Extreme tooth enlargement in a new Late Cretaceous rhabdodontid dinosaur from Southern France

Rhabdodontidae is a successful clade of ornithopod dinosaurs, characteristic of Late Cretaceous continental faunas in Europe. A new rhabdodontid from the late Campanian, of southern France, Matheronodon provincialis gen. et sp. nov., is characterized by the extreme enlargement of both its maxillary and dentary teeth, correlated to a drastic reduction in the number of maxillary teeth (4 per generation in MMS/VBN-02-102). The interalveolar septa on the maxilla are alternately present or resorbed ventrally so as to be able to lodge such enlarged teeth. The rhabdodontid dentition and masticatory apparatus were adapted for producing a strict and powerful shearing action, resembling a pair of scissors. With their relatively simple dentition, contrasting with the sophisticated dental batteries in contemporary hadrosaurids, Matheronodon and other rhabdodontids are tentatively interpreted as specialized consumers of tough plant parts rich in sclerenchyma fibers, such as Sabalites and Pandanites.


Description
MMS/VBN-02-102 is a particularly robust right maxilla (rostrocaudal length of 22 cm and maximal height of 10 cm; Fig. 1). The rostral process is particularly short and oriented craniodorsally. In Rhabdodon, the rostral process is much more elongated, forming a horizontal styloid premaxillary process that is nearly as long as the body of the maxilla 9 ; this styloid process is also well developed in Zalmoxes 3 . The rostral border of MMS/VBN-02-102 is not eroded, so preservation cannot explain the shortening of its rostral process. In dorsal view, the rostral third of the maxilla is medially broadened to form a triangular and concave rostrodorsal shelf that likely supported the caudolateral (maxillary) process of the premaxilla; this shelf is absent in Rhabdodon 9 and Zalmoxes 3 . The medial surface of the rostrodorsal shelf forms an elongated and flattened surface, likely marking the contact with the vomer and/or with the paired maxilla 3 (Fig. 1c). Caudally to this surface, the dorsal half of the medial side of the maxilla is dorsoventrally convex, forming dorsal bar as also observed in Rhabdodon 9 . At mid-length, the dorsal surface of the bar forms a prominent and laterally-oriented palatine process, with a deep groove along its mediocaudal side forming the articulation with the palatine (Fig. 1c). Caudally to the palatine process, the dorsomedial side of the dorsal bar is pitted and grooved by the articular surface for the pterygoid. The dorsal bar is separated from the flattened dentigerous ventral half by a continuous transverse sulcus (Fig. 1c), corresponding to the neurovascular groove that usually connects the alveolar foramina in ornithopods 10 . Caudally to the rostral process, the dorsolateral margin of the maxilla forms a prominent, dorsocaudally-inclined, dorsal process. The dorsal portion of this process is expanded along both its rostral and caudal edges; the caudal expansion is more important, forming a triangular caudodorsal wing that participates in the rostrodorsal margin of the elliptical antorbital fossa (Fig. 1b). The dorsal process is much more developed in Matheronodon than in Zalmoxes 3 ; in Rhabdodon, the dorsal process looks rostrocaudally narrower and is more vertically oriented, as observed in MC-QR9 9 . A large wing-like jugal process extends caudolaterally from the base of the dorsal process up to the caudal quarter of the bone (Fig. 1a,b). It is mediolaterally thin, separated from the main body of the maxilla by a wide and deep sulcus. Along its dorsal edge, a small triangular process marks the caudal end of the antorbital fenestra. A similar process is also present, though it is proportionally higher, in Rhabdodon 9 . A short, but strong ectopterygoid ridge connects the caudal end of the jugal process to the main body of the maxilla (Fig. 1a,b). As in Zalmoxes 3 , the lateral surface of the maxilla overhangs the tooth row and forms a large labial recess. A dozen foramina are more or less aligned along the dorsal part of the dentigerous portion of the maxilla (Fig. 1b) and likely transmitted neurovascular bundles from the maxillary division of the trigeminal nerve and the maxillary artery 3 . The rostralmost foramen, just in front of the rostral process, is the largest. Ventrally, the tooth row is relatively straight and composed of only eight alveoli, despite of the large size of this specimen. Up to 10 alveoli are present in much smaller Zalmoxes specimens 3 and 11 in MC-QR9, of similar size as MMS/VBN-02-102, from the Campanian of Quarante (Hérault, southern France) and referred to as Rhabododon 9 . The interalveolar septa are alternately present or nearly completely resorbed, so the base of the maxillary dental battery in fact contains four tooth positions, each of which is composed of two partially fused alveoli (Fig. 1d). Alveolar resorption has not been observed in Rhabdodon 9 and Zalmoxes 3 . The functional maxillary teeth of MMS/VBN-02-102 are not preserved, but the crowns of the replacement teeth emerge from the alveoli. CT-scans of MMS/VBN-02-102 reveal the presence of two generations of replacement teeth (Fig. 2). The first generation emerges from the rostral half of each paired alveolus. The development of the maxillary teeth remained typically reptilian in pattern, progressing in wave-like fashion from the caudal part to the rostral part of the battery 11 . A second generation of replacement teeth is present in the caudal half of each paired alveolus. It can therefore be concluded that successive generations of teeth alternatively emerged from the rostral, and then from the caudal half of the paired fused alveoli. However, the tooth arrangement and replacement pattern in Matheronodon is not radically different from those in more advanced iguanodontians 11 . Because of the enlargement of the maxillary crowns (see below), teeth within each fused alveolus were largely imbricated, the rostral one covering the distal one in labial view (Fig. 2). Although they are only partially erupted and a large portion is consequently still embedded within the maxilla, more than 18 vertical ridges are visible along the labial surface of the second and third maxillary crowns (Fig. 1e), and the total number of labial ridges is consequently higher than 25 (see also Fig. 2).
