Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK.
Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, UK.
Proc Biol Sci. 2023 Mar 29;290(1995):20230160. doi: 10.1098/rspb.2023.0160. Epub 2023 Mar 15.
Skeletal pneumaticity is a key feature of extant avian structure and biology, which first evolved among the non-flying archosaurian ancestors of birds. The widespread presence of air-filled bones across the postcranial skeleton is unique to birds among living vertebrates, but the true extent of skeletal pneumaticity has never been quantitatively investigated-hindering fundamental insights into the evolution of this key avian feature. Here, we use microCT scans of fresh, frozen birds to directly quantify the fraction of humerus volume occupied by air across a phylogenetically diverse taxon sample to test longstanding hypotheses regarding the evolution and function of avian skeletal pneumatization. Among other insights, we document weak positive allometry of internal air volume with humeral size among pneumatized humeri and provide strong support that humeral size, body mass, aquatic diving, and the presence or absence of pneumaticity all have independent effects on cortical bone thickness. Our quantitative evaluation of humeral pneumaticity across extant avian phylogeny sheds new light on the evolution and ontogenetic progression of an important aspect of avian skeletal architecture, and suggests that the last common ancestor of crown birds possessed a highly pneumatized humerus.
骨骼气腔性是现生鸟类结构和生物学的一个关键特征，它首先在鸟类的非飞行恐龙祖先中演化而来。在现生脊椎动物中，空气填充骨骼广泛存在于后肢骨骼中，这是鸟类所特有的，但骨骼气腔性的真实程度从未被定量研究过，这阻碍了对这一关键鸟类特征进化的基本了解。在这里，我们使用新鲜冷冻鸟类的 microCT 扫描，直接量化了在一个系统发育多样化的分类群样本中肱骨体积被空气占据的比例，以检验关于鸟类骨骼气腔化的进化和功能的长期假说。除了其他见解之外，我们记录了充气肱骨中内部空气体积与肱骨大小之间的弱正异速生长，并有力地支持了肱骨大小、体重、水生潜水以及气腔化的存在与否对皮质骨厚度都有独立影响的假说。我们对现生鸟类系统发育中肱骨气腔性的定量评估，为鸟类骨骼结构的一个重要方面的进化和个体发育进展提供了新的认识，并表明冠群鸟类的最后共同祖先拥有一个高度充气的肱骨。
