Field of Science

A New Nearly Complete Articulated Skeleton of the Bipedal Pseudosuchian Poposaurus gracilis from the Upper Triassic of Utah

Back in 2003 I was fortunate to be part of a research team that discovered a mostly complete articulated skeleton of Poposaurus gracilis from the Chinle Formation of Utah. Uncovering such an amazing specimen is definitely one of the highlights of my career, especially the moment when we realized how amazingly complete and articulated the specimen was. Below is a photo of the articulated hind limb and pelvis at that moment during the excavation.

I've seen the specimen a couple of times at Yale in the process of preparation and the specimen is definitely a beauty. Furthermore is is extremely important because prior to this find P. gracilis was only known from the pelvis, hind limbs, and some vertebrae.  Unfortunately, the skull of this new specimen was unrecoverable, but the rest provides much information including an articulated forelimb and the pes (foot). I should also add that we met a lot of good friends on this trip.

The first paper out on this specimen is not a detailed osteological study but rather introduces the specimen and provides a detailed determination of the proposed locomotor abilities of this taxon, concluding from numerous lines of evidence (phylogenetic, extant phylogenetic bracketing, morphological) that Poposaurus was an obligatory biped. Morphological support for bipedality in P. gracilis includes a fully erect posture, relatively short forelimbs, elongate pes, five sacral vertebrae, partially open acetabulum, and an elongate tail. Gauthier et al. also provide an in-depth argument that the development of bipedality in Archosauria represents a major evolutionary innovation in vertebrate history.

Finally this paper gives a preview of Sterling Nesbitt's upcoming (in press, out very very shortly) extremely detailed phylogenetic analysis of the Archosauria (from his PhD dissertation).  Sterling's analysis provides some pretty surprising (and well-supported) placements of various taxa that will turn some heads and cause a good bit of discussion, especially regarding the revised definitions of some beloved (by some) clade names.

Although I am told that the Poposaurus paper is officially out, it is not up yet on either the Yale Peabody Museum website or the BioOne site, but should be soon. I'll post the link as soon as it does. In the meantime you can read more about the initial find here. I'm also looking forward to the complete osteological description of this amazng specimen.

Gauthier, J. A., Nesbitt, S. J., Schachner, E. R., Bever, G. S., and W. G. Joyce. 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.

Abstract - We introduce a spectacular new specimen of a Late Triassic stem crocodilian identified as Poposaurus gracilis. It is part of a poorly known group, Poposauroidea, that, because of its striking similarities with contemporaneous stem avians (“dinosaurs”), has long puzzled archosaur paleontologists. Observed vertebrate locomotor behaviors, together with exceptional preservation of distinctive anatomical clues in this fossil, enable us to examine locomotor evolution in light of new advances in phylogenetic relationships among Triassic archosaurs. Because this stem crocodilian is unambiguously an archosaur, a diapsid, a tetrapod and a choanate sarcopterygian, we can safely infer major components of its locomotor behavior. These inferences, together with form-function constraints, suggest that P. gracilis was a fleet-footed, obligately erect-postured, striding biped. That behavior seems to have been superimposed on the ancestral archosaur’s innovative locomotor repertoire, which includes the capacity to “high walk.” These novelties persist in a recognizable form in archosaurs for at least 245 million years and are widely distributed across Earth’s surface in diverse ecological settings. They thus qualify as evolutionary innovations regardless of significant differences in diversification rates among extant diapsid reptiles.


  1. Thanks for the alert; I've known about the specimen for awhile, but haven't seen much of it yet and am looking forward to it. Also looking forward to Sterling's big archo-revision!

  2. That's great to hear, can't wait to see the paper. What's more, the Yale site you linked to has a Triceratops reconstruction without any cheeks!


  3. I'm going to be pedantic - a couple bits of the skull were actually recovered; I know, because I found them! Still, a few jaw fragments isn't that spectacular :)

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  5. What this it mean 'stem crocodilian'?
    Stem crocodylomorph or stem suchian or something else?

  6. This specimen came from BLM lands, specifically Grand Staircase-Escalante. My job is to make sure everyone knows that!

  7. @Anonymous - By stem crocodile they mean that it is near the base of the crown group, Pan-Crocodylia.

    @220mya - it was definitely a great find, and at the end of the day too!

    @Alan - GSENM rules!

  8. "... concluding from numerous lines of evidence (phylogenetic, extant phylogenetic bracketing, morphological) that Poposaurus was an obligatory biped."

    How can EPB tell you anything about any stem-croc's -pedality, when the two closest extant outgroups are extant crocs (quadrupedal) and birds (bipedal)?

    Not that I dispute the bipedality finding for a moment -- I just don't see how EBP can help you reach that conclusion (or any conclusion).

  9. I'm glad it's finally out too, thanks to Bill and Randy who were there and knew right away what it was. I think, I think...yes, it's a poposaur! That was a pretty fun day, wasn't it? And thanks to Alan and the BLM for making it possible.

  10. @Mike,
    What they deduce from EPB is that Poposaurus was at least able to perform a 'high walk' and thus had an upright posture. Then they proceed to build upon this using morphological characters to argue for obligate bipedality. I have to admit that they build a good case.


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