Data from: A bird-like skull in a Triassic diapsid reptile increases heterogeneity of the morphological and phylogenetic radiation of Diapsida

The Triassic Period saw the first appearance of numerous amniote lineages (e.g. Lepidosauria, Archosauria, Mammalia) that defined Mesozoic ecosystems following the end Permian Mass Extinction, as well as the first major morphological diversification of crown-group reptiles. Unfortunately, much of our understanding of this event comes from the record of large-bodied reptiles (total body length > 1 m). Here we present a new species of drepanosaurid (small-bodied, chameleon-like diapsids) from the Upper Triassic Chinle Formation of New Mexico. Using reconstructions of micro-computed tomography data, we reveal the three-dimensional skull osteology of this clade for the first time. The skull presents many archaic anatomical traits unknown in Triassic crown-group reptiles (e.g. absence of bony support for the external ear), whereas other traits (e.g. toothless rostrum, anteriorly directed orbits, inflated endocranium) resemble derived avian theropods. A phylogenetic analysis of Permo-Triassic diapsids supports the hypothesis that drepanosaurs are an archaic lineage that originated in the Permian, far removed from crown-group Reptilia. The phylogenetic position of drepanosaurids indicates the presence of archaic Permian clades among Triassic small reptile assemblages and that morphological convergence produced a remarkably bird-like skull nearly 100 Myr before one is known to have emerged in Theropoda.

三叠纪（Triassic Period）见证了众多羊膜类支系（amniote lineages，如鳞龙超目Lepidosauria、主龙类Archosauria、哺乳纲Mammalia）的首次出现——这些支系塑造了二叠纪末大灭绝后的中生代生态系统，同时也是冠群爬行动物（crown-group reptiles）首次发生大规模形态辐射的关键时期。遗憾的是，当前学界对这一演化事件的认知大多基于体长超过1米的大体型爬行动物的化石记录。本文报道了一件产自美国新墨西哥州上三叠统钦利组（Upper Triassic Chinle Formation）的镰龙科（drepanosaurid）新物种，该类群为小型、形似变色龙的双孔类（diapsids）。通过显微计算机断层成像（micro-computed tomography）数据的三维重建，我们首次揭示了该类群的三维头骨骨骼解剖学特征。其头骨兼具诸多在三叠纪冠群爬行动物中未被报道过的原始解剖特征（例如外耳无骨性支撑结构），同时部分特征（如无齿吻部、朝前的眼眶、膨大的颅腔）与进步的鸟类兽脚类（avian theropods）高度相似。针对二叠-三叠纪双孔类的系统发育分析支持如下假说：镰龙类是起源于二叠纪的原始支系，与冠群爬行纲（crown-group Reptilia）的亲缘关系极为疏远。镰龙科的系统发育位置表明，三叠纪小型爬行动物组合中存在原始的二叠纪支系，且形态趋同演化催生了极为类似鸟类的头骨结构——这一现象比已知兽脚亚目（Theropoda）中出现同类头骨形态的时间早了近1亿年。