Synthetic analysis of trophic diversity and evolution in Enantiornithes with new insights from Bohaiornithidae

Version 1 accompanies the reviewed preprint of this work, Version 2 accompanies the revised work, Version 3 reflects changes made during the editorial process.

Extended abstract:
The “opposite birds” Enantiornithines were the dominant birds of the Mesozoic, but our understanding of their ecology is still tenuous. In particular, diets of enantiornithine species have remained speculative until recently. We introduce new data on the enantiornithine family Bohaiornithidae, famous for their large size and strong teeth and claws. Alongside previously-published data on the earlier-diverging pengornithids and later-diverging longipterygids, we comment on the breadth of enantiornithine ecology and potential patterns in which it evolved.
Body mass, mechanical advantage, finite element analysis, and traditional morphometrics are compared between bohaiornithids and living birds. We find bohaiornithids ecologically diverse: Bohaiornis and Parabohaiornis resemble living plant-eating birds; Longusunguis resembles raptorial carnivores; Zhouornis resembles both fruit-eating birds and generalist feeders; and Shenqiornis and Sulcavis plausibly ate fish, plants, or a mix of both. This ecological diversity is wider than any other enantiornithine family studied previously.
With these new data, there is support for enantiornithines inhabiting nearly every trophic level. We predict the ancestral enantiornithine bird to have been a generalist. This suggests the ecological diversity of enantiornithine birds represents specialisation in taking foods their ancestors were already eating, rather than many dramatic changes in diet. However, more quantitative data from across the enantiornithine tree is needed to refine this prediction. By the Early Cretaceous, enantiornithine birds had diversified into a variety of ecological niches in a similar way to crown birds after the K-Pg extinction, adding to the body of evidence that traits unique to crown birds cannot completely explain their ecological success.

本版次（Version 1）伴随本研究的评审预印本，第二版次（Version 2）伴随修订后的作品，第三版次（Version 3）反映了编辑过程中的变更。

扩展摘要：
“对偶鸟”恩提奥鸟类是中生代的统治鸟类，但我们对它们生态学的认识仍显薄弱。特别是，恩提奥鸟类的饮食构成至今仍属推测性质。本研究引入了关于大型化、强健牙齿和爪子的恩提奥鸟科（Bohaiornithidae）的新数据。结合先前发表的关于早期分支的鹏鸟科（pengornithids）和晚期分支的长翼鸟科（longipterygids）的数据，我们对恩提奥鸟类的生态范围及其演化过程中的潜在模式进行了评论。

在比较波海鸟科与现存鸟类之间的体重、力学优势、有限元分析和传统形态计量学时，我们发现波海鸟科生态多样性丰富：波海鸟（Bohaiornis）和副波海鸟（Parabohaiornis）与现存的植食性鸟类相似；长趾鸟（Longusunguis）与捕食性肉食鸟类相似；周鸟（Zhouornis）与食果鸟类及杂食性鸟类相似；而沈其鸟（Shenqiornis）和凹喙鸟（Sulcavis）可能以鱼类、植物或两者的混合物为食。这种生态多样性比先前研究的任何恩提奥鸟科都更为广泛。

借助这些新数据，我们有理由相信恩提奥鸟类几乎占据了所有营养级。我们预测原始恩提奥鸟类可能是一种泛食性鸟类。这表明恩提奥鸟类生态多样性反映了在其祖先已食用的食物种类上的专门化，而非饮食构成的许多显著变化。然而，为了完善这一预测，需要来自恩提奥鸟类谱系更多定量数据。在早白垩纪，恩提奥鸟类已分化为各种生态位，其方式与K-Pg灭绝后冠鸟类（crown birds）的分化相似，这进一步丰富了证据，表明独特的冠鸟类特征不能完全解释其生态成功。