Revised phylogenetic analysis of the Aetosauria (Archosauria: Pseudosuchia); assessing the effects of incongruent morphological character sets

Abstract

Aetosauria is an early-diverging clade of pseudosuchians (crocodile-line archosaurs) that had a global distribution and high species diversity as a key component of various Late Triassic terrestrial faunas. It is one of only two Late Triassic clades of large herbivorous archosaurs, and thus served a critical ecological role. Nonetheless, aetosaur phylogenetic relationships are still poorly understood, owing to an overreliance on osteoderm characters, which are often poorly constructed and suspected to be highly homoplastic. A new phylogenetic analysis of the Aetosauria, comprising 27 taxa and 83 characters, includes more than 40 new characters that focus on better sampling the cranial and endoskeletal regions, and represents the most comprenhensive phylogeny of the clade to date. Parsimony analysis recovered three most parsimonious trees; the strict consensus of these trees finds an Aetosauria that is divided into two main clades: Desmatosuchia, which includes the Desmatosuchinae and the Stagonolepidinae, and Aetosaurinae, which includes the Typothoracinae. As defined Desmatosuchinae now contains Neoaetosauroides engaeus and several taxa that were previously referred to the genus Stagonolepis, and a new clade, Desmatosuchini, is erected for taxa more closely related to Desmatosuchus. Overall support for some clades is still weak, and Partitioned Bremer Support (PBS) is applied for the first time to a strictly morphological dataset demonstrating that this weak support is in part because of conflict in the phylogenetic signals of cranial versus postcranial characters. PBS helps identify homoplasy among characters from various body regions, presumably the result of convergent evolution within discrete anatomical modules. It is likely that at least some of this character conflict results from different body regions evolving at different rates, which may have been under different selective pressures.

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

“…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, C. chathamensis lacks spinose lateral armor.…”

“…The best preserved teeth are essentially round in cross-section and cylindrical (straightsided) 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.…”

“…We divide this discussion into a brief summary of our interpretation of the skull and lower jaws, which are infrequently found with aetosaur specimens and not well understood, and a section on the osteoderms. The osteoderm discussion is critical because osteoderms are extremely important in understanding the taxonomy of aetosaurs (many taxa are known solely from osteoderms- Desojo et al, 2013;Heckert et al, 2015;Parker, 2016), and indeed, despite the completeness of this specimen, it is the osteoderms that verify its assignment to Coahomasuchus.…”

“…Detailed locality data for NCPALEO1902 are on file at NCSM. Heckert et al-New species of Coahomasuchus from North Carolina Parker, 2016), approximately two-thirds of the presacral skeleton is preserved. Dorsal and ventral osteoderms are largely in articulation and some of the appendicular osteoderms covering the forelimbs are preserved (Figs.…”

“…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).…”