Pritchard Adam C, Nesbitt Sterling J
Department of Anatomical Sciences, Stony Brook University, Stony Brook, NY 11794, USA.
Department of Geology and Geophysics, Yale University, 210 Whitney Avenue, New Haven, CT 06520-8109, USA.
R Soc Open Sci. 2017 Oct 11;4(10):170499. doi: 10.1098/rsos.170499. eCollection 2017 Oct.
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.
三叠纪时期出现了众多羊膜动物谱系（如鳞龙超目、主龙形下纲、哺乳纲），这些谱系在二叠纪末大灭绝之后定义了中生代生态系统，同时冠群爬行动物也首次出现了重大的形态多样化。不幸的是，我们对这一事件的了解大多来自大型爬行动物（全长>1米）的记录。在此，我们描述了一种来自新墨西哥州上三叠统钦尔组的镰龙科新物种（小型、变色龙般的双孔类动物）。通过对微计算机断层扫描数据的重建，我们首次揭示了该类群的三维头骨骨骼结构。该头骨呈现出许多三叠纪冠群爬行动物所没有的古老解剖特征（如没有外耳道的骨质支撑），而其他特征（如无齿的吻部、向前的眼眶、膨大的脑颅）则类似于衍化的鸟类兽脚亚目恐龙。对二叠纪 - 三叠纪双孔类动物的系统发育分析支持了这样一种假说，即镰龙是一个起源于二叠纪的古老谱系，与冠群爬行纲关系甚远。镰龙科的系统发育位置表明，在三叠纪小型爬行动物组合中存在古老的二叠纪类群，并且形态趋同产生了一个与鸟类极为相似的头骨，这比已知在兽脚亚目中出现类似头骨的时间早了近1亿年。
