A new species ofCoahomasuchus(Archosauria, Aetosauria) from the Upper Triassic Pekin Formation, Deep River Basin, North Carolina

Abstract

Abstract.-We describe a new species of the aetosaur Coahomasuchus, C. chathamensis, based on an incomplete, but largely articulated, anterior portion of a skeleton recovered from a quarry in the Upper Triassic Pekin Formation of Chatham County, North Carolina. This is only the second documented occurrence of Coahomasuchus, with the other being the holotype of C. kahleorum Heckert and Lucas, 1999 from the Upper Triassic Colorado City Formation of Texas. Although much of the specimen is the same size as the holotype of C. kahleorum, the dorsal paramedian osteoderms of the North Carolina taxon are considerably (~1.3×) wider than homologous counterparts in C. kahleorum, and the ventral thoracic osteoderms are also rectangular (~1.5× wider than long), rather than square, presumably to accommodate the wider body. This is a rare instance where two articulated specimens of closely related aetosaur species are available for direct comparison of homologous osteoderms. Isolated osteoderms with similar ornamentation from the same locality indicate that C. chathamensis may have been one of the earliest aetosaurs to attain the broad osteoderm proportions (width:length >3.5:1) otherwise known solely from later branching, spinose taxa such as Typothorax. The co-occurrence of Lucasuchus and Coahomasuchus in both North Carolina and Texas supports past correlations indicating an Otischalkian (Carnian) age for these strata and demonstrates that plesiomorphic, non-spinose aetosaur genera were not necessarily endemic to a single basin in North America.

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

“…With maximum paramedian osteoderm widths of 90 mm in the holotype specimen of C. chathamensis (Heckert et al, ), and a sectioned osteoderm 70 mm wide in this study, it is apparent from the histological data that C. chathamensis grew rapidly when compared to aetosaurs of similar size. For example, the holotype specimen of Aetosauroides scagliai (PVL 2073—92 mm maximum paramedian osteoderm width) has five unambiguous LAGs (Taborda et al, ).…”

“…We also note that the medial flange of paramedian osteoderms of both species of Coahomasuchus have an ornamentation comprised primarily of pitting that is much closer to the anastomosing pattern than radial sensu Taborda et al () (Heckert and Lucas, , fig. 7a; Heckert et al, , figs. 6.1–3).…”

“…All fossil material of C. chathamensis , including the holotype specimen and referred materials from the holotype locality (NCPALEO 1902), are housed at NCSM in the vertebrate paleontology collections. These specimens were all collected from a stratigraphically narrow (~30 m thick) layer within the Upper Triassic Pekin Formation of the Chatham Group, Newark Supergroup, North Carolina (Heckert et al, , ). These are among the stratigraphically lowest Upper Triassic aetosaurs from North America (Whiteside et al, ; Heckert et al, ).…”

“…These specimens were all collected from a stratigraphically narrow (~30 m thick) layer within the Upper Triassic Pekin Formation of the Chatham Group, Newark Supergroup, North Carolina (Heckert et al, , ). These are among the stratigraphically lowest Upper Triassic aetosaurs from North America (Whiteside et al, ; Heckert et al, ). The fossil preparation of the specimens was performed at NCSM as described by Heckert et al ().…”

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

“…Aetosaurs comprise a clade of heavily armored, quadrupedal herbivorous to faunivorous pseudosuchian-line archosaurs ( Parker, 2016a ; Heckert, Fraser & Schneider, 2017 ) known from Upper Triassic deposits across Pangea. To date, fossil specimens have been recovered from every modern continent except Antarctica and Australia ( Heckert & Lucas, 2000 ; Desojo et al, 2013 ; Schoch & Desojo, 2016 ; Heckert, Fraser & Schneider, 2017 ). As one of the few lineages of non-dinosauromorph archosauromorphs to have evolved herbivory during the Triassic ( Desojo et al, 2013 ), aetosaurs represented a key component of Late Triassic ecosystems.…”