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）见证了诸多羊膜动物支系（amniote lineages）的首次出现，其中包括鳞龙超目（Lepidosauria）、主龙类（Archosauria）与哺乳纲（Mammalia），这些类群共同定义了二叠纪末大灭绝后的中生代生态系统；同时，三叠纪也是冠群爬行动物（crown-group reptiles）首次发生主要形态辐射的时期。遗憾的是，当前学界对这一演化事件的认知大多源自对大体型爬行动物（体长＞1米）的化石记录研究。本研究报道了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，小型、避役型双孔类动物）新物种。借助显微计算机断层扫描（micro-computed tomography）数据重建，我们首次揭示了该支系的三维头骨骨骼解剖学特征。该物种的头骨兼具诸多三叠纪冠群爬行动物未曾记录的原始解剖性状，例如无外耳骨性支撑结构；而部分特征，如无齿吻部、朝前的眼眶、膨大的颅腔，则与衍征鸟类兽脚类（derived avian theropods）高度相似。针对二叠-三叠纪双孔类（Permo-Triassic diapsids）的系统发育分析（phylogenetic analysis）支持下述假说：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类群中曾存在原始的二叠纪支系；同时形态趋同（morphological convergence）催生了极具鸟类特征的头骨结构，这一演化事件比已知兽脚亚目（Theropoda）出现类似头骨形态早了近1亿年。