The estimated Permo–Carboniferous origin of caecilians leaves a gap exceeding 70 million years between putative Palaeozoic relatives and Eocaecilia 1. Of the nine tetrapod lineages surviving from the Triassic to the present day 21, caecilians have the most depauperate fossil record, with only 11 total occurrences 22 of these, only Rubricacaecilia monbaroni 23 and Eocaecilia micropodia 11, 12 represent unambiguous stem caecilians. 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. The provenance of these fossils suggests a Pangaean equatorial origin for caecilians, implying that living caecilian biogeography reflects conserved aspects of caecilian function and physiology 19, in combination with vicariance patterns driven by plate tectonics 20. 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 orbits 17 and the tentacular organ 18. 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. 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. ![]() The estimated Palaeozoic era gymnophionan–batrachian molecular divergence 1 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. Living amphibians (Lissamphibia) include frogs and salamanders (Batrachia) and the limbless worm-like caecilians (Gymnophiona).
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