The osteology of the Late Triassic reptile Scleromochlus taylori from μCT data

Abstract

Scleromochlus taylori is one of the most enigmatic members of the herpetofauna from the Lossiemouth Sandstone Formation (Upper Triassic) of Elgin (Moray, Scotland). For many years it was thought to be closely related to pterosaurs and dinosaurs, but the anatomy of this animal is difficult to interpret because of the notoriously poor preservation of the six available specimens, which comprise void space in the sandstone after the bones were destroyed by diagenesis. Historically, these fossils have been studied using physical molds, which provide only incomplete, and potentially distorted, information. Due to these uncertainties, interpretations of the anatomy, phylogenetic relationships, and paleobiology of Scleromochlus taylori have remained contentious. Here, we use microcomputed tomographic (μCT) techniques to redescribe and illustrate the osteology of Scleromochlus in detail, building upon a short redescription of keystone features of the anatomy that we recently published. We digitally visualize, describe, and figure previously inaccessible—and thus unaltered—portions of its skeleton, as well as providing new observations on the exposed parts of each specimen. This work reveals many novel features of the skull, mandible, trunk, tail, girdles, forelimb, and hindlimb (particularly of the manus, femur, and pes), demonstrating that historic molding techniques failed, in some cases, to accurately capture the anatomy of Scleromochlus. Our review sheds light on some of the most controversial aspects of Scleromochlus morphology showing that this taxon retains plesiomorphic features of Avemetatarsalia in the postcranial skeleton, alongside a suite of synapomorphies diagnostic of pterosauromorphs (the broad clade of pterosaurs and taxa more closely related to them than dinosaurs), particularly one subgroup, the lagerpetids. Consistent with recent work, our updated phylogenetic analyses (Maximum Parsimony and Bayesian Inference) demonstrate that Scleromochlus taylori is an avemetatarsalian archosaur that is recovered firmly in an early diverging position within Pterosauromorpha, as a member of Lagerpetidae, thus shedding important information on the origin of pterosaurs, the first group of vertebrates to evolve powered flight.

The dawn of an Era: New contributions on comparative and functional anatomy of Triassic tetrapods

“…Recent studies have demonstrated the close relationship between pterosaurs and lagerpetids, elucidating one of the most interesting macroevolutionary processes in vertebrate history. In this context, Foffa et al (2024, this issue) provide the thus far most detailed description of the iconic avemetatarsalian Scleromochlus taylori . Classically considered a key player in the origin of pterosaurs (and dinosaurs), the small reptile is recovered by the authors as a member of Lagerpetidae, indeed contributing to the understanding of the early evolution of Pterosauromorpha.…”

Comments on the pelvic girdle anatomy of Lagerpeton chanarensis Romer, 1971 (Archosauria) and its implications on the posture and gait of early pterosauromorphs

“…The interpretation that Lagerpeton represented the earliest-diverging member of the dinosaur lineage was unanimously followed by subsequent authors that studied dinosaur origins (e.g., Bittencourt et al, 2015;Brusatte et al, 2010;Cabreira et al, 2016;Ezcurra, 2006Ezcurra, , 2016Irmis et al, 2007;Langer et al, 2010Langer et al, , 2013Nesbitt, 2011;Nesbitt et al, 2010;Novas, 1996). More recently, Lagerpeton and its kin (i.e., Lagerpetidae) were re-interpreted as the sister group to Pterosauria, forming the clade Pterosauromorpha with them (e.g., Baron, 2021;Ezcurra, Nesbitt, Bronzati, et al, 2020;Foffa et al, 2022Foffa et al, , 2024Kellner et al, 2022;Müller et al, 2023).…”