Anatomy and Cranial Functional Morphology of the Small-Bodied Dinosaur Fruitadens haagarorum from the Upper Jurassic of the USA

BackgroundHeterodontosaurids are an important but enigmatic and poorly understood early radiation of ornithischian dinosaurs. The late-surviving heterodontosaurid Fruitadens haagarorum from the Late Jurassic (early Tithonian) Morrison Formation of the western USA is represented by remains of several small (Fruitadens is hypothesized to represent one of the smallest known ornithischian dinosaurs. Methodology/Principal FindingsWe describe the cranial and postcranial anatomy of Fruitadens in detail, providing comparisons to all other known heterodontosaurid taxa. High resolution micro-CT data provides new insights into tooth replacement and the internal anatomy of the tooth-bearing bones. Moreover, we provide a preliminary functional analysis of the skull of late-surviving heterodontosaurids, discuss the implications of Fruitadens for current understanding of heterodontosaurid monophyly, and briefly review the evolution and biogeography of heterodontosaurids. Conclusions/SignificanceThe validity of Fruitadens is supported by multiple unique characters of the dentition and hindlimb as well as a distinct character combination. Fruitadens shares highly distinctive appendicular characters with other heterodontosaurids, strengthening monophyly of the clade on the basis of the postcranium. Mandibular morphology and muscle moment arms suggest that the jaws of late-surviving heterodontosaurids, including Fruitadens, were adapted for rapid biting at large gape angles, contrasting with the jaws of the stratigraphically older Heterodontosaurus, which were better suited for strong jaw adduction at small gapes. The lack of wear facets and plesiomorphic dentition suggest that Fruitadens used orthal jaw movements and employed simple puncture-crushing to process food. In combination with its small body size, these results suggest that Fruitadens was an ecological generalist, consuming select plant material and possibly insects or other invertebrates.

背景：
畸齿龙科（Heterodontosaurids）是一类兼具重要研究价值却又神秘难解、认知程度极低的早期鸟臀目（Ornithischia）恐龙辐射演化类群。来自美国西部晚侏罗世（提通阶早期）莫里逊组（Morrison Formation）的晚存续畸齿龙类哈氏果齿龙（Fruitadens haagarorum），以多件小型个体的遗骸为化石记录，果齿龙被推测为已知体型最小的鸟臀目恐龙之一。

研究方法与主要发现：
我们详细描述了哈氏果齿龙的颅部与颅后解剖结构，并与所有已知畸齿龙类类群开展对比研究。高分辨率显微CT（micro-CT）数据为牙齿替换现象以及承载牙齿的骨骼内部解剖结构提供了全新的研究视角。此外，我们对晚存续畸齿龙类的颅骨进行了初步功能分析，探讨了哈氏果齿龙对当前畸齿龙类单系群（monophyly）认知的启示，并简要回顾了畸齿龙类的演化历程与生物地理分布格局。

结论与意义：
哈氏果齿龙的分类有效性得到了多组独特的齿系与后肢特征，以及独特的特征组合的支持。哈氏果齿龙与其他畸齿龙类共享极具辨识度的附肢特征，进一步基于颅后结构强化了该演化支（clade）的单系性结论。下颌形态与肌肉力矩臂（muscle moment arms）数据表明，包括哈氏果齿龙在内的晚存续畸齿龙类的颌骨适配于大张口角度下的快速咬合，这与地层年代更早的畸齿龙（Heterodontosaurus）的颌骨形成鲜明反差：后者更适合在小张口角度下实现强有力的下颌内收。无磨蚀面（wear facets）与原始齿系的特征表明，哈氏果齿龙采用垂直颌骨运动与简单的穿刺-压碎方式处理食物。结合其小型体型，上述结果显示哈氏果齿龙属于生态广适性物种，可取食特定植物材料，同时也可能捕食昆虫或其他无脊椎动物。