A redescription of Orovenator mayorum (Sauropsida, Diapsida) using high‐resolution μCT, and the consequences for early amniote phylogeny

Abstract

The earliest known neodiapsid Orovenator mayorum from the lower Permian of Oklahoma is redescribed using high‐resolution μCT, revealing remarkable details of the skull anatomy. Our findings are relevant to both palaeoecology (suggesting burrowing and nocturnality) and phylogeny. Orovenator and other sauropsids share at least 16 character states with varanopids, many of which were not recognized by previous studies. These include a rounded subnarial shelf of the premaxilla, a posterodorsal extension of the external naris, the asymmetrical bifurcation of the anterior vomer, and a prominent dorsomedial shelf of the surangular. This exceptional degree of similarity between Orovenator and varanopids (a nominally synapsid clade) questions our current understanding of relationships among early amniotes. We test this by including Orovenator in a phylogenetic data matrix used in an earlier study to differentiate between early diapsids and synapsids, and find a monophyletic clade of Orovenator + varanopids within Diapsida. Recent phylogenetic research on early amniote evolution has focused on resolving intra‐clade affiliations rather than the interrelationships of major taxonomic groups. Nevertheless, the relative incompleteness of existing phylogenetic character lists for early amniotes can only be remedied by detailed cross‐clade assessment. We therefore suggest that early amniote relationships require further scrutiny before we can confidently accept or reject our new phylogenetic hypothesis.

Peer Review #1 of "New information on the early Permian lanthanosuchoid Feeserpeton oklahomensis based on computed tomography (v0.1)"

“…8) 331 varanopid taxa (Archaeovenator, Mycterosaurus) included in the analysis were recovered as 332 being more closely related to the reptile taxa than to the synapsids. This is an atypical result that 333 warrants further investigation in the future, but is at least partly in line with the results of recent 334 work that has recovered varanopids within Reptilia (Ford & Benson, 2018). In the case of our 335 study this result could potentially be the result of the relatively narrow focus of our analysis.…”

The many faces of synapsid cranial allometry

“…Unfortunately, our sample size for "pelycosaurs" is too small to allow detailed investigations of allometric patterns within the major subclades: despite an expansive postcranial record, complete, three-dimensionally preserved "pelycosaur" crania are rare. Further investigation and, ultimately, a larger number of complete cranial specimens will be needed to further test whether the apparent CREA-like pattern was indeed a ubiquitous feature of the grade (and possibly the ancestral character state for Synapsida), or an artifact of combining data from multiple clades that may have had distinct allometric relationships (and indeed may not all be closely related; see Ford and Benson 2018).…”

Inner ear morphology of diadectomorphs and seymouriamorphs (Tetrapoda) uncovered by high‐resolution x‐ray microcomputed tomography, and the origin of the amniote crown group

“…Thirdly, traditional branching patterns in several lineages of primitive crown amniotes have been challenged. As an example, Ford & Benson () put forward a new phylogenetic hypothesis in which varanopids, usually regarded as a clade of basal synapsids, are nested within diapsids. Fourthly, the identity of the closest relatives of crown amniotes from among the diverse array of early tetrapods has long been debated (e.g.…”

“…In most phylogenetic analyses, however, diadectomorphs form the monophyletic sister taxon to crown amniotes (but see Ruta & Coates ) and are often employed as the ‘default’ outgroup to polarize characters near the roots of this clade (e.g. Müller & Reisz ; Benson ; Ford & Benson ). To the best of our knowledge, Berman (, ) was the first author to retrieve diadectomorphs as the sister taxon to synapsids based upon a formal character‐based analysis.…”