Müller and Garcia (Biology Letters).tnt from A paraphyletic ‘Silesauridae' as an alternative hypothesis for the initial radiation of ornithischian dinosaurs

Whereas ornithischian dinosaurs are well known from Jurassic and Cretaceous deposits, deciphering the origin and early evolution of the group remains one of the hardest challenges for palaeontologists. So far, there are no unequivocal records of ornithischians from Triassic beds. Here, we present an alternative evolutionary hypothesis that suggests consideration of traditional ‘silesaurids' as a group of low-diversity clades representing a stem group leading to core ornithischians (i.e. unambiguous ornithischians, such as <i>Heterodontosaurus tucki</i>). This is particularly interesting because it fills most of the ghost lineages that emerge from the Triassic. Following the present hypothesis, the lineage that encompasses the Jurassic ornithischians evolved from ‘silesaurids' during the Middle to early Late Triassic, while typical ‘silesaurids' shared the land ecosystems with their relatives until the Late Triassic, when the group completely vanished. Therefore, Ornithischia changes from an obscure to a well-documented clade in the Triassic and is represented by records from Gondwana and Laurasia. Furthermore, according to the present hypothesis, Ornithischia was the first group of dinosaurs to adopt an omnivorous/herbivorous diet. However, this behaviour was achieved as a secondary step instead of an ancestral condition for ornithischians, as the earliest member of the clade is a faunivorous taxon. This pattern was subsequently followed by sauropodomorph dinosaurs. Indeed, the present scenario favours the independent acquisition of an herbivorous diet for ornithischians and sauropodomorphs during the Triassic, whereas the previous hypotheses suggested the independent acquisition for sauropodomorphs, ornithischians, and ‘silesaurids'.

尽管鸟臀目恐龙（Ornithischia）在侏罗纪与白垩纪地层中已有大量确凿的化石记录，但厘清该类群的起源与早期演化仍是古生物学家面临的最棘手挑战之一。截至目前，三叠纪地层中尚未发现任何可明确归属于鸟臀目的化石记录。
本文提出一种全新的演化假说，主张将传统分类中的‘西里龙类（silesaurids）’视为一支多样性较低的干群支系，代表通向核心鸟臀类的演化节点——核心鸟臀类即明确的鸟臀目恐龙，如塔克异齿龙（*Heterodontosaurus tucki*）。这一假说极具研究价值，因其填补了三叠纪时期出现的绝大多数幽灵支系空白。
根据本假说，侏罗纪鸟臀目恐龙的演化支系于中三叠世至晚三叠世早期由‘西里龙类’演化而来；而典型的西里龙类与其近亲共同占据陆地生态系统，直至晚三叠世该类群彻底灭绝。
由此，鸟臀目在三叠纪的演化记录从原本鲜为人知的类群转变为证据充分的支系，其化石分布覆盖冈瓦纳大陆（Gondwana）与劳亚大陆（Laurasia）。
此外，依据本假说，鸟臀目是首批采取杂食/植食性食性的恐龙类群。但这一食性特征属于次生演化获得的性状，而非鸟臀目的祖先状态——该支系的最早成员为食虫性类群。这一演化模式随后也在蜥脚形亚目（sauropodomorph）恐龙中重现。
实际上，当前的演化场景支持鸟臀目与蜥脚形亚目在三叠纪时期独立演化出植食性食性；而此前的演化假说则认为，蜥脚形亚目、鸟臀目以及西里龙类三者均独立获得了植食性食性。