Museum of Comparative Zoology and Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA 02138, USA.
Integrative Research Center, Field Museum of Natural History, Chicago, IL 60605, USA.
Science. 2018 Sep 21;361(6408):1249-1252. doi: 10.1126/science.aar3126.
A unique characteristic of mammals is a vertebral column with anatomically distinct regions, but when and how this trait evolved remains unknown. We reconstructed vertebral regions and their morphological disparity in the extinct forerunners of mammals, the nonmammalian synapsids, to elucidate the evolution of mammalian axial differentiation. Mapping patterns of regionalization and disparity (heterogeneity) across amniotes reveals that both traits increased during synapsid evolution. However, the onset of regionalization predates increased heterogeneity. On the basis of inferred homology patterns, we propose a "pectoral-first" hypothesis for region acquisition, whereby evolutionary shifts in forelimb function in nonmammalian therapsids drove increasing vertebral modularity prior to differentiation of the vertebral column for specialized functions in mammals.
哺乳动物的一个独特特征是具有解剖上明显不同的区域的脊柱，但这个特征是如何以及何时进化的仍然未知。我们重建了已灭绝的哺乳动物先驱——合弓类动物的脊椎区域及其形态差异，以阐明哺乳动物轴向分化的进化。对羊膜动物的区域化和差异（异质性）模式的映射表明，这两个特征在合弓类动物的进化过程中都增加了。然而，区域化的开始早于异质性的增加。根据推断的同源模式，我们提出了一个“前肢优先”的区域获取假说，即非哺乳动物兽孔类动物前肢功能的进化转变，在哺乳动物的专门功能导致脊柱分化之前，就推动了脊椎的模块化增加。
