The osteology and phylogenetic position of the loricatan (Archosauria: Pseudosuchia)Heptasuchus clarki, from the ?Mid-Upper Triassic, southeastern Big Horn Mountains, Central Wyoming (USA)

Abstract

Loricatan pseudosuchians (known as “rauisuchians”) typically consist of poorly understood fragmentary remains known worldwide from the Middle Triassic to the end of the Triassic Period. Renewed interest and the discovery of more complete specimens recently revolutionized our understanding of the relationships of archosaurs, the origin of Crocodylomorpha, and the paleobiology of these animals. However, there are still few loricatans known from the Middle to early portion of the Late Triassic and the forms that occur during this time are largely known from southern Pangea or Europe. Heptasuchus clarki was the first formally recognized North American “rauisuchian” and was collected from a poorly sampled and disparately fossiliferous sequence of Triassic strata in North America. Exposed along the trend of the Casper Arch flanking the southeastern Big Horn Mountains, the type locality of Heptasuchus clarki occurs within a sequence of red beds above the Alcova Limestone and Crow Mountain formations within the Chugwater Group. The age of the type locality is poorly constrained to the Middle—early Late Triassic and is likely similar to or just older than that of the Popo Agie Formation assemblage from the western portion of Wyoming. The holotype consists of associated cranial elements found in situ, and the referred specimens consist of crania and postcrania. Thus, about 30% of the osteology of the taxon is preserved. All of the pseudosuchian elements collected at the locality appear to belong to Heptasuchus clarki and the taxon is not a chimera as previously hypothesized. Heptasuchus clarki is distinct from all other archosaurs by the presence of large, posteriorly directed flanges on the parabasisphenoid and a distinct, orbit-overhanging postfrontal. Our phylogenetic hypothesis posits a sister-taxon relationship between Heptasuchus clarki and the Ladinian-aged Batrachotomus kupferzellensis from current-day Germany within Loricata. These two taxa share a number of apomorphies from across the skull and their phylogenetic position further supports ‘rauisuchian’ paraphyly. A minimum of three individuals of Heptasuchus are present at the type locality suggesting that a group of individuals died together, similar to other aggregations of loricatans (e.g., Heptasuchus, Batrachotomus, Decuriasuchus, Postosuchus).

Widespread azendohsaurids (Archosauromorpha, Allokotosauria) from the Late Triassic of western USA and India

“…data; Stocker 2013), and the age of the Heptasuchus locality is poorly constrained (Nesbitt et al . 2020). Therefore, correlations to the Otischalkian based on the Wyoming Hyperodapedon occurrence or using Heptasuchus are questionable and likely to be incorrect, and any correlations should be restricted to be with the type locality of Parasuchus bransoni in the Popo Agie Formation of Fremont County, Wyoming.…”

“…The exact age of these deposits is not clear and could be entirely older than that of the bone‐bearing localities of the Chinle Formation and Dockum Group (Stocker 2013; Nesbitt et al . 2020).…”

A new pseudosuchian archosaur,Mambawakale ruhuhugen. et sp. nov., from the Middle Triassic Manda Beds of Tanzania

“…We assessed the phylogenetic position of Mambawakale ruhuhu using the early archosaur matrix of Nesbitt [6] as a framework, and following the modifications of characters, scores and terminal taxa presented by Nesbitt et al . [35]. This modified matrix incorporates the largest published taxon and character sampling for early pseudosuchians, as well as an extensive sample of early avemetatarsalians.…”

“…Following Nesbitt et al . [35], we a priori excluded the following terminal taxa: Lewisuchus admixtus and Pseudolagosuchus majori (combined into Lewisuchus / Pseudolagosuchus following [6,12], Prestosuchus loricatus paralectotype, Prestosuchus chiniquensis lectotype, Prestosuchus chiniquensis paralectotype, Prestosuchus chiniquensis type series, UFRGS PV 156T, UFRGS PV 152T and CPEZ 239b were all combined in the Prestosuchus chiniquensis ALL terminal taxon following Desojo et al . [36] and Nundasuchus songeaensis and Pagosvenator candelariensis were excluded based on rationale from Nesbitt et al .…”

“…[36] and Nundasuchus songeaensis and Pagosvenator candelariensis were excluded based on rationale from Nesbitt et al . [35]. This resulted in a primary dataset following taxon deletion with 90 taxa.…”