New data and diagnosis for the Arctic ceratopsid dinosaur <i>Pachyrhinosaurus perotorum</i>

The pachyrostran centrosaurine dinosaur <i>Pachyrhinosaurus perotorum</i> is the geologically youngest (Maastrichtian, 70–68.5 Ma) centrosaurine, and latitudinally highest distributed ceratopsid yet known. Continued preparation of material collected from the type locality, the Kikak-Tegoseak Quarry, has produced more examples of cranial material from multiple individuals, including partial skulls and incomplete parietals. The original reconstruction of the type parietal was incorrect, and the element is similar to that of other <i>Pachyrhinosaurus</i> species in bearing medially directed epiparietal 2 processes along its posterior margin. <i>Pachyrhinosaurus perotorum</i> is diagnosed by an upturned tip of the rostrum; a dorsally shifted rostral bone lacking a sharply downturned, parrot-like beak; an enlarged median ridge at the posterior end of the nasal boss; and, tentatively, a posterior sulcus on epiparietal 2 and a canal passing dorsoventrally through the base of epiparietal 2. A cladistic phylogenetic analysis incorporating new data from this and other recent studies of centrosaurine relationships recovers a monophyletic <i>Pachyrhinosaurus</i> clade. <i>Pachyrhinosaurus perotorum</i> and <i>P. canadensis</i> are found to be sister taxa, united by the presence of an extra ossification on the lateral surface of the rostrum between the narial fossa and nasal boss, and by enlarged supraorbital bosses that contact or nearly contact the posterior end of the nasal boss. Parietal and squamosal frill ornamentations alone do not adequately address the variables in craniofacial morphology needed to distinguish between species of <i>Pachyrhinosaurus</i>.厚吻型尖角龙亚科（Centrosaurinae）恐龙佩罗托厚鼻龙（*Pachyrhinosaurus perotorum*）是目前已知地质年代最年轻的尖角龙亚科物种（马斯特里赫特阶，7000万~6850万年前），同时也是纬度分布最高的角龙科（Ceratopsidae）物种。对采自模式产地基卡克-特戈塞克采石场（Kikak-Tegoseak Quarry）的标本进行持续修复后，获得了来自多个个体的更多颅骨材料，包括部分头骨与不完整的顶骨。该物种的模式顶骨最初复原存在错误，其结构与其他厚鼻龙（*Pachyrhinosaurus*）物种相似，在后缘带有指向内侧的第2顶骨突。佩罗托厚鼻龙的鉴别特征包括：吻部尖端上翘；位置背侧迁移的吻骨不具备尖锐下弯的鹦鹉状喙；鼻隆后端带有扩大的正中脊；暂定鉴别特征为第2顶骨突上存在后沟，以及贯穿该顶骨突基部的背腹向管道。整合本次研究及其他近期尖角龙亚科亲缘关系研究的新数据所开展的分支系统发育分析结果显示，厚鼻龙属（*Pachyrhinosaurus*）为单系演化支。佩罗托厚鼻龙与加拿大厚鼻龙（*P. canadensis*）被恢复为姊妹群，二者共享的共源性状包括：吻部外侧、鼻孔窝与鼻隆之间存在额外骨化结构；扩大的眶上隆接触或近乎接触鼻隆的后端。仅依靠顶骨与鳞状骨颈盾的纹饰，不足以充分解决区分厚鼻龙物种所需的颅面形态变量问题。