Osteology of the Late Triassic Bipedal Archosaur Poposaurus gracilis (Archosauria: Pseudosuchia) from Western North America

Abstract

Poposaurus gracilis is a bipedal pseudosuchian archosaur that has been poorly understood since the discovery of the holotype fragmentary partial postcranial skeleton in 1915. Poposaurus. gracilis is a member of Poposauroidea, an unusually morphologically divergent clade of pseudosuchians containing taxa that are bipedal, quadrupedal, toothed, edentulous, and some individuals with elongated thoracic neural spines (i.e., sails). In 2003, a well preserved, fully articulated, and nearly complete postcranial skeleton of P. gracilis was discovered with some fragmentary cranial elements from the Upper Triassic Chinle Formation of Grand Staircase‐Escalante National Monument of southern Utah, USA. The aim of this work is to describe the osteology of this specimen in detail and compare P. gracilis to other closely related pseudosuchian archosaurs. The open neurocentral sutures throughout the majority of the vertebral column, the small size of this individual, and the presence of seven evenly spaced cyclic growth marks in the histologically sectioned femur indicate that this specimen was a skeletally immature juvenile, or subadult when it died. The pes of P. gracilis contains multiple skeletal adaptations and osteological correlates for soft tissue structures that support a hypothesis of digitigrady for this taxon. When coupled with the numerous postcranial characters associated with cursoriality, and the many anatomical traits convergent with theropod dinosaurs, this animal likely occupied a similar ecological niche with contemporaneous theropods during the Late Triassic Period. Anat Rec, 303:874–917, 2020. © 2019 American Association for Anatomy

Revision ofErpetosuchus(Archosauria: Pseudosuchia) and new erpetosuchid material from the Late Triassic ‘Elgin Reptile’ fauna based on μCT scanning techniques

“…13O: M. I). This condition contrasts with the well-developed tubercle positioned approximately at the midshaft that is present in phytosaurs, ornithosuchids and aetosaurs (Sereno 1991; Parrish 1993; Nesbitt 2011).…”

“…The moulds of three undetermined metatarsals are preserved between BGS GSM 91081 and BGS GSM 91080, close to the distal end of tibia and fibula (Figs 2, 14K). Little can be said about them other than they are unfused, considerably shorter than both the tibia and fibula (approximate maximum length of the longest element is ~14.5 mm), and, thus, are not as elongated as those of most avemetatarsalian archosaurs (Sereno 1991; Nesbitt 2011).…”

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

“…Second, the deposits that Heptasuchus clarki was found in are likely not the same as the Popo Agie Formation from the western portion of Wyoming and the deposits that Heptasuchus clarki was found in are likely older than that of the Popo Agie Formation and hence Poposaurus gracilis. Third, with an abundance of new specimens of Poposaurus gracilis from partial skeletons (Weinbaum & Hungerbühler, 2007) to nearly complete and articulated postcranial remains (Gauthier et al, 2011;Schachner et al, 2019), and comparative skull material (Parker & Nesbitt, 2013), it is clear that Poposaurus gracilis and Heptasuchus clarki are different taxa.…”

“…Namely, it is not clear if these “rauisuchians” represent a natural group (traditional hypothesis; Brusatte et al, 2008 ; 2010 ), a grade leading to crocodylomorphs ( Nesbitt, 2011 ), or a combination of subclades and grades spread across Pseudosuchia ( Nesbitt, 2011 ; Nesbitt et al, 2013 ). Luckily, over the past 20 years, huge headway has been made in uncovering their anatomy and relationships through the discovery of new taxa (e.g., Batrachotomus kupferzellensis ; Gower, 1999 ; Gower & Schoch, 2009 ; Postosuchus alisonae , Peyer et al, 2008 ; Decuriasuchus quartacolonia , De França, Ferigolo & Langer, 2011 ; De França et al, 2013 ; Viveron haydeni , Lessner et al, 2016 ; Mandasuchus tanyauchen , Butler et al, 2018 ), or new specimens of previously named taxa (e.g., Arizonasaurus babbitti , Nesbitt, 2003 , 2005 ; Prestosuchus chiniquensis , Da Silva et al, 2018 ; Mastrantonio et al, 2019 ; Poposaurus gracilis , Schachner et al, 2019 ) and revised and detailed descriptions (e.g., Rauisuchus tiradentes , Lautenschlager & Rauhut, 2015 ; Postosuchus kirkpatricki , Weinbaum, 2011 , 2013 ; Luperosuchus fractus , Nesbitt & Desojo, 2017 ; Prestosuchus chiniquensis , Desojo, Baczko & Rauhut, 2020 ; Ticinosuchus ferox , Lautenschlager & Desojo, 2011 ).…”

Alligators employ intermetatarsal reconfiguration to modulate plantigrade ground contact

“…Indeed, pterosaurian ( Padian, 1983 ) and dinosaurian ( Holtz, 1995 ; Jannel et al, 2019 ) lineages, along with few croc-line taxa [e.g. Poposaurus gracilis ( Farlow et al, 2014 ; Schachner et al, 2020 )], are suggested to be digitigrade and rely on the toes to maximize ground contact. As many taxa throughout Archosauria exhibit varying degrees of adduction or fusion along the length of the metatarsus, we suggest that this character is likely to be an indicator of the degree of metatarsus conformation possible within the pes.…”