Isolated maxillary teeth ( Fig. 3a-d) were found associated to MMS/VBN-02-102, but it cannot be ascertained that they belong to the same individual, as their size is rather heterogeneous and the fossils were obviously transported over a short distance 8 . However, all have the same morphology and also closely resemble those inside MMS/VBN-02-102, so they evidently belong to the same taxon. All are particularly high (around 6 cm in MMS/VBN-09-149) and mesiodistally broad (around 5 cm in MMS/VBN-09-150). Their crown is typically cleaver-shaped, as also observed in other rhabdodontids including Zalmoxes 3 , Mochlodon 4 , and Rhabdodon 9 . In labial view, the crown is more elevated distally than mesially and the cutting apical edge is oblique and straight. Enamel covers the crown on all sides, but it is much thicker labially than lingually. As also observed in the holotype MMS/VBN-02-102, the labial surface of all recovered maxillary teeth is ornamented by more than 25 vertical and parallel ridges, all subequal in size. All the ridges reach the apical edge of the crown, forming tiny denticles, and then extend farther along the apical portion of the lingual side of the unworn crowns. Some of the ridges are bifurcated at the base of the labial side. The number of labial ridges is much higher in Matheronodon than in other rhabdodontids: a maximum of 19, 13, and 12 labial ridges can be observed in Zalmoxes 3 , Mochlodon 4 , and Rhabdodon 9 , respectively. As already observed 3,9 , there is apparently no significant correlation between tooth width and number of ridges on the maxillary teeth in rhabdodontids. Basally, the labial enamel surface of the maxillary teeth of Matheronodon is bordered by a thin crenulated cingulum that curves apically along its distal margin. A single wear facet can be observed in MMS/VBN-09-149a, forming an angle of approximately 60° with the horizontal plane, as also observed in the most heavily worn teeth of Zalmoxes 3 and Rhabdodon 9 . Scratches on the worn dentine surface are parallel and vertically oriented, as also observed in Zalmoxes 3 and Mochlodon 9 . The root of the maxillary tooth is particularly robust, higher than the crown, and lingually curved. Well-developed replacement tooth grooves along both the mesial and distal sides of the roots indicate that functional and replacement teeth were closely imbricated.
Three large dentary teeth ( Fig. 3e-g) were also found in Velaux-La Bastide Neuve, close to the holotype maxilla. As in other rhabdodontids, unworn dentary crowns have a leaf-shaped, lingual aspect, dominated by a prominent primary ridge, slightly distal to the midline of the crown. As in Zalmoxes 3,12 , the thickly-enameled-lingual side of the crown is covered, on either side of the median ridge, by a dozen vertical subsidiary ridges. Subsidiary ridges are less numerous in Rhabdodon (up to 8 on either side of the primary ridge 9 ) and Mochlodon (4-7 on either side 4 ). The subsidiary ridges are slightly divergent and, as it is also the case for the maxillary teeth, they extend onto the apical part of the labial side of the crown, forming small denticles as they cross the apical edge. Unlike in Mochlodon 4 , the subsidiary ridges reach the basal margin of the enameled surface of the lingual side of the crown. The enamel is much thinner on the labial side than on the lingual side of the crown. Because these dentary teeth were found in the same fossiliferous pocket as the holotype maxilla and are also characterized by more numerous subsidiary ridges than in Rhabdodon as it is also the case for the maxillary teeth, it is reasonable to refer them to Mochlodon.

