Heckert et al.—New species of Coahomasuchus from North Carolina


see also Parker, 2016) independently redescribed the lateral osteoderms and several other aspects of C. kahleorum as well. The taxonomic significance of isolated osteoderms has


been hotly debated, but many taxa remain known solely from osteoderms, and osteoderm characters provide abundant infor- mation used in cladistic hypotheses of aetosaur relationships (Heckert and Lucas, 1999, 2000; Parker, 2007, 2016; Heckert et al., 2015). Therefore, the preservation of two carapaces of Coahomasuchus provides an opportunity to test hypotheses regarding the nature of variation in osteoderms. Certainly there is variation in the shape and proportions of dorsal paramedian osteoderms to accomplish changes in carapace shape posteriorly through the column. However, we note that all of the osteoderms described here have the same fundamental orna- mentation pattern, and that this particular pattern has only been reported for Coahomasuchus (Heckert and Lucas, 1999; Desojo et al., 2013; Parker, 2016). Similarly, the laterals, which are relatively plesiomorphic, are still remarkably consistent in shape and ornamentation pattern between the two skeletons. We interpret this to show that the markedly greater width of C. chathamensis is a real taxonomic feature, not a result of intraspecific variation based on age or sex.


Discussion


Coahomasuchus chathamensis is important because it repre- sents a new taxon closely related to C. kahleorum, which is otherwise only known with certainty from NMMNH locality 3357 in the Colorado City Formation of west Texas. Impor- tantly, as documented previously, C. chathamensis is definitely wider-bodied than C. kahleorum and the isolated, but referred, osteoderms from NCPALEO1902 indicate that the maximum size of Coahomasuchus is definitely larger than either preserved holotype specimen. Cranial material of aetosaurs is not particularly well


known, so this new record is important as it permits additional insights into the skull of Coahomasuchus not available to Lucas and Heckert (1999) or Desojo and Heckert (2004). If the right quadratojugal in NCSM 23618 is correctly identified, then the broad contact between the squamosal and the quadratojugal as depicted in Stagonolepis and many other aetosaurs is not present in C. chathamensis (Fig. 7). However, it should be pointed out that this region of the skull is rather poorly preserved in the majority of specimens of other described taxa, including Longosuchus, Aetosaurus and Neoaetosauroides, and we sus- pect that the restorations of these taxa may have been influenced by the condition reported in Stagonolepis (Walker, 1961). Coahomasuchus chathamensis is notable for its relatively


wide osteoderms as preserved in both the anterior carapaces of the holotype as well as in several of the isolated osteoderms (e.g., NCSM 16444-1—Fig. 6.2). Indeed, NCSM 16444-1, while incomplete, has a W:L ratio of 3.6:1. This is comparable to the ratios of known wide-bodied taxa such as Typothorax, Paratypothorax, and Tecovasuchus (Typothoracisinae of Parker, 2007; Desojo et al., 2013; emended to Typothoracinae by Parker, 2016), all of which have multiple paramedian osteoderms with W:L ratios exceeding 3.5:1 and some as wide as 4:1 (Heckert and Lucas, 2000; Martz and Small, 2006; Desojo et al., 2013). Unlike these taxa, however,


175


C. chathamensis lacks spinose lateral armor. Because of the incomplete nature of the Pekin Formation fossils, we only ten- tatively note that it is possible that Coahomasuchus marks an early acquisition of a wider body plan that is probably inde- pendent of (and convergent with) that of typothoracisines. Because Coahomasuchus is older than any of the other wide- bodied taxa, this marks the earliest evidence we have of an aetosaur evolving a relatively wide body, with paramedian osteoderms attaining W:L ratios of as much as 3.5:1. Interest- ingly, Parker (2016, fig. 8A) recovers Coahomasuchus in a somewhat later branching position, closely related to Typothorax and Paratypothorax,socoding C. chathamensis identically to C. kahleorum except for Parker’s (2016) character 64, which would code as “2” like Typothorax and Paratypothorax,would almost certainly strengthen that relationship.


Phylogeny.—As we prepared to submit this article two new phylogenetic hypotheses of aetosaurs appeared in the literature (Parker, 2016; Schoch and Desojo, 2016). Of these, we discuss Schoch and Desojo’s (2016) analysis, which we find problematic, first, before addressing how Coahomasuchus chathamensis may affect Parker’s (2016) analysis. Like most recent phylogenies, Schoch and Desojo (2016)


effectively used the character matrix of Parker (2007) as updated by some later authors (Parker et al., 2008; Parker and Martz, 2010; Desojo et al., 2012; da-Silva et al., 2014). Although Schoch and Desojo (2016) did add seven new cranial characters to the analysis, they chose not to incorporate updates and corrections to the main body of the matrix we published when we named Gorgetosuchus (Heckert et al., 2015), in spite of the fact that Gorgetosuchus was published well prior to their paper. This is important, because not only did we add Gorgetosuchus to the taxon list, but we included a total of eleven scoring changes for Lucasuchus, Longosuchus, Typothorax, Redondasuchus, and Coahomasuchus, the latter based on our observations of the material described here. Thus, Schoch and Desojo (2016) effectively reused the da-Silva et al. (2014) version of the matrix without Gorgetosuchus or eight of the eleven coding changes we recommended (nor did they cite Heckert et al. [2015], so it is unclear why they made the changes they did). In light of the omissions of Schoch and Desojo (2016) we are skeptical of their results, particularly their finding of Coahomasuchus as a “wildcard taxon” that prevented them from achieving a well-resolved tree in their main analysis. Of the seven characters novel to Schoch and Desojo’s


(2016) analysis, we can only update one scoring, coding Coahomasuchus as (1) for the their character 43 (squamosal included or excluded from the infratemporal fenestra by the postorbital-quadratojugal contact). The posterior process of the postorbital we have described here articulates with the quadratojugal and effectively excludes the squamosal from the infratemporal fenestra (Fig. 7). Parker’s (2016) analysis built off his (2014) dissertation


and incorporates many more characters (83) than any previous study. Unlike Schoch and Desojo (2016), he incorporated Gorgetosuchus and at least some of the character scoring changes we (Heckert et al., 2015) advocated, with the caveat that he also overhauled many of the characters. We therefore consider his analysis more robust generally and far more likely


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