Heckert et al.—New species of Coahomasuchus from North Carolina
articulated specimen, are similar to each other and, although they vary in shape, the ornamentation is consistent and they unequivocally represent the same species. The carapace is not from a mature specimen as the largest paramedian osteoderm from the holotype is only about two-thirds the size of NCSM 21137, the largest disassociated osteoderm. The articulated carapace.—Both columns of dorsal para-
median osteoderms are at least partially exposed in dorsal view, with the left column more complete than the right. All are demonstrably wider than long, and most display some degree of gentle transverse arching. As is the case in the holotype of Coahomasuchus kahleorum, it is difficult to see any ornamenta- tion on the cervical and anterior thoracic paramedians. When present, it consists almost entirely of elongate, subradial grooves with little or no pitting, and all the pitting is confined to the medial side of the center of ossification. Similarly, the bosses or dorsal eminences (“center of ossification”) are extremely reduced and only just discernable in posterior view. Each boss is extremely low and rounded, and antero-posteriorly longer than wide so it forms a keel. The actual relief of the structure is so low that it is easier to feel the topography than to see, let alone illustrate, it, but its shape is clearly delineated in the slightly disarticulated (most posterior) left osteoderm in Figure 2.1 (identified as 14? in Fig. 3.1). The boss is always located much closer to the midline than the lateral margin. The first two rows of paramedian osteoderms are well
171 The first preserved lateral is actually embedded with the
skull elements, situated dorsal to the right postorbital (Fig. 5.3, 5.4). This osteoderm is relatively D-shaped, with a straight medial margin and a rounded lateral margin. The ornamentation is not discernable, but, like the first dorsal paramedian, it lacks an anterior bar. Portions of at least two lateral osteoderms are slightly exposed posterior to the skull on the left side. Further posteriorly, the edges of three laterals are apparent
lateral to left dorsal paramedians 10–12 (Figs. 2.1, 3.1). The four laterals on the right side (only 3 of which are visible in dorsal aspect) probably pertain to the nearby paramedians, tentatively interpreted as rows 10–13, but exact matches are not clear (Figs. 2.1, 3.1). These laterals are very slightly asymmetrical, with a weakly defined center of ossification slightly medial to the middle of the osteoderm. This forms an elongate ridge that separates the osteoderm into a narrower dorsal flange and wider lateral flange. Ventral osteoderms.—Anterior to the pectoral girdle the
preserved except that the first osteoderm in the right column is displaced posteriorly onto the second osteoderm (Figs. 2.1, 3.1, 4.1, 4.2). The first pair of paramedians appears to correspond to “nuchal” osteoderms, in that they lack lateral osteoderms, and probably sat directly on the parietal in life. These nuchals are relatively wide, but similar to the anteriormost osteoderms in the holotype of C. kahleorum (Heckert and Lucas, 1999, fig. 3). The third row of paramedian osteoderms is displaced
ventrally on the left side but well exposed on the right, where the radially distributed, elongate grooves are among the best- preserved on the specimen. Beginning with what we interpret as the fourth row, pairs are again easily identified for four rows (through the seventh row). Posterior to this the cervical to anterior dorsal left osteoderms are the best preserved, and articulated for another six rows (rows 8–13; Fig. 3.1). The eighth(?) right dorsal paramedian is displaced and rotated into the body cavity, and parts of three right dorsal paramedians that are displaced and covered by matrix are posterior to it (Fig. 3.1). All of one (the fourteenth?) and parts of two (rows 15 and 16?) other left dorsal paramedians are preserved, but detached from the more anterior osteoderms. All or part of four more right dorsal paramedians appear to represent this region of the body (approximately rows 11–14?) and these are closely associated with four essentially square, articulated right lateral osteoderms. The most posterior osteoderms are the widest (approximately 90mm wide). Lateral osteoderms.—Relatively few lateral osteoderms are
preserved and visible. Throughout the specimen, lateral osteoderms possess a more-or-less radial ornamentation of comparatively deeply incised pits emanating from near the center of the osteoderm surface. All are essentially flat and this ornamentation is deeper and better defined than in the dorsal paramedians.
ventral armor comprises more rounded, less polygonal osteo- derms that are deeply incised with a radial pattern of ovate pits (Figs. 2.2, 3.2). After a transitional zone of two rows of slightly more quadrangular osteoderms that more or less corresponds to the pectoral girdle (exposed anterior to the head of the left humerus—Fig. 3.2), the thoracic ventral armor consists of at least six and as many as eight columns of osteoderms. These are all rectangular and slightly wider than long. Articulation impedes precise measurement, but typical width:length ratios are ~1.5–2. Where preserved, all possess a radial pattern of pits emanating from the center of the osteoderm. This patterning is much fainter (smaller, less deeply incised pits) on the thorax than it is on the more anterior ventral osteoderms. The fifth through seventh row of osteoderms posterior to the pectoral girdle indicate the presence of eight columns, exposed as the fourth column on the right side (Fig. 3.2). These osteoderms are much more deeply incised and are extremely similar to the lateral osteoderms exposed on the opposite side. These rows are incompletely preserved, and the quality of preservation diminishes posteriorly so that only a single osteoderm from each of the tenth and eleventh rows is evident. The primary difference between the ventral armor of
NCSM 23168 and the holotype skeleton of C. kahleorum is that the ventral osteoderms of NCSM 23168 are more rectangular and less square (compare to Heckert and Lucas, 1999, fig. 4). Although C. kahleorum has as many as 10 columns of osteoderms, the rows with 10 columns are more posterior than the ones preserved here, so the presence of only eight rows in C. chathamensis may be an artifact of preservation. The NCSM specimen also appears to have more organized
pre-pectoral armor lacking the hexagonal ventral osteoderms of C. kahleorum, although this difference could well be tapho- nomic in nature if those rows are simply not preserved in the NCSM specimen. Appendicular osteoderms.—There are appendicular osteo-
derms on both the dorsal and ventral surfaces of the right side, suggesting that the right forelimb is largely complete. There is a string of at least fourteen approximately rhomboid appendicular osteoderms associated with the right forelimb on the dorsal side (Figs. 2.1, 3.1). These osteoderms overlap one another but have a
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