Birds are some of the most diverse organisms on Earth, with species inhabiting a wide variety of niches across every major biome. As such, birds are vital to our understanding of modern ecosystems. Unfortunately, our understanding of the evolutionary history of modern ecosystems is hampered by knowledge gaps in the origin of modern bird diversity and ecosystem ecology. A crucial part of addressing these shortcomings is improving our understanding of the earliest birds, the non-avian avialans (i.e. non-crown birds), particularly of their diet. The diet of non-avian avialans has been a matter of debate, in large part because of the ambiguous qualitative approaches that have been used to reconstruct it. Here we review methods for determining diet in modern and fossil avians (i.e. crown birds) as well as non-avian theropods, and comment on their usefulness when applied to non-avian avialans. We use this to propose a set of comparable, quantitative approaches to ascertain fossil bird diet and on this basis provide a consensus of what we currently know about fossil bird diet. While no single approach can precisely predict diet in birds, each can exclude some diets and narrow the dietary possibilities. We recommend combining (i) dental microwear, (ii) landmark-based muscular reconstruction, (iii) stable isotope geochemistry, (iv) body mass estimations, (v) traditional and/or geometric morphometric analysis, (vi) lever modelling, and (vii) finite element analysis to reconstruct fossil bird diet accurately. Our review provides specific methodologies to implement each approach and discusses complications future researchers should keep in mind. We note that current forms of assessment of dental mesowear, skull traditional morphometrics, geometric morphometrics, and certain stable isotope systems have yet to be proven effective at discerning fossil bird diet. On this basis we report the current state of knowledge of non-avian avialan diet which remains very incomplete. The ancestral dietary condition in non-avian avialans remains unclear due to scarce data and contradictory evidence in Archaeopteryx. Among early non-avian pygostylians, Confuciusornis has finite element analysis and mechanical advantage evidence pointing to herbivory, whilst Sapeornis only has mechanical advantage evidence indicating granivory, agreeing with fossilised ingested material known for this taxon. The enantiornithine ornithothoracine Shenqiornis has mechanical advantage and pedal morphometric evidence pointing to carnivory. In the hongshanornithid ornithuromorph Hongshanornis only mechanical advantage evidence indicates granivory, but this agrees with evidence of gastrolith ingestion in this taxon. Mechanical advantage and ingested fish support carnivory in the songlingornithid ornithuromorph Yanornis. Due to the sparsity of robust dietary assignments, no clear trends in non-avian avialan dietary evolution have yet emerged. Dietary diversity seems to increase through time, but this is a preservational bias associated with a predominance of data from the Early Cretaceous Jehol Lagerstätte. With this new framework and our synthesis of the current knowledge of non-avian avialan diet, we expect dietary knowledge and evolutionary trends to become much clearer in the coming years, especially as fossils from other locations and climates are found. This will allow for a deeper and more robust understanding of the role birds played in Mesozoic ecosystems and how this developed into their pivotal role in modern ecosystems.

中文翻译：

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基于现代和化石证据的早期鸟类的饮食及其重建的新框架

鸟类是地球上最多样化的生物之一，其物种栖息在每个主要生物群落的各种生态位中。因此，鸟类对于我们理解现代生态系统至关重要。不幸的是，我们对现代生态系统进化史的理解受到现代鸟类多样性起源和生态系统生态学知识空白的阻碍。解决这些缺点的一个关键部分是提高我们对最早的鸟类、非鸟类鸟类（即非冠鸟）的了解，特别是它们的饮食。非鸟类鸟类的饮食一直是一个有争议的问题，很大程度上是因为重建它的定性方法不明确。在这里，我们回顾了确定现代鸟类和化石鸟类（即冠鸟）以及非鸟类兽脚亚目动物饮食的方法，并评论了它们在应用于非鸟类鸟类时的有用性。我们用它来提出一套可比较的、定量的方法来确定化石鸟类的饮食，并在此基础上就我们目前对化石鸟类饮食的了解达成共识。虽然没有一种方法可以精确预测鸟类的饮食，但每种方法都可以排除某些饮食并缩小饮食的可能性。我们建议结合 ( i ) 牙齿微磨损、( ii ) 基于地标的肌肉重建、( iii ) 稳定同位素地球化学、( iv ) 体重估计、( v ) 传统和/或几何形态测量分析、( vi ) 杠杆建模和(七)有限元分析准确重建化石鸟类饮食。我们的评论提供了实施每种方法的具体方法，并讨论了未来研究人员应记住的复杂情况。我们注意到，目前对牙齿中磨损、头骨传统形态测量、几何形态测量和某些稳定同位素系统的评估形式尚未被证明能够有效识别化石鸟类饮食。在此基础上，我们报告了目前对非鸟类饮食的了解仍然非常不完整。由于始祖鸟的数据稀缺和相互矛盾的证据，非鸟类鸟类的祖先饮食状况仍不清楚。在早期的非鸟类尾柱类中，孔子鸟的有限元分析和机械优势证据表明其是食草动物，而萨佩鸟鸟仅具有表明谷食性的机械优势证据，这与该类群已知的化石摄入材料一致。对映鸟科鸟胸科Shenqiornis具有机械优势和足足形态测量证据，表明其属于肉食性动物。红山鸟科 Ornithuromorph红山鸟只有机械优势证据表明食谷，但这与该分类群中胃石摄入的证据一致。机械优势和摄入的鱼类支持松林鸟科鸟形亚鸟的肉食性。由于健全的饮食分配的稀疏性，非鸟类饮食进化尚未出现明显的趋势。饮食多样性似乎随着时间的推移而增加，但这是一种与早白垩世 Jehol Lagerstätte 的主导数据相关的保存偏差。通过这个新框架以及我们对当前非鸟类饮食知识的综合，我们预计饮食知识和进化趋势在未来几年将变得更加清晰，特别是随着来自其他地点和气候的化石的发现。这将使人们更深入、更深入地了解鸟类在中生代生态系统中所扮演的角色，以及鸟类如何发展成为现代生态系统中的关键角色。