Updated April 22, 2014 as poposaurs now nest as basal archosaurs, not phytodinosaurs.
Poposaurus and the Poposauridae
Dr. Maurice G. Mehl (1915) described the very first poposaur, Poposaurus (Fig. 1), but it was based only on scraps. Even so, he was able to report, “Everything in the structure of the form so far studied indicates a well-muscled creature light in weight, possibly bipedal in gait occasionally, and most assuredly swift in movement.” Later, citing Nopcsa (1921, 1928), Colbert (1961) remarked, “[Poposaurus] has been commonly regarded as an ornithischian dinosaur of undoubted Triassic age.” In his conclusion Colbert (1961) considered Poposaurus particularly baffling before suggesting it was a theropod. More complete poposaurs would not be described until several decades later. A more complete Poposaurus would not be described until Gauthier et al. (2011) who nested Poposaurus as a “stem crocodilian”, between two sail-back archosaurs, Xilosuchus and Lotosaurus, close to rauisuchians plus crocodylomorphs.
The Poposauridae: The Traditional List
According to Nesbitt (2011) and Gauthier et al. (2011) other poposaurs include Qianosuchus, Arizonasaurus, Xilosuchus, Lotosaurus, Sillosuchus, Shuvosaurus and Effigia. This clade nested as sisters to the Rauisuchia + Crocodylomorpha. Outgroup taxa included Gracilisuchus, Turfanosuchus, Ticinosuchus, Revueltosaurus and the Aetosauria.
The Poposauridae: The Heretical List
Here only the following taxa nested as poposaurs: Poposaurus, Shuvosaurus, Effigia, Turfanosuchus, Silesaurus and Lotosaurus. See figure 1.
Figure 1. Poposaurs to scale and in phylogenetic order (top to bottom). Sacisaurus is at the base. Silesaurus and Lotosaurus are derived. Poposaurus is one of the largest, along with Lotosaurus. Pisanosaurus does not belong here. It is a basal ornithischian.
Dino-Like Crocs? Or Dinos with an Ankle Issue?
Poposaurs are widely considered to be dinosaur-like crocs, nesting with the Rauisuchia + Crocodylomorpha. In particular, Effigia (Fig. 1) was considered to exhibit “extreme convergence” with the body plan of dinosaurs (Nesbitt and Norell 2006).
By contrast, the present large reptile study, many times larger than the Gauthier (2011) or Nesbitt (2011) studies, found that poposaurs nested as basal archosaurs derived from as sited to Turfanosuchus. That solves all sorts of problems. The problem with earlier studies stemmed from a lack of considering the possibility of convergence in these clades with regard to the development of the calcaneal tuber, which also developed by convergence in the unrelated phytosaurs and to a lesser extent in chanaresuchids, including Lagerpeton, which we looked at earlier.
Chatterjee (1985) coined Rauisuchia to incorporate Rauisuchidae and Poposauridae. That term definition is retained here, so birds and crocs are also members of the Rauisuchia. Benton and Clark (1988) used Prestosuchus and Ticinosuchus to represent Rauisuchidae and Postosuchus to represent the Poposauridae. Nesbitt (2011) considered poposaurs monophyletic and a clade within the rauisuchians.
The Postosuchus Pelvis Problem
Postosuchus was considered a poposaurid by several writers, based largely on the pelvis presented by Chatterjee (1985). Long and Murray (1995) showed that that pelvis did not belong to Postosuchus but to an unidentified poposaurid, hence the confusion.
Was Poposaurus an herbivore?
Poposaurus had tiny and gracile fingers with tiny unguals on a short arm, so it was not grasping prey. Rather this clue suggests herbivory as in all other poposaurs. The cervicals were more robust. The vertebral spines were all higher and expanded anteroposteriorly. The tail was deeper proximally and longer distally. The ilium extended over the femur in the manner of a rauisuchian pelvis for additional support. The pubis was longer than the femur. A calcaneal spur or tuber developed extending posterodorsally.
Shuvosaurus and Effigia were more gracile overall with a smaller, toothless skull. The scapula was more robust. The toes were more slender, but the toe unguals were larger.
