Poposaurus gracilis Mehl, 1915

Nesbitt, Sterling J., 2011, The Early Evolution Of Archosaurs: Relationships And The Origin Of Major Clades, Bulletin of the American Museum of Natural History 2011 (352), pp. 1-292 : 28-29

Poposaurus gracilis Mehl, 1915

AGE: Carnian–early Norian, Late Triassic.

OCCURRENCE: Popo Agie Formation, Wyoming ; Blue Mesa Member of the Chinle Formation , Arizona ; Mesa Redondo Member of the Chinle Formation , Arizona ; Tecovas Formation of the Dockum Group , Texas ; Monitor Butte Member of the Chinle Formation , southern Utah .

HOLOTYPE: FMNH 357, two dorsal vertebrae, one caudal vertebra, a left ilium, the proximal portion of a left femur, a right femur, distal portion of the ischia.

REFERRED MATERIAL: TTU-P 10419, vertebrae, pelvic elements; TMM 43683-1, vertebrae and nearly complete pelvis; various UCMP elements from A269 (see Long and Murry, 1995); YPM 57100, nearly complete skeleton lacking the skull.

REMARKS: Poposaurus gracilis was named from a fragmentary specimen consisting of pelvic elements, the femora, and a few vertebrae ( Mehl, 1915). The differences in morphology from other Triassic archosaurs led various authors to identify P. gracilis as an ornithischian ( Nopsca, 1921), a stegosaur ( Huene, 1950), a theropod ( Colbert, 1961), and a pseudosuchian ( Walker, 1969). New specimens of P. gracilis and other closely related taxa confirmed the pseudosuchian affinity of the taxon ( Galton, 1977; Long and Murry, 1995; Nesbitt and Norell, 2006; Weinbaum and Hungerbühler, 2007). With the exception of a nearly complete skeleton lacking the skull ( Joyce and Gauthier, 2006), nearly all specimens of P. gracilis consist of pelvic material, a few vertebrae, and partial limbs ( Weinbaum and Hungerbühler, 2007).

The element that was identified as the pubes in the holotype (FMNH 357) is actually the ischium; therefore, the pubis is not represented in the holotype material. The element that was once identified at the pubis bears a large distal expansion (5 pubic boot), and this expansion has greatly influenced the interpretation of its relationships in older ( Colbert, 1961) and more recent ( Weinbaum and Hungerbühler, 2007) studies. Indeed, the ischium bears a greatly enlarged distal expansion. Ironically, new specimens confirm that a large distal expansion (5 pubic boot) is present in P. gracilis (TMM 43683-1; YPM 57100).

Dawley et al. (1979) described Heptasuchus , another ‘‘rauisuchian’’ from the same formation (Popo Agie Formation) as the holotype of P. gracilis . Later, Zawiskie and Dawley (2003) hypothesized that the skull of Heptasuchus belongs to the body of P. gracilis . Although only a few elements (e.g., pubis, ulna) are directly comparable between the unique specimen of Heptasuchus and P. gracilis , there are important differences between the pubes. Both taxa have a distal expansion of the pubis; however, the distal expansion in Heptasuchus is robust and rounded like that of Batrachotomus rather than the mediolaterally compressed distal expansion of P. gracilis (TMM 43683-1). Furthermore, the preserved portions of the skull of Heptasuchus (maxilla, premaxilla, braincase) are much like that of Batrachotomus and not much like those of the putative close relatives of Poposaurus such as Arizonasaurus and Effigia (Nesbitt, 2007) . Furthermore, it is reasonable to assume that more than two paracrocodylomorph taxa exist in a single assemblage as demonstrated by the cooccurrence of Postosuchus and Poposaurus in the Placerias Quarry ( Long and Murry, 1995) and Postosuchus and Shuvosaurus in the Post (5 Miller) Quarry ( Long and Murry, 1995). Therefore, the hypothesis that Heptasuchus represents the skull of P. gracilis is rejected here.

Poposaurus gracilis possesses two autapomorphies: a thick lateral ridge posterior to the acetabulum and a pit on the proximal part of the ischium for reception of the convex ischial peduncle of the ilium ( Weinbaum and Hungerbühler, 2007).

KEY REFERENCES: Mehl, 1915; Colbert, 1961; Galton, 1977; Long and Murry, 1995; Weinbaum and Hungerbühler, 2007.