174


Journal of Paleontology 91(1):162–178


overlapped the squamosal (Fig. 7). In other aetosaurs, including Stagonolepis (Walker 1961, fig. 2; Sulej, 2010, fig. 4) Stenomyti (Small and Martz, 2013, fig. 4.11), and Desmatosuchus (Small, 2002, fig. 2a), the postorbital itself forms a pronounced posterior process that broadly overlaps the anterior edge of the squamo- sal. In Coahomasuchus it would appear that there was therefore a more reduced overlap of the squamosal. The occipital portion of the squamosal is also pronounced and most similar to


Aetosaurus (compare Fig. 5.1, 5.2 to Desojo and Baéz, 2007, fig. 5; Schoch, 2007, fig. 5a). Additionally, the jugal has a slender posterior process that


contrasts with the condition in Desmatosuchus (Small, 2002, fig. 2a), where it is stouter and shorter. This is suggestive of a


slightly longer lower temporal fenestra in Coahomasuchus chathamensis, a condition more reminiscent of Stagonolepis robertsoni (Walker, 1961, fig. 2) and Stenomyti (Martz and Small, 2013, fig. 11) (Fig. 7). Nevertheless our interpretations still favor a relatively small infratemporal fenestra, also seen in modern interpretations of Neoaetosauroides Desojo and Baéz, 2007, Aetosaurus (Schoch, 2007), Desmatosuchus (Small, 2002), and Paratypothorax (Schoch and Desojo, 2016), not the much larger infratemporal fenestra shown in many earlier reconstructions of aetosaurs such as Neoaetosauroides (Bonaparte, 1967, 1971). Mandible and dentition.—The broad contact of the sur-


angular over the angular is closer to the condition in Aetosaurus (e.g., Schoch, 2007, fig. 9b) than in Stagonolepis robertsoni (Walker, 1961, fig. 2c), although the surangular extends further posteriorly and ventrally in NCSM 23168 than in Aetosaurus (Fig. 7). Significantly, the ventral process of the surangular is at least as long as, or longer than, the dorsal process in Coahomasuchus chathamensis and quite unlike either Stagonolepis robertsoni (as illustrated by Walker 1961, fig. 6) or Stagonolepis olenkae (as illustrated by Sulej 2010, fig. 7) (Fig. 7). As a result the more ventral process of the surangular extends much further anteriorly than it does in either species of Stagonolepis (e.g., Walker, 1961; fig. 6) and Paratypothorax (Schoch and Desojo, 2016, fig. 6). Indeed such a condition has not been illustrated in any other aetosaur. Finally, the preserved teeth are cylindrical to weakly


recurved, and therefore plesiomorphic (Heckert and Lucas, 1999; Parker, 2007, 2016). They closely resemble those of Aetosaurus (Walker, 1961; Schoch, 2007) in lateral view, but are more cylindrical and less bulbous. The best preserved teeth are essentially round in cross-section and cylindrical (straight- sided) in labial view for approximately 80% of their total height and are recurved at the tip. This condition appears to correspond with Parker’s (2016) character state (1) for his character (35) except that the bases of the exposed teeth are essentially straight, and not conical, although we cannot be certain that these are maxillary teeth. The preserved teeth of Coahomasuchus chathamensis are less recurved than those Parker (2016) describes for the specimen he assigned to C. kahleorum (TMM 31100-437), but the comparison is complicated by the disarticulated nature of the holotype of C. chathamensis. Armor.—Although the holotypes of both species of


Coahomasuchus include skull and jaw material, neither is well preserved, and, given that osteoderm characters are often used to delineate aetosaurian taxa, it is the armor that ties these two


species in the genus Coahomasuchus. In general, the osteo- derms of NCSM 23168 strongly resemble those of NMMNH in P-18496, the holotype of C. kahleorum, but are considerably wider, with homologous paramedian osteoderms as much as 30% wider than their counterparts NMMNH P-18496. Exclu- sive of the paramedian osteoderms, NCSM 23168 is broadly comparable in size to the holotype of C. kahleorum. The proximal end of the humerus of NCSM 23168 is slightly wider (~25 to ~23mm), but the braincases are equally as wide across the basipterygoid processes (~18mm). However, comparable dorsal paramedian osteoderms are always narrower in C. kahleorum than C. chathamensis. Osteoderms in approxi- mately rows 14, 15 in C. chathamensis are more than 90mm wide, whereas they are only ~75mm wide in C. kahleorum. These differences are consistent regardless of whether the osteoderm width is measured in a straight line (dorsal view) or along the arc of the osteoderm. We are therefore confident that the two specimens represent distinct species. The ornamentation of the osteoderms is remarkably


similar, with extremely faint, subparallel grooves on the presacral paramedian osteoderms. As was the case in the holotype of Coahomasuchus kahleorum, this pattern is so faint that it is difficult to see any ornamentation on the cervical and thoracic paramedians. Similarly the dorsal eminences are extremely reduced, barely discernable in posterior view, and are expressed as very low longitudinal keels located much closer to the medial edge than the lateral one. These characteristics are apomorphic for Coahomasuchus in that the presacral para- medians of Coahomasuchus bear extremely faint ornamentation that, especially on the lateral side of the osteoderm, appears to consist almost entirely of sub parallel grooves and ridges to the exclusion of pits (Heckert and Lucas, 1999; Desojo and Heckert, 2004). We use the term “sub parallel” because the pits and ridges do not emanate in as radial a fashion as is typical in most aetosaurs, although perhaps this could be better quantified in the future. The only aetosaur with comparably faint ornamentation is Neoaetosauroides engaeus Bonaparte (1967, 1971; Desojo and Baéz, 2005), which has bosses that are both more distinct and situated closer to the middle of the osteoderm relative to Coahomasuchus and an ornamentation of more radially distributed grooves and ridges (Bonaparte, 1967, 1971; ABH pers. obs). Apachesuchus heckerti Spielmann and Lucas, 2012 has essentially no ornamentation, and is the only taxon with less distinct ornamentation than Coahomasuchus. Lateral osteoderms in Coahomasuchus chathamensis are


distinct in that the few (~3) relatively well preserved osteoderms in the holotype carapace are more deeply incised with a pattern of deeply incised pits with few, if any grooves compared to those of C. kahleorum. Many of the isolated lateral osteoderms, however, more closely match posterior (mid-thoracic, sacral, and caudal) lateral osteoderms of C. kahleorum. We note that


the asymmetrical shape of the laterals, with a narrower dorsal flange and broader lateral flange separated by a longitudinal keel was not originally reported in C. kahleorum by Heckert and Lucas (1999) but is clear from both the published illustrations (Heckert and Lucas, 1999, fig. 3) and our examination of a cast of that specimen. Indeed, we labeled such osteoderms as laterals in earlier presentations of this material (Fraser et al., 2007; Schneider et al., 2011). In his recent dissertation Parker (2014,


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