A new phylogenetic hypothesis of Tanystropheidae (Diapsida, Archosauromorpha) and other "protorosaurs", and its implications for the early evolution of stem archosaurs.

Stephan N.F Spiekman,Torsten M Scheyer,Nicholas C Fraser

doi:10.7717/peerj.11143

Stephan N.F Spiekman, Torsten M Scheyer + Show 1 more

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https://doi.org/10.7717/peerj.11143

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Journal: PeerJ	Publication Date: May 3, 2021
Citations: 25	License type: CC BY 4.0

Affiliation: University of Zurich, National Museums Scotland

Abstract

The historical clade “Protorosauria” represents an important group of archosauromorph reptiles that had a wide geographic distribution between the Late Permian and Late Triassic. “Protorosaurs” are characterized by their long necks, which are epitomized in the genus Tanystropheus and in Dinocephalosaurus orientalis. Recent phylogenetic analyses have indicated that “Protorosauria” is a polyphyletic clade, but the exact relationships of the various “protorosaur” taxa within the archosauromorph lineage is currently uncertain. Several taxa, although represented by relatively complete material, have previously not been assessed phylogenetically. We present a new phylogenetic hypothesis that comprises a wide range of archosauromorphs, including the most exhaustive sample of “protorosaurs” to date and several “protorosaur” taxa from the eastern Tethys margin that have not been included in any previous analysis. The polyphyly of “Protorosauria” is confirmed and therefore we suggest the usage of this term should be abandoned. Tanystropheidae is recovered as a monophyletic group and the Chinese taxa Dinocephalosaurus orientalis and Pectodens zhenyuensis form a new archosauromorph clade, Dinocephalosauridae, which is closely related to Tanystropheidae. The well-known crocopod and former “protorosaur” Prolacerta broomi is considerably less closely related to Archosauriformes than was previously considered.

Highlights

Non-archosauriform archosauromorphs lived during the late Permian and Triassic and belong to the archosaurian stem-lineage, the ancestral lineage of crocodylians and birds
At the base of the strict consensus tree (SCT) of both analyses a polytomy is formed by Claudiosaurus germaini, Acerosodontosaurus piveteaui, Youngina capensis, and Sauria, with Orovenator mayorum as the sister taxon to this polytomy (Figs. 33A and 34A)
It is generally considered that Orovenator mayorum is most distantly related to Sauria among our sampled operational taxonomic units (OTUs) except for the outgroup Petrolacosaurus kansensis (Ford & Benson, 2020; Reisz et al, 2011), there is no clear consensus in the relationships between Youngina capensis, Claudiosaurus germaini, and Acerosodontosaurus piveteaui (Bickelmann, Müller & Reisz, 2009)

Summary

Introduction

Non-archosauriform archosauromorphs lived during the late Permian and Triassic and belong to the archosaurian stem-lineage, the ancestral lineage of crocodylians and birds. Many members of this group were placed within either “Protorosauria” or “Prolacertiformes” These two groups generally encompassed the same taxa and the usage of one term over the other depended on the inclusion within the clade of either Protorosaurus speneri or Prolacerta broomi, or both. As Permo-Triassic non-archosauriform archosauromorphs, “protorosaurs” represent some of the earliest members of the lineage that gave rise to Archosauria and as such are important both for our understanding of early archosauromorph evolution and the acquisition of traits within the archosaur character complex. The Chinese Dinocephalosaurus orientalis and an unnamed closely related taxon represent the only known viviparous archosauromorphs (Li, Rieppel & Fraser, 2017; Liu et al, 2017)

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