Abstract

Living amphibians (Lissamphibia) include frogs and salamanders (Batrachia), and the limbless worm-like caecilians (Gymnophiona). The estimated Paleozoic 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 caecilian fossils has made their relationships to batrachians and affinities to Paleozoic tetrapods controversial. Here we report the geologically-oldest stem caecilian, a crown lissamphibian, from the Late Triassic of Arizona, USA, extending the caecilian record by ~35 Ma. 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 Pangean 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 unique to caecilians alongside features shared with batrachian and dissorophoid temnospondyls, providing new and compelling evidence supporting a single origin of living amphibians within dissorophoid temnospondyls.