Centro de Apoio à Pesquisa Paleontológica da Quarta Colônia, Universidade Federal de Santa Maria, São João do Polêsine, RS, 598, 97230-000, Brazil.
Biol Lett. 2020 Aug;16(8):20200417. doi: 10.1098/rsbl.2020.0417. Epub 2020 Aug 26.
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 ). 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'.
虽然鸟臀目恐龙在侏罗纪和白垩纪的沉积物中广为人知,但该类群的起源和早期演化仍然是古生物学家面临的最大挑战之一。到目前为止,还没有三叠纪地层中鸟臀目恐龙的确切记录。在这里,我们提出了一个替代的进化假说,即认为传统的“空尾龙类”是一个多样性较低的分支群,代表了一个通向核心鸟臀目(即明确的鸟臀目,如)的祖先群。这一点尤其有趣,因为它填补了三叠纪时期出现的大部分幽灵谱系。根据目前的假说,包含侏罗纪鸟臀目恐龙的谱系是从中三叠世到早晚三叠世从“空尾龙类”进化而来的,而典型的“空尾龙类”与它们的亲缘关系一起共享陆地生态系统,直到晚三叠世,该类群完全消失。因此,鸟臀目恐龙在三叠纪从一个模糊的分支转变为一个有充分记录的分支,在冈瓦纳和劳亚古陆都有记录。此外,根据目前的假说,鸟臀目恐龙是第一个采用杂食/草食性饮食的恐龙群体。然而,这种行为是作为一个次要步骤而不是鸟臀目恐龙的祖征实现的,因为该类群的最早成员是一个以动物为食的分类单元。随后,蜥脚形亚目恐龙也出现了这种模式。事实上,目前的情况有利于在三叠纪时期,鸟臀目恐龙和蜥脚形亚目恐龙独立获得草食性饮食,而之前的假说则表明蜥脚形亚目恐龙、鸟臀目恐龙和“空尾龙类”独立获得了草食性饮食。