Under the armor: X-ray computed tomographic reconstruction of the internal skeleton ofCoahomasuchus chathamensis(Archosauria: Aetosauria) from the Upper Triassic of North Carolina, USA, and a phylogenetic analysis of Aetosauria

Abstract

Aetosauria is a clade of heavily armored, quadrupedal omnivorous to herbivorous archosaurs known from the Late Triassic across what was the supercontinent of Pangea. Their abundance in many deposits relative to the paucity of other Triassic herbivores indicates that they were key components of Late Triassic ecosystems. However, their evolutionary relationships remain contentious due, in large part, to their extensive dermal armor, which often obstructs observation of internal skeletal anatomy and limits access to potentially informative characters. In an attempt to address this problem we reanalyzed the holotype of a recently described species of Coahomasuchus, C. chathamensis, from the Sanford sub-basin of North Carolina using computed tomography (CT). CT scans of the holotype specimen clarify preservation of the skeleton, revealing several articulated vertebrae and ribs, an isolated vertebra, left ulna, left scapula, and the right humerus, though none of the material resulted in updated phylogenetic scorings. Reexamination of aetosaur materials from the holotype locality also indicates that several isolated osteoderms and elements of the appendicular skeleton are newly referable. Based on these results, we update the Coahomasuchus chathamensis hypodigm and conduct a revised phylogenetic analysis with improved character scorings for Coahomasuchus and several other aetosaurs. Our study recovers Coahomasuchus in a polytomy with Aetosaurus and the Typothoracinae, in contrast with a recent analysis that recovered Coahomasuchus as a wild-card taxon.

Anatomical notes and discussion of the first described aetosaurStagonolepis robertsoni(Archosauria: Suchia) from the Upper Triassic of Europe, and the use of plesiomorphies in aetosaur biochronology

“…Thus presently there is strong agreement between the phylogenetic position of S. robertsoni in these studies as more closely related to Desmatosuchinae than to Typothoracinae ( Fig. 7 , 9; Parker, 2016a ; Hoffman, Heckert & Zanno, 2018 ).…”

“…Another recent phylogenetic analysis ( Fig. 10 ; Hoffman, Heckert & Zanno, 2018 ) of the Aetosauria that also builds on successive analyses from Heckert & Lucas (1999) , Parker (2007) , Desojo, Ezcurra & Kischlat (2012) , Heckert et al (2015) , and Schoch & Desojo (2016) , but does not include all currently known taxa (e.g., Stagonolepis olenkae ) also recovers S. robertsoni in Stagonolepidinae as the sister taxon to Desmatosuchinae (=Desmatosuchini of Parker, 2016a ). Interestingly, with the exclusion of S. olenkae from the analysis, the sister taxon to S. robertsoni is Calyptosuchus ( Stagonolepis ) wellesi .…”

Disparate Growth Strategies within Aetosauria: Novel Histologic Data from the Aetosaur Coahomasuchus chathamensis

“…In the holotype of C. kahleorum the preserved radius is 32 mm and estimated to be 60 mm if complete and the complete fibula is 79 mm long (Heckert and Lucas, ). The holotype of C. chathamensis does not preserve either a radius or fibula, but the partial left ulna, recovered in CT scans, is 40 mm and was likely ~60 mm when complete, indicating that the radius was also approximately 60 mm (Hoffman et al, ). Given that there are seven LAGs in the radius and four in the fibula, and that the holotype of C. chathamensis preserves at most one to two LAGS, it is apparent that the holotype does not represent a full‐grown individual, and is in fact likely to be a juvenile.…”

“…Although we understand much about the general characteristics of Aetosauria, there remain questions about this important clade in areas such as aetosaur phylogenetic relationships and patterns of growth. For example, in the last four phylogenetic analyses, the genus Coahomasuchus was recovered in four different positions within Aetosauria (Heckert et al, ; Parker, ; Schoch and Desojo, ; Hoffman et al, ).…”

Ontogenetic braincase development in Psittacosaurus lujiatunensis (Dinosauria: Ceratopsia) using micro-computed tomography

“…The integration of advanced computed tomography (CT) techniques into paleontological studies has enabled extensive, in-depth morphological reviews and non-destructive investigations into both extinct and extant taxa (Chapelle & Choiniere, 2018; Hoffman, Heckert & Zanno, 2018; Neenan & Scheyer, 2012; Walsh & Knoll, 2018). While several studies have touched upon the braincase of Psittacosaurus (Coombs, 1982; Zhou et al, 2007; Dodson, You & Tanoue, 2010; Sereno, 2010), few have used CT data to explore internal structures (Zhou et al, 2007).…”