Was Postosuchus bipedal or quadrupedal?

My post presenting a new reconstruction of Postosuchus kirkpatricki generated some discussion on whether or not Postosuchus was an obligate biped or quadrupedal. Jeff Martz’s reconstruction (below and revised from the earlier post) is based on research by Jonathan Weinbaum arguing the former (Weinbaum, 2008). In his dissertation Jonathan provides several lines of evidence which he feels clearly demonstrates that Postosuchus is bipedal. This includes a shortened forelimb with an extremely reduced manus, elongate and narrow pubes and ischia (a condition which he claims is only found in bipeds), and the presence of hyposphenes and hypantra in the dorsal vertebrae (which restrict lateral movement and were at one time believed to only occur in dinosaurs, although I showed in 2008 that they also occur in quadrupedal aetosaurians). Some more compelling evidence presented by Weinbaum (2008) is the structure of the brain endocast which is very similar to that of bipedal dinosaurs. He also used a vertebral analysis tool developed by Christian and Preuschoft (1996), which demonstrates that vertebrae are enlarged in areas of support. Quadrupeds show two enlargement areas, whereas bipeds show only one. Postosuchus shows only a single peak above the pelvis, typical for bipeds. Jonathan is not the first to suggest this. Sankar Chatterjee, in his initial description of Postosuchus (Chatterjee, 1985), also suggested that it was bipedal based on a short trunk and forelimbs. He also showed that the forelimb/hindlimb ratio in Postosuchus was similar to the theropod Ornitholestes; however, according to the published dataset (which only includes a few taxa), the ratio is much higher than in other theropods such as Coelophysis, Allosaurus, and Albertosaurus, which are bipedal as are all theropods.

Weinbaum (2008) also looked at forelimb/hindlimb length ratios, and provided a ratio of .59 for Postosuchus, which he claimed falls between that Gallimimus and Deinonychus. Despite Weinbaum’s (2008) claims, I feel that the forelimb is not really that much shorter than the hindlimb (i.e. .59), and thus not conclusive evidence. Again, however, this was a very limited dataset. I would like to see comparisons with more taxa from a variety of archosauromorph clades as both Chatterjee (1985) and Weinbaum (2008) found that Postosuchus has a similar ratio to some of the longest limbed coelurosaurs rather than all theropods in general. I do not find this compelling.

All other workers (e.g., Long and Murry, 1995) have suggested that Postosuchus is quadrupedal (see reconstruction from Wikipedia below), which is how all other non-poposaurid “rauisuchians” have been portrayed. This is mainly based on track evidence, where the ichnotaxon Cheirotherium has been ascribed to a “rauisuchian” trackmaker. Cheirotherium tracks were made by a quadrupedal non-ornthodiran archosaur with a highly reduced manus. Furthermore, Peyer et al. (2008) argued that although the manus is extremely reduced, the pectoral girdle of Postosuchus is robust suggesting that it was used in locomotion.

However, there is trackway evidence that possibly supports bipedal Postosuchus. Olsen and Huber (1998) described tracks in the Upper Triassic Pekin Formation (which includes Postosuchus) as matching well with the pes of Postosuchus and having no sign of a manus impression.

Was Postosuchus capable of bipedal movement? Based on limb ratios and other lines of evidence, yes. Was it an obligate biped? Unfortunately we cannot say for certain as the available data is ambiguous. What is needed is a similar study as that conducted by Bonnan and Senter (2007) for “prosauropods”. These authors argued that as quadrupedal animals must pronate the forelimb for it to provide propulsion, evidence for this must be found to support this type locomotion (or the opposite to support bipedalism). Until a similar study is conducted for Postosuchus, I would say that the case is far from closed.

REFERENCES

Bonnan, M. F., and P. Senter. 2007. Were the basal sauropodomorph dinosaurs Plateosaurus and Massospondylus habitual quadrupeds? Special Papers in Palaeontology 77:139-155.

Chatterjee, S. 1985. Postosuchus, a new thecodontian reptile from the Triassic of Texas
and the origin of tyrannosaurs. Philosophical Transactions of the Royal Society of London, Series B 309, 395-460; London.

Christian, A., and H. Preuschoft. 1996. Deducing the body posture of extinct large vertebrates from the shape of the vertebral column. Palaeontology 39:801-812.

Long, R. A., and P. A. Murry. 1995. Late Triassic (Carnian and Norian) tetrapods from the southwestern United States. New Mexico Museum of Natural History and Science, Bulletin 4:1–254.

Olsen, P. E. and Huber, P., 1998, The oldest Late Triassic footprint assemblage from North America (Pekin Formation, Deep River basin, North Carolina, USA). Southestern Geology, v. 38, no. 2, p. 77-90.

Peyer, K., Carter, J. G., Sues, H.-D., Novak, S. E., and P. E. Olsen. 2008. A new suchian archosaur from the Upper Triassic of North Carolina. Journal of Vertebrate Paleontology 28:363-381.

Weinbaum, J. C. 2008. Review of the Triassic reptiles Poposaurus gracilis and Postosuchus kirkpatricki (Reptilia: Archosauria). Unpublished PhD dissertation, Texas Tech University, 183.