Tooth microstructure
A coronal section in a maxillary tooth (Fig. 4) shows a jagged pattern of ridges and grooves on the labial surface (Fig. 4b,h), corresponding to the numerous vertical ridges, and a rather smooth surface on the lingual side  Table S1) demonstrate that the ridges on the labial side of the crown have thicker enamel than the grooves in between. The enamel itself has a wavy enamel Schmelzmuster (Fig. 4f,g), similar to hadrosaurid dinosaurs 13,14 . Unlike in hadrosaurids 14 , the enamel is present around the entire crown of unworn teeth, but thinner on the lingual side of the sectioned specimen. Measurements on a well-preserved side of the tooth (Fig. 4c) show that Matheronodon has significantly thicker (Supplementary Table S1; independent t-test, N = 11, p < 0.001) enamel (avg. 179 µm) on its dental ridges, compared to the enamel on the central keel of Edmontosaurus teeth (avg. 155 µm, cf. also 'Anatosaurus' 13 ).
The dentine of the sectioned tooth (Fig. 4d,e) does not show any clear variation in the orientation of dentine tubules as in hadrosaurids 14 . We could also observe a high number of incremental Von Ebner lines (50 to 100 IVELs, Fig. 4e), indicating a long development time for a single tooth 15 . Unfortunately, the preservation state of the tooth did not allow a precise count of Von Ebner lines in the dentine.

Discussion
Among basal iguanodontians, Rhabdodontidae (all iguanodontians more closely related to Rhabdodon priscus than to Parasaurolophus walkeri 16 ) form a clade mostly endemic to Europe during the Cretaceous. The following dental characters, regarded as unambiguous synapomorphies for Rhabdodontidae 3-5 can be observed in Matheronodon: dentary teeth with multiple (more than 10) evenly-spaced accessory ridges around the primary ridge, and maxillary teeth with multiple ridges of similar size and devoid of a primary ridge. Besides Matherodon provincialis, three rhabdodontid genera and six species have currently been named: Rhabdodon priscus from the Campanian-Maastrichtian of France and Spain 1 , R. septimanicus from the Campanian-Maastrichtian of The maxilla MMS/VBN-02-102 and the teeth that were found in the same fossiliferous pocket at Velaux-La Bastide Neuve are quite different from those described in Rhabdodon, also from late Campanian-early Maastrichtian deposits from Southern France, and in other rhabdodontids, so they clearly belong to a new taxon:  Fig. 3a) showing an occlusal surface with a serrated (black arrows) slicing edge. the rostral process is short and rostrodorsally oriented, although it is much more elongated, horizontal and styloid in Rhabdodon; the dorsal shelf is much broader on the rostral half of the maxilla; the dorsal process is wider and caudally inclined in MMS/VBN-02-102; alveolar resorption has not been observed in Rhabdodon so far; the number of labial ridges is much higher on the maxillary teeth in Matheronodon (>25) than in Rhabdodon (<12); and the number of subsidiary ridges is also higher on the dentary teeth in Matheronodon (a dozen or more on either side of the primary ridge) than in Rhabdodon (up to 8 on either side of the primary ridge).
The sophistication of their feeding apparatus has long been identified as a key element in the evolutionary success and diversification of ornithopods 19,20 . Basal, non-iguanodontian ornithopods are characterized by relatively low tooth count (14-17 positions), low, labiolingually-compressed tooth crowns that are bulbous to sub-triangular in shape, and ornamented by numerous apicobasal ridges [21][22][23] . The marginal denticles are relatively large and triangular 20 . More advanced iguanodontians, such as Camptosaurus dispar 24 , Iguanodon bernissartensis 25 , Mantellisaurus atherfieldensis 26 , and Ouranosaurus nigeriensis 27 , are characterized by a higher tooth count (around 20 positions), proportionally higher, shield-like tooth crowns, a reduced number of apicobasal ridges, and proportionally reduced marginal denticles. The evolutionary explosion of hadrosauroid iguanodontians during the Late Cretaceous was accompanied by the development of true dental batteries, consisting of up to 60 closely packed tooth families. In advanced hadrosaurids, each tooth family is made of three up to seven successional teeth. Tooth miniaturization is accompanied in hadrosaurids by proportionally higher tooth crowns and by a further reduction of ridges to a single median carina (one or two faint subsidiary ridges are occasionally present) and of the marginal denticles 11 . Enamel restriction to the sides of crowns promoted self-sharpening 14 .
