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.
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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
Poposaurus gracilis is the type species for the clade Poposauroidea, a group of unusual pseudosuchians who tried all sorts of things, such as sail-backs, beaks, aquatic life, and in Poposaurus gracilis’ case: bipedalism.
Similar to a theropod dinosaur, Poposaurus had reduced front limbs and walked only on its hind limbs. It was fairly large, about 13 feet long… though half of that was its abnormally long tail. It would have walked with a very horizontal posture (though I have illustrated this one as rearing up to look around.) It was a carnivore living in the late Triassic of the southwestern USA.
Since its discovery in 1904, Poposaurus has been classified in nearly every group of archosaur. At first it was assumed to be a phytosaur. Some classified it as an ornithiscian dinosaur, some as a sauropodomorph, one paleontologist even considered it an early stegosaur! In 1961 it was classified as a theropod, particularly a Carnosaur, and then in 1977 it was finally placed within pseudosuchia. In 1995, 2007, and 2011, new Poposaurus fossils were discovered that confirmed this and further narrowed down where it belonged on the pseudosuchian tree.
#ArchosaurArtApril Day 12 - #poposaurus . For some reason the name of this archosaur sound really funny to me, but I probably wouldn't laugh if I encountered one. This aninal was four metres long and related to crocodilians! . . . . #archosaur#sketch #sketchbook #smltart #paleoart #animalart#animals #animalartist #drawing #pencildrawing#pencilart #traditionaldrawing #traditionalartist#traditionalart #dailydrawing #dailyart #instart#instartist #artistsoninstagram https://www.instagram.com/p/BwRCiXsJ3gD/?utm_source=ig_tumblr_share&igshid=12kml84coj9d8
Poposaurus is one of those prehistoric reptiles that no one is exactly sure how to classify, and it has been shifted around to a few different groups. It seems like the general consensus now is that this bipedal archosaur is most likely a Rauisuchian, along with awesome creatures like Postosuchus and Polonosuchus.
Unlike most other prehistoric archosaurs (a big ass possibly polyphyletic group of reptiles that includes crocodylomorpha), this creature was bipedal. There were a few other bipedal early archosaurs (here’s a good article by Brian Switek).
They’re known from fossils found throughout the American West, and they were up to 4 m long. As you might assume to look at them, they were swift predators, probably capable of taking down larger and fast moving prey.
Find out more about their relatives here:
https://en.wikipedia.org/wiki/Poposauroidea
illustrations by Smokeybjb and Jeff Martz/National Park Service
