Living amphibians (Lissamphibia) include frogs and salamanders (Batrachia) and the limbless worm-like caecilians (Gymnophiona). The estimated Palaeozoic era gymnophionan–batrachian molecular divergence¹ suggests a major gap in the record of crown lissamphibians prior to their earliest fossil occurrences in the Triassic period^2,3,4,5,6. Recent studies find a monophyletic Batrachia within dissorophoid temnospondyls^7,8,9,10, but the absence of pre-Jurassic period caecilian fossils^11,12 has made their relationships to batrachians and affinities to Palaeozoic tetrapods controversial^1,8,13,14. Here we report the geologically oldest stem caecilian—a crown lissamphibian from the Late Triassic epoch of Arizona, USA—extending the caecilian record by around 35 million years. These fossils illuminate the tempo and mode of early caecilian morphological and functional evolution, demonstrating a delayed acquisition of musculoskeletal features associated with fossoriality in living caecilians, including the dual jaw closure mechanism^15,16, reduced orbits¹⁷ and the tentacular organ¹⁸. The provenance of these fossils suggests a Pangaean equatorial origin for caecilians, implying that living caecilian biogeography reflects conserved aspects of caecilian function and physiology¹⁹, in combination with vicariance patterns driven by plate tectonics²⁰. These fossils reveal a combination of features that is unique to caecilians alongside features that are shared with batrachian and dissorophoid temnospondyls, providing new and compelling evidence supporting a single origin of living amphibians within dissorophoid temnospondyls.

现存的两栖动物 (Lissamphibia) 包括青蛙和火蜥蜴 (Batrachia) 以及无肢蠕虫状的蛔虫 (Gymnophiona)。估计的古生代裸子动物-蛙类分子差异^{1表明，在三叠纪时期2,3,4,5,6}最早的化石出现之前，冠软体动物的记录存在重大差距。最近的研究发现双足类短椎动物^7,8,9,10中存在单系蛙类，但由于缺乏前侏罗纪时期的蛔虫化石^{11,12 ，}它们与蛙类的关系以及与古生代四足动物的亲缘关系存在争议^1,8,13,14. 在这里，我们报告了地质学上最古老的有茎盲目动物——一种来自美国亚利桑那州晚三叠世的冠软体动物——将盲目动物的记录延长了大约 3500 万年。这些化石阐明了早期盲肠动物形态和功能进化的节奏和模式，证明了在活的盲肠动物中延迟获得与化石相关的肌肉骨骼特征，包括双颌闭合机制 15,16 、减少的眼眶¹⁷和^触手器官¹⁸。这些化石的出处表明，盲目动物的盘古大陆赤道起源，意味着活的盲目生物地理学反映了盲目功能和生理学的保守方面¹⁹, 结合由板块构造²⁰驱动的变异模式。这些化石揭示了 caecilians 独有的特征组合，以及与 batrachian 和 dissorophoid temnospondyls 共有的特征，提供了新的和令人信服的证据支持 dissorophoid temnospondyls 中现存两栖动物的单一起源。