Beijing Museum of Natural History, Beijing 100050, China.
Committee on Evolutionary Biology, The University of Chicago, Chicago, Illinois 60637, USA.
Nature. 2017 Aug 17;548(7667):291-296. doi: 10.1038/nature23476. Epub 2017 Aug 9.
Stem mammaliaforms are Mesozoic forerunners to mammals, and they offer critical evidence for the anatomical evolution and ecological diversification during the earliest mammalian history. Two new eleutherodonts from the Late Jurassic period have skin membranes and skeletal features that are adapted for gliding. Characteristics of their digits provide evidence of roosting behaviour, as in dermopterans and bats, and their feet have a calcaneal calcar to support the uropagatium as in bats. The new volant taxa are phylogenetically nested with arboreal eleutherodonts. Together, they show an evolutionary experimentation similar to the iterative evolutions of gliders within arboreal groups of marsupial and placental mammals. However, gliding eleutherodonts possess rigid interclavicle-clavicle structures, convergent to the avian furculum, and they retain shoulder girdle plesiomorphies of mammaliaforms and monotremes. Forelimb mobility required by gliding occurs at the acromion-clavicle and glenohumeral joints, is different from and convergent to the shoulder mobility at the pivotal clavicle-sternal joint in marsupial and placental gliders.
哺乳形类动物是中生代哺乳动物的先驱，它们为最早的哺乳动物历史时期的解剖进化和生态多样化提供了关键证据。来自晚侏罗世的两种新的离兽类具有适应滑翔的皮肤膜和骨骼特征。它们的指骨特征提供了栖息行为的证据，类似于皮翼目动物和蝙蝠，而它们的脚有跟骨距骨来支撑类似于蝙蝠的中距骨。新的会飞的分类群在系统发育上与树栖离兽类有关。它们共同展示了一种类似于有袋类和胎盘类哺乳动物树栖类群中滑翔动物的迭代进化的进化实验。然而，滑翔离兽类具有刚性的锁骨间-锁骨结构，类似于鸟类的叉骨，并且它们保留了哺乳形类动物和单孔目动物的肩带原始特征。滑翔所需的前肢活动性发生在肩峰-锁骨和盂肱关节处，与有袋类和胎盘类滑翔动物中关键锁骨-胸骨关节处的肩部活动性不同但趋同。
