A new pachycephalosaurid from the Hell Creek Formation, Garfield County, Montana, U.S.A.

A partial skull of a pachycephalosaurid from the Upper Cretaceous Hell Creek Formation, Montana, is interpreted as a new taxon, <i>Platytholus clemensi</i> gen. et sp. nov<i>.</i> MOR 2915 does not fit into an ontogenetic continuum of known pachycephalosaurids from the Hell Creek Formation, Montana, and contemporaneous sediments from the Western Interior. Comparisons to known ontogimorphs of <i>Sphaerotholus</i> and <i>Pachycephalosaurus</i> preclude including this specimen into an ontogenetic series of either taxon. We hypothesize that MOR 2915 is a new species based on a relatively low, broad dome at this advanced ontogenetic age that is neither round nor oval in dorsal view, distinct but fused lateral cranial elements fully incorporated into the dome without any dorsal lobe differentiation, and individual tab-like tubercle ornamentation dorsolaterally. Phylogenetic analysis posits that <i>Platytholus clemensi</i> is a <i>Prenocephale</i>-grade taxon deeply nested within Pachycephalosaurinae, but it is not a member of Pachycephalosaurini. <i>Platytholus clemensi</i> is intermediate in size between the other contemporaneous pachycephalosaurids in the Hell Creek Formation and suggests a diverse set of taxa-partitioned ecological niches by body size. We confirm a well organized, major internal vascular network using high resolution computed tomography. Foramina present on the orbital roofs indicate these canals penetrated the entire ceiling of the orbits within the frontal and supraorbital bones. Abundant neurovascular canals passing through the dome to the ectocranial surface indicate a keratinous structure of some kind, possibly with a vertical structural framework, was present on the dome. We review the history of the head-butting hypothesis and associated behavioral implications.

产自美国蒙大拿州上白垩统地狱溪组的一件肿头龙科（Pachycephalosauridae）部分头骨，被确立为一新属新种*Platytholus clemensi* gen. et sp. nov.。MOR 2915无法归入已知的、产自蒙大拿州地狱溪组以及北美西部内陆同期沉积物中的肿头龙科个体发育连续序列。通过与已知的球头龙属（*Sphaerotholus*）和厚头龙属（*Pachycephalosaurus*）的个体发育形态进行对比，该标本无法被归入这两个类群的任何一个个体发育序列中。基于该个体发育晚期阶段的颅顶圆拱相对低平宽阔、背视既非圆形亦非椭圆形，颅骨侧部结构清晰但已愈合完全融入拱顶且无背叶分化，以及背外侧分布的片状独立结节纹饰等特征，我们推断MOR 2915代表一新物种。系统发育分析显示，*Platytholus clemensi*属于倾头龙属（*Prenocephale*）级类群，嵌套于肿头龙亚科（Pachycephalosaurinae）的较深演化位置，但并不属于肿头龙族（Pachycephalosaurini）。*Platytholus clemensi*的体型介于地狱溪组中其他同期肿头龙科类群之间，表明该地区存在通过体型划分生态位的多样肿头龙类群组合。我们通过高分辨率计算机断层扫描确认，该标本拥有一套结构规整的大型内部血管网络。眶顶处的孔道结构表明，这些血管管道贯穿额骨与眶上骨，穿透了整个眶腔顶部。大量穿过颅顶直达颅外表面的神经血管管道表明，颅顶表面曾存在某种角质结构，可能还具备垂直结构支撑框架。我们回顾了头部顶撞假说的研究历史及其相关的行为学意义。