Silesaurus had a smaller skull on an elongated neck. The torso was relatively smaller and the legs were relatively longer. Lotosaurus became secondarily quadrupedal with a larger body, shorter neck and a dorsal sail supported by neural spines.
Poposaurs developed early into a variety of niches. Some had a slender build and a bipedal configuration with an elongated neck and an herbivorous or toothless dentition. A calcaneal spur developed in this clade convergent with the situation in derived rauisuchians, which were also heavily built, short-legged bipeds, and in crocodylomorphs as they became quadrupeds. The longer-legged poposaurids had smaller spurs. Poposauridae was a monophyletic clade that did not survive into the Jurassic.
Brusatte SL , Benton MJ , Desojo JB and Langer MC 2010. The higher-level phylogeny of Archosauria (Tetrapoda: Diapsida), Journal of Systematic Palaeontology, 8:1, 3-47.
Casamiquela RM 1967. Un nuevo dinosaurio ornitisquio triásico (Pisanosaurus mertii; Ornithopoda) de la Formación Ischigualasto, Argentina. Ameghiniana 4 (2): 47–64.
Colbert EH 1961. The Triassic Reptile, Poposaurus. Fieldiana 14(4):59-78. online pdf
Dzik J 2003. A beaked herbivorous archosaur with dinosaur affinities from the early Late Triassic of Poland. Journal of Vertebrate Paleontology 23: 556-574.
Gauthier JA, Nesbitt SJ, Schachner ER, Bever GS and Joyce WG 2011. The bipedal stem crocodilian Poposaurus gracilis: inferring function in fossils and innovation in archosaur locomotion. Bulletin of the Peabody Museum of Natural History 52:107-126.
Hunt AP 1989. A new ornithischian dinosaur from the Bull Canyon Formation (Upper Triassic) of east-central New Mexico. In Lucas, S. G. and A. P. Hunt (Eds.), Dawn of the age of dinosaurs in the American Southwest 355–358.
Irmis RB, Nesbitt SJ, Padian K, Smith ND, Turner AH, Woody D and Downs A 2007. A Late Triassic dinosauromorph assemblage from New Mexico and the rise of dinosaurs. Science 317 (5836): 358–361. doi:10.1126/science.1143325. PMID 17641198.
Mehl MG 1915. Poposaurus gracilis, a new reptile from the Triassic of Wyoming. Journal of Geology 23:516–522.
Nesbitt SJ 2003. Arizonasaurus and its implications for archosaur divergence
Sterling J. Nesbitt Proceedings of the Royal Society, London B (Suppl.) 270, S234–S237. DOI 10.1098/rsbl.2003.0066
Nesbitt SJ and Norell MA 2006. Extreme convergence in the body plans of an early suchian (Archosauria) and ornithomimid dinosaurs (Theropoda). Proceedings of the Royal Society B 273:1045–1048. online
Nesbitt S 2007. The anatomy of Effigia okeeffeae (Archosauria, Suchia), theropod-like convergence, and the distribution of related taxa. Bulletin of the American Museum of Natural History, 302: 84 pp. online pdf
Nesbitt SJ, Irmis RB, Parker WG, Smith ND, Turner AH and Rowe T 2009. Hindlimb osteology and distribution of basal dinosauromorphs from the Late Triassic of North America. Journal of Vertebrate Paleontology 29 (2): 498–516. doi:10.1671/039.029.0218
Nopcsa, F von 1921. Zur systematischen Stellung von Poposaurus (Mehl). Zentralbl. Min. Geol. Paleont., p. 348.
Nopcsa, F von 1928. The genera of reptiles. Palaeobiologica, 1, pp. 163-188.
Parker WG., et al. 2005. The Pseudosuchian Revueltosaurus callenderi and its implications for the diversity of early ornithischian dinosaurs. In Proceedings of the Royal Society London B 272(1566):963–969.
Weinbaum JC and Hungerbuhler A 2007. A Revision of Poposaurus gracilis (Archosauria: Suchia) based on two new specimens from the Late Triassic of the southwestern USA. Palaeontologische Zeitschrift 81(2):131-145.
Zhang F-K 1975. A new thecodont Lotosaurus, from Middle Triassic of Hunan. Vertebrata PalAsiatica 13:144-147.
AMNH Effigia webpage