The Early Evolution of Rhynchosaurs

Martín D Ezcurra,Felipe Montefeltro,Richard J Butler

doi:10.3389/fevo.2015.00142

Abstract

The rhynchosaurian archosauromorphs are an important and diverse group of fossil tetrapods that first appeared during the Early Triassic and probably became extinct during the early Late Triassic (early Norian). Here, the early evolution of rhynchosaurs during the Early and early Middle Triassic (Induan−Anisian: 252.2−242 Mya) is reviewed based on new anatomical observations and the implications of these observations for the taxonomy, phylogenetic relationships and macroevolutionary history of the group. A quantitative phylogenetic analysis recovered a paraphyletic genus Rhynchosaurus, with “Rhynchosaurus” brodiei more closely related to hyperodapedontines than to Rhynchosaurus articeps. Therefore, a new genus is erected, resulting in the new combination Langeronyx brodiei. A body size analysis found two independent increases in size in the evolutionary history of rhynchosaurs, one among stenaulorhynchines and the other in the hyperodapedontine lineage. Maximum likelihood fitting of phenotypic evolution models to body size data found ambiguous results, with body size evolution potentially interpreted as either fitting either a non-directional Brownian motion model or a stasis model. A Dispersal−Extinction−Cladogenesis analysis reconstructed the areas that are now South Africa and Europe as the ancestral areas of Rhynchosauria and Rhynchosauridae, respectively. The reconstruction of dispersal events between geographic areas that are broadly separated paleolatitudinally implies that barriers to the dispersal of rhynchosaurs from either side of the paleo-Equator during the Middle Triassic were either absent or permeable.

Highlights

The Permo-Triassic mass extinction occurred ∼252 million years ago, and produced a dramatic change in the composition of floral and faunal communities (Raup and Sepkoski, 1982; Erwin, 1994; Looy et al, 2001; Benton and Twitchett, 2003; Fröbisch, 2013; Benton and Newell, 2014; Smith and Botha-Brink, 2014)
Pre-Triassic rhynchosaurs are unknown, the Permian origin of the clade is suggested by phylogenetic topologies, with a ghost lineage extending across the Permo-Triassic boundary, into the late Permian (Ezcurra et al, 2014)
The inclusion of N. colletti resulted in a large polytomy at the base of Rhynchosauria, which includes all non-rhynchosaurid rhynchosaurs

Summary

Introduction

The Permo-Triassic mass extinction occurred ∼252 million years ago, and produced a dramatic change in the composition of floral and faunal communities (Raup and Sepkoski, 1982; Erwin, 1994; Looy et al, 2001; Benton and Twitchett, 2003; Fröbisch, 2013; Benton and Newell, 2014; Smith and Botha-Brink, 2014). Rhynchosaurs diversified in the aftermath of this extinction Many of these lineages replaced clades that flourished during the Paleozoic but vanished or were decimated at the end-Permian mass extinction event (e.g., pareiasaurs, non-mammalian synapsids, such as gorgonopsians and anomodonts). The Triassic witnessed the evolution of numerous amniote groups that occur only within this time interval and which became extinct at or before the end-Triassic mass extinction event (e.g., traversodontids, doswelliids, tanystropheids, aetosaurs, phytosaurs; Abdala and Ribeiro, 2010; Liu and Olsen, 2010; Desojo et al, 2013; Stocker and Butler, 2013; Sues et al, 2013; Pritchard et al, 2015). The rhynchosaur genus Hyperodapedon is the most commonly found tetrapod within the oldest dinosaurbearing assemblages (e.g., the Ischigualasto, Santa María 2, lower Maleri and Pebbly Arkose formations), and, as a result, has been broadly used for global biostratigraphic correlations (Langer, 2005; Lucas, 2010; Martínez et al, 2011)

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Conclusion