Aetosauria: a clade of armoured pseudosuchians from the Upper Triassic continental beds

Abstract

Aetosauria is a clade of obligately quadrupedal, heavily armoured pseudosuchians known from Upper Triassic (late Carnian–Rhaetian) strata on every modern continent except Australia and Antarctica. As many as 22 genera and 26 species ranging from 1 to 6 m in length, and with a body mass ranging from less than 10 to more than 500 kg, are known. Aetosauroides scagliai was recently recovered as the most basal aetosaur, placed outside of Stagonolepididae (the last common ancestor of Desmatosuchus and Aetosaurus). Interrelationships among the basal aetosaurs are not well understood but two clades with relatively apomorphic armour – the spinose Desmatosuchinae and the generally wide-bodied Typothoracisinae – are consistently recognized. Paramedian and lateral osteoderms are often distinctive at the generic level but variation within the carapace is not well understood in many taxa, warranting caution in assigning isolated osteoderms to specific taxa. The aetosaur skull and dentition varies across taxa, and there is increasing evidence that at least some aetosaurs relied on invertebrates and/or small vertebrates as a food source. Histological evidence indicates that, after an initial period of rapid growth, lines of arrested growth (LAGs) are common and later growth was relatively slow. The common and widespread Late Triassic ichnogenus Brachychirotherium probably represents the track of an aetosaur.

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.…”

“…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.…”

“…Skull and jaws.-Skulls of aetosaurs remain relatively rare in the Upper Triassic fossil record (Desojo et al, 2013), and are much rarer than those of coeval taxa such as phytosaurs (e.g., Long and Murry, 1995). Thus, the holotype skull of Coahomasuchus chathamensis, while incomplete, somewhat disarticulated, and difficult to interpret, retains important information.…”

“…Desojo and Baéz (2007) were the first modern workers to systematically evaluate the skulls of multiple taxa and show that there was considerable diversity of aetosaur skull morphologies, an idea further developed by Desojo et al (2013). We are not prepared to present a reconstruction of the entire skull of Coahomasuchus based on the new material, but detail insights it provides on the skull roof, temporal region, and lower jaws here (Fig.…”

“…With the exception of Australia and Antarctica, there are aetosaur records for every modern continent (Desojo et al, 2013). Aetosaurs are important as they are one of very few Triassic lineages in the large post-Permian archosauromorph radiation (other than rhynchosaurs) that exhibits any indication of herbivory.…”