Journal of Paleontology, 91(1), 2017, p. 162–178 Copyright © 2016, The Paleontological Society 0022-3360/16/0088-0906 doi: 10.1017/jpa.2016.130


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


Andrew B. Heckert,1 Nicholas C. Fraser,2,3 and Vincent P. Schneider4


1Department of Geology, Appalachian State University, ASU Box 32067, Boone, North Carolina 28608-2067, USA ⟨heckertab@appstate.edu⟩ 2National Museums Scotland, Chambers Street, Edinburgh EH1 1JF, UK ⟨nick.fraser@nms.ac.uk⟩ 3Virginia Museum of Natural History, 23 Starling Avenue, Martinsville, Virginia 24112, USA 4North Carolina Museum of Natural Sciences, 11 West Jones Street, Raleigh, North Carolina 27601-1079 USA ⟨vpsch@nc.rr.com⟩


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 rela- ted 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 plesio- morphic, non-spinose aetosaur genera were not necessarily endemic to a single basin in North America.


Introduction


The aetosaurs form a group of moderate- to large-sized (~1–4m body length) quadrupedal herbivorous to faunivorous armored archosaurs restricted to strata of Late Triassic age worldwide. With the exception of Australia and Antarctica, there are aeto- saur records for every modern continent (Desojo et al., 2013). Aetosaurs are important as they are one of very few Triassic lineages in the large post-Permian archosauromorph radiation (other than rhynchosaurs) that exhibits any indication of herbivory. Since Heckert and Lucas (2000) systematically revised the group, recognizing 10 valid genera and 13 species, many new taxa have been recognized (e.g., Lucas et al., 2002, 2006, 2007; Zeigler et al., 2002; Parker, 2005, 2007; Martz and Small, 2006; Parker et al., 2008; Sulej, 2010; Desojo et al., 2012; Spielmann and Lucas, 2012; Small and Martz, 2013; Heckert et al., 2015), such that the recent reviewby Desojo et al. (2013) recognized as many as 22 genera and 26 species. Because aetosaur armor is highly distinctive, many of these taxa are known largely to entirely from associated osteoderms, sometimes to the exclusion of other skeletal elements. Indeed, both skulls and articulated skeletons are extremely rare. In this paper, we describe an incomplete skeleton, including much of a skull, and some associated aetosaur fossils collected from a quarry operation in southeastern Chatham County, North


Carolina, that represent a new species of the aetosaur Coaho- masuchus and comment on their evolutionary and biostrati- graphic significance.


Geologic setting


The specimen described here comes from NCSM locality NCPALEO1902, located in a brick quarry in southeastern


Chatham County, North Carolina (detailed locality data are on file at NCSM) (Fig. 1.1). Olsen (1997) and Weems and Olsen (1997) referred all Triassic sedimentary rocks in North Carolina to the Chatham Group of the Newark Supergroup. Newark Supergroup sediments were deposited in a series of rift basins extending from the present-day Gulf Coast to Canada. In North Carolina, the easternmost of these basins exposed at the surface is the Deep River Basin, which is subdivided into the northern Durham, the central Sanford, and the southern Wadesboro sub- basins (Olsen et al., 1991; Fig. 1.1). The Sanford sub-basin, where NCPALEO1902 is located, is a half-graben with the primary bounding fault on the east side, and the brick quarry is in the northeastern-most portion of the Sanford sub-basin, just south of the Colon cross-structure, a basement feature that separates it from the Durham sub-basin to the north (Olsen et al., 1991; Fig. 1.1, 1.2).


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