Wednesday, April 27, 2011

Poposaurus and the future of 'Crurotarsi'

Like many rauisuchians, Poposaurus has been plagued with a lack of fossil material. However, a recent find in the Upper Triassic Chinle Formation of Utah has helped shed some light into this early archosaur. Although a skull still remains to be found, paleontologists have managed to uncover  an almost complete post-cranial skeleton. What is also interesting is the phylogenetic analysis associated with the new Poposaurus material. The most startling result is that Phytosauria falls out of the crocodile side of the archosaur tree and becomes the sister taxon of Archosauria. Depending on your definition of Crurotarsi, this could mean that dinosaurs are crurotarsans. However, under the Sereno 2005 definition, Crurotarsi is safe. For now ;-). Rauisuchia may also be in trouble based on the new phylogeny, which preserves Poposauroidea, but not Rauisuchidae. A more detailed phylogeny by Sterling Nesbitt will be coming out soon in the Bulletin of the American Museum of Natural History.

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.  

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 even regardless of significant differences in diversification rates among extant diapsid reptiles.


  1. Was the "Sereno, 2005" definition ever actually published though? Most of his new dinosaurian definitions only appeared online on TaxonSearch.

  2. For the sake of this blog, let us hope that Paul neglected to publish his definition so it can be trimmed of Phytosauria. "Pseudosuchia: The Forgotten Archosaurs" is intolerable.

    However, this would mean that you, Susan, must go back and delete all phytosaur-related posts posthaste, a Great Paleo-Purge if you will. "Phytosaurs have never been crurotarsans," saith the Ministry.

    Alternatively, you could expand to include dinosaur coverage or call your blog "The Forgotten Crurotarsans," which, under the old definition, was all of them.

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  4. Poposaurus looks more like the tyrannosaurs from movies than actual tyrannosaurs do. 0_0

    Are we sure it was a predator, though?

  5. There's a paper coming out soon that will address a lot of those issues. Until recently, there wasn't much skull material for Poposaurus, so things like artists' reconstructions and diet had to be inferred.