“西里龙”（恐龙的一个近亲）的一个新属和新种的一个近乎完整的骨架的发现，将三叠纪中期爬行动物化石记录中一些松散环节联系了起来。新发现的种是“鸟颈类主龙”（Ornithodira）已知最早的成员之一，这个鸟类分支还包括恐龙和飞龙，其生活年代可以追溯到两大“祖龙”类别“鸟颈类”和“鳄鱼类”分化后不久。

齿系和其他特征表明，“西里龙”并不像人们可能预料的那样是两条腿的食肉动物，而是体型更大的食草动物。但其真正的意义在于年代早，这表明，我们对恐龙和飞龙演化的最早阶段仍然知之甚少。

原始出处：

Nature 464, 95-98 (4 March 2010) | doi:10.1038/nature08718

Ecologically distinct dinosaurian sister group shows early diversification of Ornithodira

Sterling J. Nesbitt1,2,9, Christian A. Sidor3, Randall B. Irmis4,5, Kenneth D. Angielczyk6, Roger M. H. Smith7 & Linda A. Tsuji8

The early evolutionary history of Ornithodira (avian-line archosaurs) has hitherto been documented by incomplete (Lagerpeton 1) or unusually specialized forms (pterosaurs and Silesaurus 2). Recently, a variety of Silesaurus-like taxa have been reported from the Triassic period of both Gondwana and Laurasia, but their relationships to each other and to dinosaurs remain a subject of debate3, 4, 5. Here we report on a new avian-line archosaur from the early Middle Triassic (Anisian) of Tanzania. Phylogenetic analysis places Asilisaurus kongwe gen. et sp. nov. as an avian-line archosaur and a member of the Silesauridae, which is here considered the sister taxon to Dinosauria. Silesaurids were diverse and had a wide distribution by the Late Triassic, with a novel ornithodiran bauplan including leaf-shaped teeth, a beak-like lower jaw, long, gracile limbs, and a quadrupedal stance. Our analysis suggests that the dentition and diet of silesaurids, ornithischians and sauropodomorphs evolved independently from a plesiomorphic carnivorous form. As the oldest avian-line archosaur, Asilisaurus demonstrates the antiquity of both Ornithodira and the dinosaurian lineage. The initial diversification of Archosauria, previously documented by crocodilian-line archosaurs in the Anisian6, can now be shown to include a contemporaneous avian-line radiation. The unparalleled taxonomic diversity of the Manda archosaur assemblage indicates that archosaur diversification was well underway by the Middle Triassic or earlier.