Mann Arjan, Pardo Jason D, Maddin Hillary C
Department of Paleobiology, National Museum of Natural History, Smithsonian Institution, Washington DC, USA.
Department of Comparative Biology and Experimental Medicine, University of Calgary, Calgary, Alberta, Canada.
Nat Ecol Evol. 2022 May;6(5):614-621. doi: 10.1038/s41559-022-01698-y. Epub 2022 Mar 28.
Among living tetrapods, many lineages have converged on a snake-like body plan, where extreme axial elongation is accompanied by reduction or loss of paired limbs. However, when and how this adaptive body plan first evolved in amniotes remains poorly understood. Here, we provide insights into this question by reporting on a new taxon of molgophid recumbirostran, Nagini mazonense gen. et sp. nov., from the Francis Creek Shale (309-307 million years ago) of Illinois, United States, that exhibits extreme axial elongation and corresponding limb reduction. The molgophid lacks entirely the forelimb and pectoral girdle, thus representing the earliest occurrence of complete loss of a limb in a taxon recovered phylogenetically within amniotes. This forelimb-first limb reduction is consistent with the pattern of limb reduction that is seen in modern snakes and contrasts with the hindlimb-first reduction process found in many other tetrapod groups. Our findings suggest that a snake-like limb-reduction mechanism may be operating more broadly across the amniote tree.
在现存的四足动物中，许多谱系都演化出了类似蛇的身体形态，其特征是身体轴向极度拉长，同时成对的四肢缩小或消失。然而，这种适应性身体形态在羊膜动物中最初是何时以及如何演化的，目前仍知之甚少。在此，我们通过报道一种来自美国伊利诺伊州弗朗西斯溪页岩（距今3亿900万至3亿700万年前）的新型平卧鼻类蚓螈——马佐内纳吉尼蚓螈（Nagini mazonense），为这个问题提供了新的见解。该物种表现出极度的身体轴向拉长和相应的肢体缩小。这种蚓螈完全没有前肢和肩带，因此代表了在羊膜动物系统发育中最早出现的一个类群完全失去肢体的情况。这种前肢先于后肢的肢体缩小模式与现代蛇类中观察到的肢体缩小模式一致，与许多其他四足动物类群中后肢先于前肢的缩小过程形成对比。我们的研究结果表明，类似蛇类的肢体缩小机制可能在羊膜动物谱系中更为广泛地存在。
