Data from: 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 exceed the length of the forelimbs, coracoids with wide sternal margins, and small teeth throughout the upper and lower jaws. Reinterpretation of this new taxon, Eogranivora edentulata gen. et sp. nov, helps to clarify trophic driven patterns of tooth loss within the Ornithuromorpha. Apparent loss of the hallux may represent the first such occurrence in a Mesozoic bird and suggests a highly terrestrial lifestyle.产自义县组（Yixian Formation）的STM35-3标本，是目前已知唯一一件留存有直接食谷性证据的早白垩世鸟胸类（Ornithuromorpha）化石。其嗉囊（crop）内保存有大量种子，而疑似存于肌胃（ventriculus）中的胃石（gastroliths）则表明，该物种与现生食谷鸟类类似，配备砂囊（gastric mill）以适配其食性。该标本最初被归入长冠红山鸟（Hongshanornis longicresta），后者隶属于一类体型小巧、后肢修长的基干鸟胸类演化支（clade）——红山鸟科（Hongshanornithidae）。后肢比例特征曾推断红山鸟科为涉禽，这一生态推论与红山鸟属以种子为食的直接证据存在一定矛盾。但通过对STM35-3标本的细致重新检视，可发现其实为一类与红山鸟科亲缘关系较远的新物种：其前肢修长且长度超过后肢，喙突（coracoids）粗壮且形态狭长，同时拥有纤细无齿的吻部。与之形成鲜明对比的是，所有红山鸟科成员的后肢长度均远超前肢，喙突的胸骨缘宽阔，且上下颌均分布有小型牙齿。对这一新类群Eogranivora edentulata gen. et sp. nov的重新解读，有助于阐明鸟胸类演化中由营养驱动的牙齿丢失模式。该标本似乎缺失拇趾（hallux），这可能是中生代鸟类中首次出现的此类特征，同时也暗示该物种具有高度陆生的生活方式。