first use of a silicon DD-MZM to generate QPSK signals in the O-band. This modulation scheme has been evaluated first on a LiNbO3 dual-drive modulator. A power penalty of 5 dB is obtained when compared with standard IQ modulation, linked to the presence of intensity ripples when generating the signal. Interestingly the additional power penalty when using the silicon dual-drive modulator is only 1.5 dB, due to a reduced electro-optical bandwidth of the tested device in comparison with the LiNbO3 modulator. Such penalty could be further decreased thanks to pre-emphasis of the electrical drive voltages or with an optical pulse carver [START_REF] Krause | Demonstration of 20-Gb/s DQPSK With a Single Dual-Drive Mach-Zehnder Modulator[END_REF][START_REF] Peng | Evaluation of the inherent ripple effects on the performance of an optical DQPSK signal generated by using only one MZM[END_REF]. Future works will be focused on improvement of the RF design to exceed the 6-dB electro-optical bandwidth of 15-GHz. 25-Gbaud (i.e. 50-Gb/s) QPSK modulation can thus be expected in the short term.

As a main advantage, this modulator scheme allows using only a single Mach-Zehnder interferometer, with 2×Vπ peakto-peak voltage instead of 4×Vπ in a standard IQ modulator. Such reduction of the global power consumption to generate the QPSK signal, can be a key advantage for future short distance applications.

Laurent Bramerie biography not available at the time of publication

Charles Baudot (Senior Member, IEEE) obtained his MSc degree in microelectronics form Université Pierre et Marie Curie, Paris, France in 2004 and his PhD degree from Nanyang Technological University, Singapore in 2010. He joined STMicroelectronics in Singapore in 2005 as an R&D engineer to explore alternative technologies to silicon. In the course of that activity he worked on molecular electronics, carbon nanotube based electronics and printed electronics. In 2010 he moved to STMicroelectronics, France as an advanced R&D senior staff engineer where his activity is devoted to the development of advanced silicon photonic device and process integration.

Mohamed Chaibi biography not available at the time of publication

Sonia

Fig. 1 .

 1 Fig. 1. Cross section of the silicon phase-shifter: (a) schematic and (b) SEM image. Architecture of a DD-MZM: (a) schematic and (b) actual layout.

Fig. 2 .

 2 Fig. 2. Normalized small signal electro-optical response as a function of the applied frequency.

Fig. 3 .

 3 Fig. 3. Eye and constellation diagrams of a QPSK signal generated with (a) an IQ modulator and (b) a DD-MZM.

Fig. 4 .

 4 Fig. 4. Experimental set-up for QPSK operation of the DD-MZM.

Fig. 5 .

 5 Fig. 5. QPSK constellation diagrams at 10 Gbaud after and before DSP for (a) silicon DD MZM, (b) LiNbO3 DD-MZM and (c) LiNbO3 IQ modulator (Prx = -22 dBm).

Fig. 6 .

 6 Fig. 6. Measured BER as a function of the average received power for QPSK signal generation.

  Messaoudène received her Diploma of Higher Education from Université Joseph Fourier, Grenoble (France) in 2000. She then worked at STMicroelectronics, Crolles (France) during 2 years on dry etching process. From 2003 to 2009 she worked at Tronics, Crolles (France) on electrical and dynamic sensor testing. She currently works as a process integration technician at STMicroelectronics, Crolles (France) on the follow up of advanced silicon photonics process R&D. Nathalie Vulliet received her M.Sc. degree in 1996 from INP Grenoble, France. In 1989, she joined Thomson Semiconductors, Grenoble and worked on the electrical characterization of CMOS and BICMOS process development during 4 years. She was then involved in the reliability analysis of advanced CMOS technologies with SGS-Thomson Microelectronics (currently STMicroelectronics), Central R&D, Crolles, France. In 2002, she joined the Advanced Devices team to be involved in the collaborative R&D between STMicroelectronics and CEA-LETI working on the process development of multichannel advanced MOSFET. In 2011, she joined the Advanced Device Technologies team of the Silicon Technology Development department and since then she works on the process integration of silicon photonic technologies in the 300mm Crolles wafer foundry. Laurent Vivien received the Ph.D. degree in physics from the Polytechnique School, Palaiseau Cedex, France, in 2001. His Ph.D. Thesis was on nonlinear optical properties of carbon nanotubes for optical limiting.Between 2001 and 2003, he held a postdoctoral position with the Institute of Fundamental Electronics (IEF), Orsay, France, where studied single-mode and polarization-insensitive structures in silicon-on-insulators and the coupling from sub-micrometric waveguides to single mode fiber for optical telecommunication applications. Since 2003 he has joined the CNRS at the Institute of Fundamental Electronics where his activities are related to passive (waveguide, splitters, 90° turns, optical couplers) and active (Si-based optical modulators and Ge on Si photodetectors) micro and nanophotonic devices on silicon for optical interconnects and optical fiber communications.

  From 2011 until September 2013, he was working in the design of coherent receivers in III-V technology at the Photonics & RF research laboratory at Málaga University. In September 2013, he joined the Silicon Photonics research group at Paris-Sud University, where he worked on the development of silicon transceivers. In October 2016, he defended his PhD entitled "High speed optical modulation, advanced modulation formats and mode division multiplexing in Silicon Photonics" at Paris-Sud University. Since October 2017 he works in the Photonics Research Labs at Polytechnic University of Valencia developing optical links with Spatial Division Multiplexing for fronthaul mobile networks. His research interests comprise devices based on periodic structures and analog and digital optical communication systems including polarization diversity and space division multiplexing.

	Diego	Pérez	Galacho	received	the	MSc	in
	Telecommunication Engineering from Málaga University
	(Spain) in 2011.

Frederic Boeuf biography not available at the time of publication

Christophe Peucheret received the graduate engineering degree from Telecom Bretagne, Brest, France, the M.Sc. degree in microwaves and optoelectronics from University College London, London, U.K., and the Ph.