New clade of enigmatic early archosaurs yields insights into early pseudosuchian phylogeny and the biogeography of the archosaur radiation.

Richard J Butler,Corwin Sullivan,Jun Liu,Agustina Lecuona,Roland B Sookias,Martín D Ezcurra

doi:10.1186/1471-2148-14-128

Abstract

BackgroundThe origin and early radiation of archosaurs and closely related taxa (Archosauriformes) during the Triassic was a critical event in the evolutionary history of tetrapods. This radiation led to the dinosaur-dominated ecosystems of the Jurassic and Cretaceous, and the high present-day archosaur diversity that includes around 10,000 bird and crocodylian species. The timing and dynamics of this evolutionary radiation are currently obscured by the poorly constrained phylogenetic positions of several key early archosauriform taxa, including several species from the Middle Triassic of Argentina (Gracilisuchus stipanicicorum) and China (Turfanosuchus dabanensis, Yonghesuchus sangbiensis). These species act as unstable ‘wildcards’ in morphological phylogenetic analyses, reducing phylogenetic resolution.ResultsWe present new anatomical data for the type specimens of G. stipanicicorum, T. dabanensis, and Y. sangbiensis, and carry out a new morphological phylogenetic analysis of early archosaur relationships. Our results indicate that these three previously enigmatic taxa form a well-supported clade of Middle Triassic archosaurs that we refer to as Gracilisuchidae. Gracilisuchidae is placed basally within Suchia, among the pseudosuchian (crocodile-line) archosaurs. The approximately contemporaneous and morphologically similar G. stipanicicorum and Y. sangbiensis may be sister taxa within Gracilisuchidae.ConclusionsOur results provide increased resolution of the previously poorly constrained relationships of early archosaurs, with increased levels of phylogenetic support for several key early pseudosuchian clades. Moreover, they falsify previous hypotheses suggesting that T. dabanensis and Y. sangbiensis are not members of the archosaur crown group. The recognition of Gracilisuchidae provides further support for a rapid phylogenetic diversification of crown archosaurs by the Middle Triassic. The disjunct distribution of the gracilisuchid clade in China and Argentina demonstrates that early archosaurs were distributed over much or all of Pangaea although they may have initially been relatively rare members of faunal assemblages.

Highlights

The origin and early radiation of archosaurs and closely related taxa (Archosauriformes) during the Triassic was a critical event in the evolutionary history of tetrapods
T. dabanensis has been considered a euparkeriid archosauriform [11], an archosauriform close to but outside Archosauria [18], a non-pseudosuchian archosauriform of uncertain affinities [12], a very early pseudosuchian [19], or a “rauisuchiform” pseudosuchian [15], whereas Y. sangbiensis has been identified as a non-archosaurian archosauriform [13] that may represent the sister taxon to Archosauria [18]
We highlight some new observations on the anatomy of G. stipanicicorum below in drawing comparisons to Turfanosuchus dabanensis and Yonghesuchus sangbiensis

Summary

Introduction

The origin and early radiation of archosaurs and closely related taxa (Archosauriformes) during the Triassic was a critical event in the evolutionary history of tetrapods. The timing and dynamics of this evolutionary radiation are currently obscured by the poorly constrained phylogenetic positions of several key early archosauriform taxa, including several species from the Middle Triassic of Argentina (Gracilisuchus stipanicicorum) and China (Turfanosuchus dabanensis, Yonghesuchus sangbiensis). These species act as unstable ‘wildcards’ in morphological phylogenetic analyses, reducing phylogenetic resolution. The most detailed and extensive revision of early archosaur phylogeny recovered both G. stipanicicorum and T. dabanensis close to the base of Suchia [2], but both taxa were highly unstable and acted as phylogenetic ‘wildcards’, obscuring early pseudosuchian relationships [2,20] This analysis did not include Y. sangbiensis. Determining the affinities of all three enigmatic taxa is one of the key challenges remaining in understanding early archosaur phylogeny, and in understanding the broader dynamics and palaeobiogeography of this evolutionary radiation

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