1. Department of Comparative Biology and Experimental Medicine, Faculty of Veterinary Medicine, University of Calgary, 3330 Hospital Drive, Calgary, Alberta T2N 4N1, Canada
2. Department of Biology, University of Toronto at Mississauga, 3359 Mississauga Road, Mississauga, Ontario L5L 1C6, Canada
3. Redpath Museum, McGill University, 859 Sherbrooke Street West, Montréal, Québec H3A 2K6, Canada
4. Department of Biology, California State University at San Bernardino, 5500 University Parkway, San Bernardino, California 92407-2307, USA

Correspondence to: Jason S. Anderson¹ Correspondence and requests for materials should be addressed to J.S.A. (Email: janders@ucalgary.ca).

Abstract

The origin of extant amphibians (Lissamphibia: frogs, salamanders and caecilians) is one of the most controversial questions in vertebrate evolution, owing to large morphological and temporal gaps in the fossil record^{1, 2, 3}. Current discussions focus on three competing hypotheses: a monophyletic origin within either Temnospondyli^{4, 5, 6, 7} or Lepospondyli^{8, 9, 10}, or a polyphyletic origin with frogs and salamanders arising among temnospondyls and caecilians among the lepospondyls^{11, 12, 13, 14, 15, 16}. Recent molecular analyses are also controversial, with estimations for the batrachian (frog–salamander) divergence significantly older than the palaeontological evidence supports^{17, 18}. Here we report the discovery of an amphibamid temnospondyl from the Early Permian of Texas that bridges the gap between other Palaeozoic amphibians and the earliest known salientians^{19, 20} and caudatans²¹ from the Mesozoic. The presence of a mosaic of salientian and caudatan characters in this small fossil makes it a key taxon close to the batrachian (frog and salamander) divergence. Phylogenetic analysis suggests that the batrachian divergence occurred in the Middle Permian, rather than the late Carboniferous as recently estimated using molecular clocks^{18, 22}, but the divergence with caecilians corresponds to the deep split between temnospondyls and lepospondyls, which is congruent with the molecular estimates.

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