Department of Earth Sciences, The University of Hong Kong, Pokfulam, Hong Kong SAR, China.
School of Life Sciences, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong SAR, China.
BMC Biol. 2022 May 12;20(1):101. doi: 10.1186/s12915-022-01294-3.
Birds are key indicator species in extant ecosystems, and thus we would expect extinct birds to provide insights into the nature of ancient ecosystems. However, many aspects of extinct bird ecology, particularly their diet, remain obscure. One group of particular interest is the bizarre toothed and long-snouted longipterygid birds. Longipterygidae is the most well-understood family of enantiornithine birds, the dominant birds of the Cretaceous period. However, as with most Mesozoic birds, their diet remains entirely speculative.
To improve our understanding of longipterygids, we investigated four proxies in extant birds to determine diagnostic traits for birds with a given diet: body mass, claw morphometrics, jaw mechanical advantage, and jaw strength via finite element analysis. Body mass of birds tended to correspond to the size of their main food source, with both carnivores and herbivores splitting into two subsets by mass: invertivores or vertivores for carnivores, and granivores + nectarivores or folivores + frugivores for herbivores. Using claw morphometrics, we successfully distinguished ground birds, non-raptorial perching birds, and raptorial birds from one another. We were unable to replicate past results isolating subtypes of raptorial behaviour. Mechanical advantage was able to distinguish herbivorous diets with particularly high values of functional indices, and so is useful for identifying these specific diets in fossil taxa, but overall did a poor job of reflecting diet. Finite element analysis effectively separated birds with hard and/or tough diets from those eating foods which are neither, though could not distinguish hard and tough diets from one another. We reconstructed each of these proxies in longipterygids as well, and after synthesising the four lines of evidence, we find all members of the family but Shengjingornis (whose diet remains inconclusive) most likely to be invertivores or generalist feeders, with raptorial behaviour likely in Longipteryx and Rapaxavis.
This study provides a 20% increase in quantitatively supported fossil bird diets, triples the number of diets reconstructed in enantiornithine species, and serves as an important first step in quantitatively investigating the origins of the trophic diversity of living birds. These findings are consistent with past hypotheses that Mesozoic birds occupied low trophic levels.
鸟类是现存生态系统中的关键指示物种，因此我们期望已灭绝的鸟类能为我们提供有关古代生态系统的本质的见解。然而，许多已灭绝鸟类的生态学方面，特别是它们的饮食，仍然不清楚。特别有趣的一类是奇异的有齿长吻长翼鸟。长翼鸟科是白垩纪时期优势鸟类——手盗龙类中最被了解的一科。然而，与大多数中生代鸟类一样，它们的饮食仍然完全是推测性的。
为了增进我们对手盗龙类的了解，我们研究了现生鸟类中的四个指标，以确定给定饮食鸟类的诊断特征：体重、爪形态测量学、下颚机械优势以及通过有限元分析得出的下颚强度。鸟类的体重往往与它们的主要食物来源的大小相对应，肉食动物和草食动物都按体重分为两个亚组：肉食动物中的食虫动物或食植物动物，草食动物中的食谷动物+食蜜动物或食叶动物+食果动物。通过爪形态测量学，我们成功地将地栖鸟类、非猛禽栖鸟类和猛禽鸟类彼此区分开来。我们无法复制过去孤立猛禽行为亚型的结果。机械优势能够区分具有高功能指数的草食性饮食，因此对于识别化石类群中的这些特定饮食非常有用，但总体而言，它对饮食的反映效果不佳。有限元分析有效地将具有坚硬和/或坚韧饮食的鸟类与食用既不坚硬也不坚韧食物的鸟类区分开来，尽管它无法区分坚硬和坚韧的饮食。我们还重建了长翼鸟科中的每一个指标，在综合了这四条证据后，我们发现该科的所有成员，除了圣约翰鸟（其饮食仍不确定），最有可能是食虫动物或杂食动物，而在长翼鸟和 Rapaxavis 中可能存在猛禽行为。
这项研究为定量支持的化石鸟类饮食增加了 20%，将手盗龙类物种重建的饮食数量增加了两倍，并为定量研究现生鸟类的营养多样性起源提供了重要的第一步。这些发现与过去的假设一致，即中生代鸟类处于较低的营养水平。
