Interrelationships, palaeobiogeography and early evolution of Stereospondylomorpha (Tetrapoda: Temnospondyli)

Abstract

The stereospondylomorph temnospondyls form a diverse group of early tetrapods that survived the Permian–Triassic extinction event and radiated during the Triassic. They encompass Carboniferous and Permian taxa from central and eastern Europe, such as ‘archegosauroids’, and early-divergent Gondwanan forms, such as rhinesuchids. By the Early Triassic, the group reached a worldwide distribution, with stereospondyls experiencing an impressive diversification and becoming major aquatic predators in fresh water, brackish, and even marine ecosystems. The origin of Stereospondyli dates back into the Permian, but the phylogenetic relationships of stereospondylomorphs, including ‘archegosauroids’ and the first stereospondyls remain unclear, representing the focus of the present study. Incorporating new records of the group to a broad revision of the different phylogenic hypotheses, a 37-taxon sample was scored for 221 morphological characters revised from previous works. The parsimony analysis resulted three most parsimonious trees (MPTs) of 737 steps. Their strict consensus tree depicts Sclerocephalidae and Intasuchidae as early-diverging stereospondylomorphs, ‘Archegosauroidea’ as paraphyletic array of taxa and Konzhukoviidae as the sister-group of a monophyletic Stereospondyli. An early-diverging and monophyletic Rhinesuchidae is divided into Australerpetinae and Rhinesuchinae, as the sister-group of the clade containing the Permian Peltobatrachus pustulatus, Arachana nigra, and a newly named clade (Superstes = ‘survivors’) of Triassic stereospondyls, formed by Lydekkerinidae and Neostereospondyli (Capitosauria and Trematosauria). Likelihood ancestral area reconstructions and time-ranging distributions along phylogeny provided a comprehensive description of early Stereospondylomorpha palaeobiogeography history. The initial evolution of the group took place in Laurasian areas (central and eastern Europe) during the Cisuralian and Guadalupian (early-mid Permian), with a broader Pangaean distribution for Platyoposauridae and Konzhukoviidae. Stereospondyls have Africa as their ancestral area, followed by dispersions to other Gondwanan regions during the Guadalupian and Lopingian (mid-late Permian), revealing a remarkable diversity previous to the P-Tr extinction. In the Triassic, Superstes greatly expanded across the Pangaea, highlighting another significant event in the evolution of Stereospondylomorpha.