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Journal of Paleontology 91(1):162–178


longitudinal keel (probably directed proximo-distally in life) and several (~8–10) relatively coarse, distinct pits radiating away from the keel. Most are imbricated so that no single appendicular osteoderm is completely exposed, but the shape of all appears to be a rounded, slightly elongated hexagon that is as much as 15mm long and 8mm wide. The appendicular armor is evident on both limbs in ventral view,where it essentially covers the right limbcompletely and the left distal forelimb(Figs. 2.2, 3.2). These osteoderms are smaller (generally ≤10mm maximum dimen- sion), more ovoid, and lack the distinct keel on the long axis, although they do still possess severalwell-defined, albeit smaller, pits. The right limb appears to be folded, with the forelimb directed anteromedially from the elbow joint. A row of appen- dicular osteoderms anterior and medial to the left humerus may indicate the presence of the left distal forelimb.


Referred material.—Isolated fragments of various aetosaurian postcranial elements have been recovered from the locality, but only the elements assigned to NCSM 23618 can be unambigu- ously referred to C. chathamensis. All of the following specimens exhibit distinctive features of Coahomasuchus, specifically that the dorsal paramedian osteoderms are relatively wide with an extremely faint ornamentation that, lateral to a subdued boss, consists of fine, elongate, subparallel grooves and ridges. Almost all are slightly larger than those knownfromeither holotype skeleton, but given our lack of knowledge of the onto- genetic state of those skeletons, they probably represent some- what larger, presumably older individuals.Only a fewventral and appendicular osteoderms are represented in this material and we did not observe any details of these osteoderms that modified our interpretations of osteoderm homology in the holotype. Associated dorsal osteoderms.—The majority of the speci-


mens represent paramedian osteoderms and are generally much wider than long, with exceptionally low dorsal eminences (Fig. 6.1–6.4). In the case of each paramedian, the anterior edge bears a smooth facet, the anterior bar, which was overlapped by the posterior edge of the preceding osteoderm. On some paramedian osteoderms, the anteromedial corner of the anterior bar is developed into a short spur (anteromedial projection of Parker, 2016). The lateral margin lacks a strong anterolateral projection sensu Parker (2016). The ornamentation is rather weakly expressed in most specimens, and takes the form of radiating pits that are sometimes developed into shallow grooves. The development of continuous grooves is particularly evident on the lateral osteoderms and the lateral edges of the broadest paramedians, whereas strong pitting is most commonly seen on the medial portion of the paramedians. The orna- mentation radiates from a point about two thirds of the way back from the anterior edge and along the line of angulation, closer to the midline than the lateral edge of the osteoderm, the “center of ossification” of Martz and Small (2006). As in the holotype, the grooves tend to be situated on the lateral side of the angulation and almost all of the pits on the medial side. The boss is very weakly developed, but sometimes takes the form of an extre- mely low ridge that is typically situated close to the point of ornament radiation, with the highest point somewhat anterior to the posterior margin of the osteoderm. Associated paramedian osteoderms come from many parts of the skeleton, including the posterior cervical or anterior


dorsal series (Fig. 6.1), an anterior dorsal paramedian (Fig. 6.2), and a more thoracic or “trunk” paramedian (Fig. 6.3). Some of the osteoderms possess a slight keel or strut, but most do not, and this strut does not substantially thicken the osteoderm, sowe interpret this as retaining the plesiomorphic (unkeeled) condi- tion of Parker (2016) and previous workers. In the preserved specimens it appears that the anterolateral portion of the paramedian would overlap the lateral osteoderm. Where preserved, the posterolateral corner forms a right angle, so that the lateral margin is relatively straight in dorsal view (NCSM 21707; Fig. 6.1)—the “cut-off” corner in NCSM 16444-1 (Fig. 6.2) is an artifact—the specimen is broken there. Lateral osteoderms.—In the assemblage of isolated osteo-


derms, there are many fewer lateral osteoderms preserved than paramedians, just as was the case with the articulated holotype specimen. Relatively few of these are as equidimensional as those described previously. Instead, these isolated laterals match the posterior laterals (especially caudal laterals) of C. kahleorum (Heckert and Lucas, 1999). Specifically, they are longer than wide, with a narrower dorsal flange and a wider lateral flange, the latter bearing a well-developed pattern of elongate pits, grooves, and ridges emanating anteriorly and laterally from a longitudinal keel that projects slightly posteriorly as a small projection, although it is less spinous than the lateral “spikes” of many aetosaurs. In the holotype of C. kahleorum laterals posterior to approximately row 9 are markedly asymmetrical, with a narrower dorsal flange and a wider lateral flange (Heckert and Lucas, 1999, fig. 3; Parker, 2016). Pitting is distributed radially and emanates from a longitudinal dorsal ridge; many of these osteoderms are gently flexed beneath this keel as well. The ornamentation of the lateral osteoderms is generally more deeply incised than on comparably long dorsal paramedians. One lateral morphotype is characterized by a weakly


developed longitudinal ridge that terminates in a slightly raised boss (Fig. 6.4, 6.5). In this morphotype, there is no development whatsoever of a spike. The lateral osteoderms on the articulated specimen also lack any well-developed spike, and the association of this morphotype of lateral osteoderm with the paramedians described above is unequivocal. Several specimens were freed from thematrix in one piece, so that thematrix provides a natural mold of the external surface of the osteoderm (Fig. 6.5).


Etymology.—The specific epithet, chathamensis, is for Chatham County, North Carolina, where this and many other Triassic fossils have been recovered.


Materials.—In addition to the holotype,we tentatively refermany other osteoderms and associated postcrania fromthe same locality to C. chathamensis. These include the following (all are NCSM specimens): Dorsal paramedian osteoderms: 16444-1, right pos- terior presacral dorsal paramedian (Fig. 6.2); 16444-2, left? pos- terior presacral dorsal paramedian; 16472, dorsal paramedian; 19302, right posterior presacral dorsal paramedian; 19675, right thoracic dorsal paramedian osteoderm in matrix block (Fig. 6.3); 21602, incomplete right dorsal paramedian; 21707, anterior right dorsal paramedian (Fig. 6.1). Lateral paramedian osteoderms: 19751, left lateral; 20829, right lateral; 21145, left(?) lateral; 21179, right lateral osteoderm in matrix (Fig. 6.4, 6.5): 21181, right cervical(?); 20739, left lateral. Ventral osteoderms: 19303,


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