Matheronodon

Matheronodon (meaning "Matheron tooth") is a genus of rhabdodontid ornithopod dinosaur from the late Cretaceous Period of the Grès à Reptiles Formation in France. The genus contains a single species, M. provincialis, which is known from a single maxilla and associated teeth. It was named by Pascal Godefroit and colleagues in 2017. The teeth of "Matheronodon" are large but few in number. The teeth are also in an unusual arrangement, emerging alternatingly from one of a pair of fused tooth sockets in its mouth. In life, the teeth would have functioned like a pair of scissors, allowing "Matheronodon" to feed on the tough leaves of monocot plants.
Description.
Like other rhabodontids, "Matheronodon" would have been a bipedal herbivore. The length of the genus has been estimated at by its lead describer Pascal Godefroit.
The maxilla of "Matheronodon" is a short, robust bone. The front portion is particularly shortened and also angled upwards, differentiating "Matheronodon" from other rhabdodontids. It measures long, and high. When viewed from the top, the front portion is triangular, and forms a broad rostrodorsal shelf. Both "Rhabdodon" and "Zalmoxes" lack the shelf. There is a bar-like structure on the interior surface, the dorsal bar, which is also present in "Rhabdodon". Immediately below this bar, there is a horizontal groove, or transverse sulcus. Further behind, the upward-projecting dorsal process is wider and angled further backwards than "Rhabdodon", and larger than "Zalmoxes". From the base of this process, a "wing" extends backwards to articulate with the jugal bone. Also at the base of the dorsal process is a small triangular projection delimiting the back of the antorbital fenestra, which is shorter than "Rhabdodon".
Teeth.
Like "Rhabdodon", "Zalmoxes", and "Mochlodon", the maxillary tooth crowns of "Matheronodon" are shaped like cleavers. They are unusually large, being up to long. More than these other rhabdodontids, there are at least 25 ridges on the inner surface of each tooth. The ridges are all roughly the same size, and there is no "primary" ridge, identifying "Matheronodon" as a rhabdodontid. A single wear facet is oriented 60° below the horizontal, like "Zalmoxes" and "Mochlodon". Enamel covers all sides of the teeth, unlike hadrosaurids, being thicker on the inner surface (particularly on the ridges, compared to "Edmontosaurus"). Also unlike hadrosaurids, the tubular structure of the dentine below the enamel does not appear to vary in orientation. The scratches on the dentine's surface are vertical like "Zalmoxes" and "Mochlodon".
Even though the maxilla is large, it bears only eight tooth sockets. "Zalmoxes" has up to 10, and "Rhabdodon" has 11. The walls separating adjacent sockets appear to have been resorbed such that the sockets have fused into four pairs, which is not known in either "Rhabdodon" or "Zalmoxes". Only one functional tooth would have been present in each pair at any time. From the preserved replacement teeth, it appears that tooth replacement progressed from the back to the front of the jaw, with teeth alternatingly emerging from the front and rear sockets in each pair. The tooth of each front socket would have overlapped those from the rear socket, due to the size of the crowns.
Unlike the teeth of the maxilla, the teeth of the lower jaw are leaf-shaped, and bear a primary ridge displaced backwards from the center of the crown. However, they are also large. The subsidiary ridges flanking the primary ridge are also more numerous than "Rhabdodon" or "Mochlodon", with there being at least 12 on each side. These ridges stretch to the bottom edge of the crown, instead of stopping short as in "Mochlodon".
Discovery and naming.
All specimens of "Matheronodon" have been found at the locality of Velaux-La Bastide Neuve, in the Aix-en-Provence basin, Bouches-du-Rhône, France. The sandstones at this locality, which was discovered by Xavier Valentin in 1992, are part of the Begudian regional stage (which correlates to the late Campanian epoch, about 74 to 72 million years ago). After their discovery, the fossils were stored in the paleontology and archaeology collections of the municipality of Velaux, and labelled as belonging to the Musée du Moulin Seigneurial/Velaux-La Bastide Neuve (MMS/VBN). The type specimen is MMS/VBN-02-102, a right maxilla; associated with it are maxillary teeth (-93-34, -09-149a, -09-150, and -12-22) and dentary teeth: (-02-11, -09-43c, and -12-A002).
In 2017, the fossil material was described by Godefroit along with Géraldine Garcia, Bernard Gomez, Koen Stein, Aude Cincotta, Ulysse Lefèvre, and Valentin in a research paper published by the journal "Nature Communications". They assigned this material to the genus "Matheronodon", with the name being formed from Philippe Matheron (who described the first dinosaur fossils found in Provence, or the southeastern region of France) and the suffix "-odon", a derivative of "" (Greek for "tooth"). They also named the type and only species "M. provincialis", with "provincialis" being Latin for Provence. For the study, a CT scan of the type maxilla was conducted at the University of Liège, and a thin section of an isolated maxillary tooth was examined under a microscope.
Palaeobiology.
Inferences regarding the method of feeding in "Matheronodon" can be made from the unique morphology of its teeth. The hadrosaurs, a contemporary successful group of ornithopods, had a large number of wide, high crowned teeth in tightly-packed rows. These were used in a complex method of chewing, a culmination of continued specialization throughout the evolution of iguanodonts. Rhabdodontids, which were more basal, had a far simpler arrangement, with teeth more similar to those of less specialized ornithopods. Fewer teeth were present in the jaw, but they were taller, thinner, and individually larger. In life, they would have functioned like a pair of scissors in powerful but simple slicing bites. The microstructure of the teeth in "Mantheronodon" was also adapted for this action.
These differences would have led to different diets in the two groups of ornithopods. Where they co-occurred, they were likely ecologically partitioned. Rhabdodontids would have had more limited diets, compared to the diverse and likely cellulose-rich and perhaps conifer-based diets of hadrosaurs. "Mantheronodon" and its relatives may have fed on monocot plants, such as the palms "Sabalites" and "Pandanites", common in Europe at the time. Sclerenchyma fibres, abundant in monocot leaves but largely absent in eudicots and conifers, result in tougher plant tissue, and would have necessitated the more powerful bites of rhabdodontids to be consumed.
Palaeoecology.
Velaux-La Bastide Neuve represents a continental deposit. However, it was probably close to the shoreline, judging by the presence of claws from decapods, gastropods, and unionid mussels at the site. Fossils from the locality are well-preserved and were likely slowly transported. Other dinosaurs discovered at the site include the titanosaur "Atsinganosaurus", in addition to teeth from nodosaurid ankylosaurs as well as neoceratosaurian and dromaeosaurid theropods. Additional faunal elements include the eusuchian "Allodaposuchus", pterosaurs, the turtles "Solemys" and "Polysternon",
the hybodont elasmobranch "Meristonoides", and a gar.