Triassic stem caecilian supports dissorophoid origin of living amphibians.

Living amphibians (Lissamphibia) include frogs and salamanders (Batrachia) and the limbless worm-like caecilians (Gymnophiona). The estimated Palaeozoic era gymnophionan-batrachian molecular divergence suggests a major gap in the record of crown lissamphibians prior to their earliest fossil occurrences in the Triassic period. Recent studies find a monophyletic Batrachia within dissorophoid temnospondyls, but the absence of pre-Jurassic period caecilian fossils has made their relationships to batrachians and affinities to Palaeozoic tetrapods controversial. 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, reduced orbits and the tentacular organ. 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, in combination with vicariance patterns driven by plate tectonics. 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.