Wang Min, Lloyd Graeme T, Zhang Chi, Zhou Zhonghe
Key Laboratory of Vertebrate Evolution and Human Origins, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Beijing 100044, People's Republic of China.
CAS Center for Excellence in Life and Paleoenvironment, Beijing 100044, People's Republic of China.
Proc Biol Sci. 2021 Feb 10;288(1944):20203105. doi: 10.1098/rspb.2020.3105. Epub 2021 Feb 3.
The origin of birds from non-avian theropod dinosaurs is one of the greatest transitions in evolution. Shortly after diverging from other theropods in the Late Jurassic, Mesozoic birds diversified into two major clades-the Enantiornithes and Ornithuromorpha-acquiring many features previously considered unique to the crown group along the way. Here, we present a comparative phylogenetic study of the patterns and modes of Mesozoic bird skeletal morphology and limb proportions. Our results show that the major Mesozoic avian groups are distinctive in discrete character space, but constrained in a morphospace defined by limb proportions. The Enantiornithines, despite being the most speciose group of Mesozoic birds, are much less morphologically disparate than their sister clade, the Ornithuromorpha-the clade that gave rise to living birds, showing disparity and diversity were decoupled in avian history. This relatively low disparity suggests that diversification of enantiornithines was characterized in exhausting fine morphologies, whereas ornithuromorphs continuously explored a broader array of morphologies and ecological opportunities. We suggest this clade-specific evolutionary versatility contributed to their sole survival of the end-Cretaceous mass extinction.
鸟类起源于非鸟类兽脚亚目恐龙是进化史上最重大的转变之一。在晚侏罗世与其他兽脚亚目恐龙分道扬镳后不久，中生代鸟类分化为两个主要分支——反鸟亚纲和今鸟型类——在此过程中获得了许多以前被认为是冠群独有的特征。在这里，我们对中生代鸟类骨骼形态和肢体比例的模式及方式进行了比较系统发育研究。我们的结果表明，中生代主要鸟类类群在离散特征空间中是独特的，但在由肢体比例定义的形态空间中受到限制。反鸟类尽管是中生代鸟类中种类最多的类群，但其形态差异远小于其姐妹分支今鸟型类——即产生现存鸟类的分支，这表明在鸟类进化史上，差异和多样性是脱钩的。这种相对较低的差异表明，反鸟类的多样化特征是穷尽了精细的形态，而今鸟型类则不断探索更广泛的形态和生态机会。我们认为，这种特定分支的进化多功能性促成了它们在白垩纪末大灭绝中唯一幸存下来。
