Reinterpretation of a previously described Jehol bird clarifies early trophic evolution in the Ornithuromorpha

STM35-3 from the Yixian Formation is the only Early Cretaceous ornithuromorph preserving direct evidence of granivory. The crop contains numerous seeds and the preservation of gastroliths presumably within the ventriculus indicates this diet was paired with the presence of a gastric mill as in living granivorous birds. STM35-3 was originally referred to Hongshanornis longicresta, member of a diverse clade of small, basal ornithuromorphs with elongate hindlimbs known as the Hongshanornithidae. Hindlimb proportions suggest that hongshanornithids were wading birds, an ecological inference somewhat in conflict with direct evidence suggesting Hongshanornis fed on seeds. However, close inspection of STM35-3 reveals that the specimen represents a new species not closely related to hongshanornithids, distinguished by large forelimbs that exceed the length of the hindlimbs, robust and narrow coracoids, and a delicate edentulous rostrum. In contrast, all hongshanornithids have hindlimbs that far ...

义县组的STM35-3标本是早白垩世唯一保存了食籽性（Granivory）直接证据的鸟胸类（Ornithuromorph）。其嗉囊中含有大量种子，且推测保存在肌胃内的胃石表明，这种食性与现生食籽鸟类一样，伴随有胃磨的存在。STM35-3最初被归入长冠红山鸟（Hongshanornis longicresta），后者属于一个多样化的小型基干鸟胸类支系——红山鸟科（Hongshanornithidae），该科鸟类具有修长的后肢。后肢比例表明红山鸟科鸟类为涉禽，这一生态推断与红山鸟以种子为食的直接证据存在一定矛盾。然而，对STM35-3的详细检查显示，该标本代表一个与红山鸟科亲缘关系较远的新物种，其特征为前肢较大且长度超过后肢、喙骨粗壮狭窄，以及精致的无齿喙。相比之下，所有红山鸟科鸟类的后肢都远……