In most hadrosaurids, up to three successional teeth from each tooth row participate in the occlusal surface, forming labiolingually wide pavements across the entire chewing area. This morphological complexity is coupled, in hadrosaurids, with a histological complexity and the development of six dental tissues instead of two in basal genasaurans, for example 14 . Hadrosaurids therefore developed a complex, dual-function slicing-grinding system presumably for the consumption of moderately tough, cellulose-rich plant diet 14 .
The rhabdodontid dentition evolved in an opposite way: unlike hadrosaurids, they developed a reduced number of mesiodistally wide, but labiolingually thin teeth 4 . Extreme tooth enlargement can be observed in Matheronodon, in which the number of maxillary tooth rows is reduced to eight. The high-angled wear surface has a particularly sharp, chisel-like cutting edge. Scratches on the worn dentine surface are usually well marked, parallel and vertically oriented 3,4 . Moreover, the rhabdodontid quadrate was proportionally low and massive 3,9 , so that the jaw joint was likely not offset too far from the level of the tooth row. The rhabdodontid skull was likely triangular in dorsal view: the premaxillae and predentary of Zalmoxes are narrow, whereas the maxillae diverge strongly posteriorly, and the quadrates are laterally splayed 3 . Intracranial mobility was obviously much more limited than in hadrosaurids. In rhabdodontids, the robust jaw, strong and elevated coronoid process and large jaw adductor muscle chamber suggest that jaw closure was powerful and that the motion of lower jaw was limited by the complex jaw joint and tight predentary-dentary suture 3 . All these characters suggest that the rhabdodontid dentition and masticatory apparatus were adapted for producing a strict and powerful slicing action, resembling a pair of scissors.
The peculiar Matheronodon tooth microstructure also reflects specialization for slicing. As soon as a new tooth erupted, the ridges along the wear-resistant thicker enameled side of the crown formed a self-sharpening serrated and jagged slicing edge (Fig. 4h). Precise measurement of durability would require detailed tribological measurements 14,28 , but this is beyond the scope of the current paper. Due to the high tooth development time and large size of the teeth, the tooth replacement rate of Matheronodon was probably not as high as in hadrosaurids or ceratopsians 14,28 .
From a biomechanical point of view, enlarged blade-like teeth, as exemplified by Matheronodon, and also by the mammalian carnassial teeth, are best adapted for fracturing tough (= resistant to crack propagation) foodstuffs [28][29][30][31] . Hadrosaurids, on the contrary, developed a much more complex dentition forming a dual-function shearing-crushing system, suggestive of a broader dietary range, including leaves, fruits, seeds, and twigs [32][33][34] ; subtle variations in their tooth morphology would have allowed for diversification into more specialized ecological niches 14,31 . In any case, the important morphological differences in their dentitions likely lead to niche portioning between rhabdodontids and hadrosaurids in Europe by the Late Cretaceous time.
Besides Matheronodon, the Velaux-La Bastide Neuve locality has yielded other herbivorous dinosaur remains, including ankylosaurid elements, and abundant material of the titanosaurid sauropod Atsinganosaurus velauciensis 6 . We have not found any trace of hadrosauroids yet. Previous works based on sites in southern France have concluded that an important faunal replacement related to an important late Maastrichtian marine regression occurred in southern Europe during the Late Cretaceous: a Campanian -early Maastrichtian fauna dominated by titanosaurids and rhabdodontids was replaced by a late Maastrichtian assemblage dominated by hadrosaurids 35 . However, the presence of hadrosauroids, titanosaurids and rhabdodontids in the late Maastrichtian assemblage of Vitrolles-La-Plaine suggests that these animals were still living together in southern France during the latest Cretaceous, but maybe in different environments. Their co-occurrence in the allochthonous assemblage of Vitrolles-La-Plaine can easily be explained by taphonomic processes (hydraulic transport) that concentrated, in the same level, bones of animals living in different places 36 .
