Phylogeny, Histology and Inferred Body Size Evolution in a New Rhabdodontid Dinosaur from the Late Cretaceous of Hungary

BackgroundRhabdodontid ornithopod dinosaurs are characteristic elements of Late Cretaceous European vertebrate faunas and were previously collected from lower Campanian to Maastrichtian continental deposits. Phylogenetic analyses have placed rhabdodontids among basal ornithopods as the sister taxon to the clade consisting of Tenontosaurus, Dryosaurus, Camptosaurus, and Iguanodon. Recent studies considered Zalmoxes, the best known representative of the clade, to be significantly smaller than closely related ornithopods such as Tenontosaurus, Camptosaurus, or Rhabdodon, and concluded that it was probably an island dwarf that inhabited the Maastrichtian Haţeg Island. Methodology/Principal FindingsRhabdodontid remains from the Santonian of western Hungary provide evidence for a new, small-bodied form, which we assign to Mochlodon vorosi n. sp. The new species is most similar to the early Campanian M. suessi from Austria, and the close affinities of the two species is further supported by the results of a global phylogenetic analysis of ornithischian dinosaurs. Bone histological studies of representatives of all rhabdodontids indicate a similar adult body length of 1.6–1.8 m in the Hungarian and Austrian species, 2.4–2.5 m in the subadults of both Zalmoxes robustus and Z. shqiperorum and a much larger, 5–6 m adult body length in Rhabdodon. Phylogenetic mapping of femoral lengths onto the results of the phylogenetic analysis suggests a femoral length of around 340 mm as the ancestral state for Rhabdodontidae, close to the adult femoral lengths known for Zalmoxes (320–333 mm). Conclusions/SignificanceOur analysis of body size evolution does not support the hypothesis of autapomorhic nanism for Zalmoxes. However, Rhabdodon is reconstructed as having undergone autapomorphic giantism and the reconstructed small femoral length (245 mm) of Mochlodon is consistent with a reduction in size relative to the ancestral rhabdodontid condition. Our results imply a pre-Santonian divergence between western and eastern rhabdodontid lineages within the western Tethyan archipelago.**背景** 凹齿龙科（Rhabdodontidae）鸟脚类恐龙是晚白垩世欧洲脊椎动物群的典型组分，此前的化石记录发现于坎潘阶（Campanian）早期至马斯特里赫特阶（Maastrichtian）的陆相沉积地层中。系统发育分析将凹齿龙科置于基干鸟脚类演化位置，作为由腱龙属（Tenontosaurus）、橡树龙属（Dryosaurus）、弯龙属（Camptosaurus）以及禽龙属（Iguanodon）构成的演化支的姊妹群。近期研究认为，该类群中最具代表性的查尔摩斯龙属（Zalmoxes）体型显著小于腱龙、弯龙等近缘鸟脚类，且推断其为栖息于马斯特里赫特阶哈泰格岛（Haţeg Island）的岛屿侏儒类群。 **材料与方法/主要发现** 匈牙利西部桑托阶（Santonian）地层产出的凹齿龙科化石残骸代表了一种全新的小型类群，我们将其命名为沃氏莫德隆（Mochlodon vorosi）新种（n. sp.）。该新种与奥地利坎潘阶早期的苏塞莫德隆（M. suessi）形态最为相似，全球鸟臀目（ornithischian dinosaurs）系统发育分析结果进一步证实了二者的近缘关系。对所有凹齿龙科类群代表物种的骨组织学研究表明：匈牙利与奥地利的新种成年个体体长约1.6–1.8米；强健查尔摩斯龙（Zalmoxes robustus）与施氏查尔摩斯龙（Z. shqiperorum）的亚成体体长为2.4–2.5米；而凹齿龙属（Rhabdodon）的成年个体体型更大，可达5–6米。将股骨长度映射至系统发育分析结果后显示，凹齿龙科的祖先状态股骨长度约为340毫米，与已知查尔摩斯龙的成年股骨长度（320–333毫米）相近。 **结论与意义** 我们针对体型演化的分析并不支持查尔摩斯龙存在独有侏儒化的假说。但凹齿龙属被推断发生了独有的巨型化演化；而莫德隆属（Mochlodon）重建的股骨长度（245毫米）相较于凹齿龙科的祖先状态，符合体型缩小的演化特征。本研究结果表明，西特提斯群岛（western Tethyan archipelago）内的凹齿龙科东西支系在桑托阶之前就已发生分化。