Despite its incredible resemblance to theropod dinosaurs, Poposaurus gracilis was actually a pseudosuchian more closely related to crocodilians than to dinosaurs.
Living in what is now North America during the Late Triassic, about 237-216 million years ago, Poposaurus grew to around 4.5m long (~15′) with roughly half of that length taken up by just its long tail. With its sharp-toothed jaws, small arms, bipedal locomotion, and counterbalancing tail, it convergently evolved the same sort of body plan and ecology as carnivorous theropods – which were still in their early days at the time, and wouldn’t really become the dominant terrestrial predators until after the end-Triassic extinction.
Unlike most other pseudosuchians Poposaurus lacked bony osteoderm armor, seems to have been capable of a digitigrade posture, and its claws were flattened and somewhat hoof-like, all adaptations that suggest it was built for running after fast-moving prey.
References:
- Farlow, James O., et al. “Pedal proportions of Poposaurus gracilis: convergence and divergence in the feet of archosaurs.” The Anatomical Record 297.6 (2014): 1022-1046. https://doi.org/10.1002/ar.22863
- Parker, William G., and Sterling J. Nesbitt. “Cranial remains of Poposaurus gracilis (Pseudosuchia: Poposauroidea) from the Upper Triassic, the distribution of the taxon, and its implications for poposauroid evolution.” Geological Society London Special Publications 379. (2013): 503-523. https://www.researchgate.net/publication/258442655_Cranial_remains_of_Poposaurus_gracilis_Pseudosuchia_Poposauroidea_from_the_Upper_Triassic_the_distribution_of_the_taxon_and_its_implications_for_poposauroid_evolution
- Schachner, Emma R., Phillip L. Manning, and Peter Dodson. “Pelvic and hindlimb myology of the basal archosaur Poposaurus gracilis (Archosauria: Poposauroidea).” Journal of Morphology 272.12 (2011): 1464-1491. https://www.academia.edu/download/85559103/1e41da_48ff7dcb23cbe8588873fc8ab8c9265a.pdf
- Schachner, Emma R., et al. “Osteology of the late Triassic bipedal archosaur Poposaurus gracilis (Archosauria: Pseudosuchia) from western North America.” The Anatomical Record 303.4 (2020): 874-917. https://doi.org/10.1002/ar.24298
- Wikipedia contributors. “Poposaurus” Wikipedia, 29 Mar. 2025, https://en.wikipedia.org/wiki/Poposaurus