Recumbirostran ‘microsaurs’ are not amniotes

Amniota is a tetrapod clade that includes extant mammals and reptiles, including birds, as well as a wealth of extinct taxa extending over 318 million years of Earth’s history. For over three decades, Amniota has been treated as a crown group, but the content of stem amniotes has varied among large-scale analyses of Palaeozoic tetrapods. Nine recent phylogenetic studies on Palaeozoic tetrapods, all derived from each other, have potentially changed our perspectives on amniote origins and early evolution. The first of these analyses posited that two particular clades of Palaeozoic tetrapods, Recumbirostra and Lysorophia, both previously considered to be lepospondyl stem-amniotes, are actually amniotes. This raises the possibility that early amniotes included both small fossorial and terrestrial animals, some of which show strong evidence of limb reduction. Here we show that the strange position of these small, often fossorial and limb-reduced tetrapods in Amniota is the result of two fundamental flaws: (1) the phylogenetic analysis combines two separate data matrices that were each designed to test only patterns of relationships among early anamniote tetrapods; and (2) the absence of key amniote characters. Our reanalysis of Palaeozoic tetrapod interrelationships reinstates recumbirostrans and lysorophians outside Amniota. The results of our study impact other recent studies that have simply accepted the codings in the data matrix that resulted in the hypothesis that recumbirostrans are amniotes and raises important questions about the initial diversification of amniotes and the evolution of fossoriality and limb reduction among tetrapods.羊膜动物（Amniota）是一支四足类演化支，涵盖现生哺乳类、爬行类（含鸟类），同时包含跨越地球3.18亿年演化历史的大量已灭绝类群。三十余年来，羊膜动物一直被视为冠群（crown group），但干群羊膜动物（stem amniotes）的范畴在各项古生代四足类大型系统发育分析中始终存在差异。近期共有9项基于古生代四足类的系统发育研究，且各项研究均衍生自前序工作，这为我们重新认识羊膜动物的起源与早期演化提供了新的视角。其中第一项研究提出，两类此前被归为壳椎类（Lepospondyli）干群羊膜动物的古生代四足类演化支——缓行类（Recumbirostra）与懒螈类（Lysorophia），实际上隶属于羊膜动物。这一结论引出了新的科学可能性：早期羊膜动物同时包含小型穴居与陆生类群，其中部分类群展现出显著的肢体退化特征。本研究表明，这类小型、通常为穴居且肢体退化的四足类被错误置于羊膜动物演化支中的异常位置，源于两处根本性缺陷：(1) 系统发育分析混用了两份原本仅用于验证无羊膜四足类（anamniote tetrapods）内部亲缘关系模式的数据矩阵；(2) 缺失了关键的羊膜动物性状。我们对古生代四足类亲缘关系的重新分析，将缓行类与懒螈类重新归入羊膜动物演化支之外。本研究结果对近期那些直接采用该数据矩阵的性状编码、从而得出缓行类属于羊膜动物这一假说的其他研究具有重要影响，同时也为羊膜动物的初始辐射演化、四足类穴居习性与肢体退化的演化历程提出了关键科学问题。