A reassessment of the osteology and phylogenetic relationships of the enigmatic, large-headed reptile Sphodrosaurus pennsylvanicus (Late Triassic, Pennsylvania, USA) indicates archosauriform affinities

EZCURRA, M.D.; SUES, H.-D.

JOURNAL OF SYSTEMATIC PALAEONTOLOGY

CAMBRIDGE UNIV PRESS

Lugar: Cambridge; Año: 2022

1477-2019

Most Triassic terrestrial diapsids belong to two clades,Lepidosauromorpha and (the more diverse) Archosauromorpha. Nevertheless, thephylogenetic relationships of some Triassic diapsids have remained uncertainfor decades because of the lack of preservationof phylogenetically relevant anatomical regions or unusual combinations offeatures. One of these enigmatic forms is the small-sized Sphodrosauruspennsylvanicus from the Upper Triassic Hammer Creek Formation of the NewarkSupergroup in Pennsylvania (USA). It was first identified as a procolophonidparareptile, later as a probable rhynchosaur archosauromorph, and more recentlyas an indeterminate neodiapsid. Here we revise the anatomy of Sphodrosauruspennsylvanicus in order to include it for the first time in a quantitativephylogenetic analysis, which is focussed on Permo-Triassic neodiapsids. Sphodrosauruspennsylvanicus is recovered in this analysis as a doswelliidproterochampsian within Archosauromorpha. As a result, this taxon is added tothe list of doswelliids known from the Carnian-Norian of the eastern andsouthwestern USA. Previous authors recognized that the most unusual feature of Sphodrosauruspennsylvanicus is its proportionally verylarge skull. Phylogenetic Generalized Least Squares regressions confirmed that Sphodrosauruspennsylvanicus has a larger skull than the vast majority of Permo-Triassicdiapsids. Optimization in the phylogeny of the skull width versus presacrallength ratio shows that the most likely scenario is that the extremely broadskull of Sphodrosaurus pennsylvanicus is autapomorphic, but it is notunique among archosauromorphs, being paralleled by hyperodapedontinerhynchosaurs and the proterochampsian Proterochampsa barrionuevoi. Explorationof a morphospace of linear measurements shows that Sphodrosauruspennsylvanicus shares strong similarities with the probably semi-aquaticProterochampsa barrionuevoi, suggesting that the former species may havehad a similar mode of life. A Linear Discriminant analysis of ungual functionalcategories found that the only preserved ungual of Sphodrosauruspennsylvanicus was suitable for digging or some other way of substrateprocessing.