Ceratopsians are particularly rare in Late Cretaceous deposits of Europe. Scarce remains have been described from the Coniacian-Santonian of Belgium 37 and from the Campanian of Sweden 38 . The coronosaurian neoceratopsian Ajkaceratops kozmai was discovered in the Santonian Csehbánya Fm of Iharkút in Hungary 39 , which has also yielded the rhabdodontid Mochlodon vorosi 4 . In contrast, ceratopsians are abundant in Late Cretaceous deposits of Asia and North America, whereas rhabdodontids have not been reported from these areas. Ceratopsian dentitions produced a strict shearing action 31 , as also hypothesized for rhabdodontids in the present paper. So, both the medium-sized ceratopsians and the rhabdodontids potentially fed on similar tough, woody or fibrous vegetation. What Matheronodon and Rhabdodon fed on will probably remain unknown, but this issue can be discussed based on some plant fossil assemblages known during the Campanian-Maastrichtian of Europe (see Supplementary Information for more detailed information on the composition of those assemblages): the Grünbach Formation in Austria [40][41][42] ; the Haţeg and Rusca Montană basins in western Romania 43 , which have also yielded abundant remains of the rhabdodontid Zalmoxes; the Áger, Vallcebre Coll de Nargό and Tremp basins in the southern Spanish Pyrenees [44][45][46][47][48][49][50][51][52][53][54][55][56][57][58][59][60] ; Lo Hueco in the south-western Iberian Ranges 61,62 , which has also yielded fossils of rhabdodontid and titanosaurid dinosaurs; and Fuveau Basin, Etang de Berre, and Sainte-Baume Massif in southeastern France [63][64][65] . In Europe, plant assemblages strongly vary in richness and abundance depending on the localities, but overall plant megafossils are marked by the replacement of the conifer Geinitzia by Cunninghamites and the spread of the monocots Sabalites and Pandanites, while plant microfossils are characterized by the abundance of fern spores and Normapolles-group eudicot pollen grains.
Tough woody tissue exists in diverse vascular plant groups, such as gymnosperms and most angiosperms except monocots, and is found in leaves, stems, and roots. It is made of secondary xylem, commonly called wood, which consists of water and mineral conductive cells, called tracheids and vessels, parenchyma cells and facultative sclerenchyma fibers. Sclerenchyma also occur out of secondary xylem, and are particularly abundant in some large leaves such as in some palms; in leaves and stems of many herbaceous and tree plants, sclerenchyma serves the function of support when wood is absent. The relative abundance of secondary xylem vs sclerenchyma fibers in eaten plant parts such as leaves and stems may cause large herbivores to adapt their diet and tooth morphology. Soft eudicot leaves with few or no sclerenchyma fibers can be easily crushed with a pestle in a mortar. Campanian-Maastrichtian conifer leafy stems, such as Geinitzia and Cunninghamites, and eudicot leaves and small stems may be easily and quickly beat into pulp. So they do not only require powerful blade-like teeth. In contrast, many monocot leaves in particular in palms such as the Campanian-Maastrichtian Sabalites longirhachis and Pandanites spp. contain much sclerenchyma fibers along the parallel veins to stiffen the large leaf laminas and petioles. Such fibers must be cut into small fragments with blade-like teeth prior to be swallowed.
It might therefore be cautiously hypothesized that rhabdodontids were adapted to preferentially feed upon tough plant parts rich in sclerenchyma fibers such as Sabalites and Pandanites. In contrast, conifers potentially constituted an important part in the diet of North American hadrosaurids. The supposed gut content of an Edmontosaurus "mummy" mainly consists of conifer needles and branches, together with deciduous angiosperm foliages and numerous small seeds or fruits 32,66 . Coprolites tentatively referred to Maiasaura primarily contain conifer stem fragments 66 . Nevertheless, there is no indication of drastic changes in the character or composition of terrestrial vegetation between the early and late Maastrichtian that could easily explain the sudden abundance of hadrosaurids and the purported contemporary decline of rhabdodontids in western Europe. Correlating faunistic and floristic remains highly speculative in the current state of our knowledge.

Methods
Computed tomography (CT) of the maxilla was done at the veterinary faculty of the University of Liege. A 16 multi-slice CT scan was used (Siemens, Somatom 16, Erlangen, Germany) with the following parameters; 120 kV and 46 mA under a Dental 0.75 H60s protocol. The slices of 0.5 mm in depth were digitally reconstructed using Amira 5.3.3 and Drishti v2.6.3.
For investigating tooth microstructure, we sectioned a slightly crushed and isolated maxillary tooth of Matheronodon. The specimen was embedded in epoxy resine (Araldite 2020) and sectioned and ground to a thickness of ~50 µm using standard lapidary methods.
Nomenclatural Act. This published work and the nomenclatural acts it contains have been registered in ZooBank, the proposed online registration system for the International Code of Zoological Nomenclature. The ZooBank life science identifiers can be resolved and the associated information viewed by appending the life science identifiers to the prefix http://zoobank.org/. The life science identifiers for this publication is 1AD18D4B-7063-43F7-A55E-19463D066D78; for Matheronodon: FF4C534D-206A-4659-8592-9154ABF4A8B5, and M. provincialis: A6D62068-15A5-4ADE-83DD-74F0ED529BAB.