Triassic stem caecilian supports dissorophoid origin of living amphibians

Abstract

Living amphibians (Lissamphibia) include frogs and salamanders (Batrachia) and the limbless worm-like caecilians (Gymnophiona). 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– 6} . Recent studies find a monophyletic Batrachia within dissorophoid temnospondyls ^{7– 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 orbits ¹⁷ and the tentacular organ ¹⁸ . 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 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.

Abstract

Analysis of fossils of the oldest known caecilian provide insights into the origin and morphological and functional evolution of caecilians.