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）见证了众多羊膜动物支系的首次出现——包括鳞龙超目（Lepidosauria）、主龙类（Archosauria）、哺乳纲（Mammalia）等——这些类群在二叠纪末大灭绝（end Permian Mass Extinction）后塑造了中生代生态系统，同时也是冠群爬行动物（crown-group reptiles）首次发生大规模形态分化的时期。遗憾的是，目前学界对这一演化事件的认知大多来自大体型爬行动物（体长＞1米）的化石记录。本文报道了产自美国新墨西哥州上三叠统钦尔组（Chinle Formation）的一种镰龙科（drepanosaurid）新物种，该类群为体型小巧、类似变色龙的双孔类（diapsids）爬行动物。通过显微计算机断层扫描（micro-computed tomography）数据重建，我们首次揭示了该类群的三维头骨骨解剖学特征。其头骨展现出诸多三叠纪冠群爬行动物所不具备的原始解剖学性状，例如缺乏外耳的骨性支撑；而另一些性状，如无齿吻部、朝前的眼眶、膨大的颅腔，则与进步的鸟类兽脚类相似。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，与冠群爬行动物亲缘关系甚远。镰龙科的系统发育位置表明，三叠纪小型爬行动物群落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，这一现象比兽脚亚目（Theropoda）中公认的鸟类头骨起源早了近1亿年。