A Triassic stem-salamander from Kyrgyzstan and the origin of salamanders
A Triassic stem-salamander from Kyrgyzstan and the origin of salamanders
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DOI：
10.1073/pnas.2001424117
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发表时间：
2020-05
期刊：
影响因子：
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通讯作者：
R. Schoch;R. Werneburg;S. Voigt
中科院分区：
文献类型：
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作者：
R. Schoch;R. Werneburg;S. Voigt
现代两栖动物的起源仍然存在争议，特别是蝾螈的化石记录仍然很差。它们微小而脆弱的骨骼很少被保存在中生代早期的岩石中，而这一时期被认为是它们的起源。在这里，我们报告了来自中亚吉尔吉斯斯坦的2.3亿年前的化石，提供了蝾螈最古老的证据。它们使我们能够重建蝾螈身体计划进化的关键步骤，与古代两栖动物temnospondyls共享许多特征。这些发现将蝾螈的岩石记录向后推了60到74 Ma，同时弥合了蝾螈、青蛙和temnospondyls之间的巨大解剖学差距。现存两栖动物的起源仍然很模糊，只有少数中生代早期的主干类群已知，而不是一个更好的化石记录，从中侏罗世开始。最近，无尾类已被追溯到早三叠世的形式和caecilians已被追溯到晚侏罗世的Eocaecilia，这两个都支持逐步收购的apomorphies。然而，从侏罗纪中期的岩石中发现的最古老的茎蝾螈，对该分支的起源几乎没有什么启示。蝾螈和其他利桑菲比亚类以及古生代四足动物之间的差距仍然相当大。在这里，我们报告了一个新的标本Triasssurus sixtelae，迄今为止神秘的四足动物中/晚三叠世吉尔吉斯斯坦，我们确定为地质最古老的干组蝾螈。这不仅揭示了蝾螈身体结构的早期进化，也揭示了整个群体的起源。新的第二个标本来自与正模标本相同的地层，即吉尔吉斯斯坦西南部的Madygen组。它揭示了一系列的蝾螈字符在这个分类群，推前的岩石记录urodeles至少60至74马（Carnian-Ekonian）。此外，这种茎蝾螈与temnospondyls，特别是鳃龙科和amphibamiforms共享近形特征。
Significance The origin of modern amphibians remains controversial, and especially the fossil record of salamanders remains poor. Their tiny, feeble skeletons are rarely preserved in rocks of the early Mesozoic era, the time frame in which they are believed to have originated. Here we report 230 million-year-old fossils from Kyrgyzstan, Inner Asia, providing the most ancient evidence of salamanders. They enable us to reconstruct crucial steps in the evolution of the salamander body plan, sharing numerous features with ancient amphibians, the temnospondyls. These finds push back the rock record of salamanders by 60 to 74 Ma and at the same time bridge the wide anatomic gap among salamanders, frogs, and temnospondyls. The origin of extant amphibians remains largely obscure, with only a few early Mesozoic stem taxa known, as opposed to a much better fossil record from the mid-Jurassic on. In recent time, anurans have been traced back to Early Triassic forms and caecilians have been traced back to the Late Jurassic Eocaecilia, both of which exemplify the stepwise acquisition of apomorphies. Yet the most ancient stem-salamanders, known from mid-Jurassic rocks, shed little light on the origin of the clade. The gap between salamanders and other lissamphibians, as well as Paleozoic tetrapods, remains considerable. Here we report a new specimen of Triassurus sixtelae, a hitherto enigmatic tetrapod from the Middle/Late Triassic of Kyrgyzstan, which we identify as the geologically oldest stem-group salamander. This sheds light not only on the early evolution of the salamander body plan, but also on the origin of the group as a whole. The new, second specimen is derived from the same beds as the holotype, the Madygen Formation of southwestern Kyrgyzstan. It reveals a range of salamander characters in this taxon, pushing back the rock record of urodeles by at least 60 to 74 Ma (Carnian–Bathonian). In addition, this stem-salamander shares plesiomorphic characters with temnospondyls, especially branchiosaurids and amphibamiforms.