Field of Science

New Volume on Triassic Archosaurs/ New Cranial Material of Poposaurus gracilis

There is a new volume coming out that is sure to be of interest to all Triassic aficionados. It's titled "Anatomy, Phylogeny and Palaeobiology of Early Archosaurs and their Kin" edited by Sterling Nesbitt, Julia Desojo, and Randall Irmis and published in the Special Publication series of the Geological Society of London. The volume stems from a symposium held in 2011 in San Juan, Argentina at the IV Congreso Latinoamericano de Paleontología de Vertebrados.  It was a great meeting and you can read some more about it here [in Spanish].

The new volume includes overviews of many archosaurian clades including Euparkeriidae, Phytosauria, "Rauisuchia", Ornithosuchidae, and of course Aetosauria. There are also a plethora of other more specific papers.  Currently the papers are being released "online first" and not all are up yet. I'm not even sure how many there are to be in the final volume, but you can consider this volume to be the " The Dinosauria" volume for non-ornithodiran archosauriforms and pseudosuchians. Keep checking back as more papers are released and at some point the printed volume should be available.

My own contribution is up. In 2003 a group from Yale University, assisted with staff from the Petrified Forest, excavated what turned out to be a nearly complete, articulated skeleton of the carnivorous pseudosuchian Poposaurus gracilis (more here and here). This specimen has been covered in several papers now (Gauthier et al., 2011; Schachner et al., 2011) and provides us with more information about poposauroids and their amazing convergence with theropod dinosaurs. Unfortunately, the skull of this specimen has eroded prior to dicovery. This was unfortunate because poposauroids show an amazing diversity of forms from presumably quadrupedal, sail-backed toothed forms (e.g., Arizonasaurus babbitti) , to bipedal edentulous forms (Effigia okeeffeae), and of course a quadrupedal sail-backed, edentulous form (Lotosaurus adentus) just to make things interesting. What is poorly understood is the congruence of the aquisition of these characters in poposauroid phylogeny. In this question, Poposaurus gracilis plays a key role as according to recent phylogenetic analyses of Archosauria (Nesbitt, 2011; Butler et al., 2011) it is a mid-grade poposauroid. It is clear from the Yale specimen that P. gracilis was bipedal and lacked a sail. A fragment of premaxilla found with the specimen suggested the presence of teeth but conformation was needed.

In 2008 Petrified Forest paleontology staff (Kate Hazlehurst and Jeff Martz) discovered a beautifully preserved ilium and pubis of Poposaurus gracilis from the base of the Sonsela Member in the park. Associated with this were a partial maxilla, dentary, and strangely a prearticular.  These elements were not complete, but they were enough to show that the skull was very similar to other poposauroids like Arizonasaurus babbitti, but more importantly it confirmed that P. gracilis was toothed.  The new paper by myself and colleague Sterling Nesbitt describes this new material and discusses its implications for specific character acquisition in the poposauroids.  Essentially we find that character acquisition is very complex and evolving quickly within the group with a strong suggestion of convegent evolution not only with theropod dinosaurs but also within the clade Poposauroidea as well.

Parker, W. G., and S. J. Nesbitt. 2013. Cranial remains of Poposaurus gracilis (Pseudosuchia: Poposauroidea) from the Upper Triassic, the distribution of the taxon, and its implications for poposauroid evolution. From: Nesbitt, S. J., Desojo, J. B. & Irmis, R. B. (eds) Anatomy, Phylogeny and Palaeobiology of Early Archosaurs and their Kin. Geological Society, London, Special Publications, 379,

Abstract - The partial postcrania of Poposaurus gracilis, a bipedal poposauroid convergent with theropod dinosaurs, has been known for nearly a century, but the skull of P. gracilis has proven elusive. P. gracilis is part of a clade of morphologically divergent pseudosuchians (poposauroids) whose members are sometimes bipedal, lack dentition (i.e. beaks) and some have elongated neural spines (i.e. sails). However, the timing and acquisition of these character states is unknown given the uncertainty of the skull morphology of the ‘mid-grade’ poposauroid P. gracilis. Here, we present the first confirmed skull remains of P. gracilis directly associated with diagnostic pelvic elements that overlap with the holotype. The incomplete skeleton (PEFO 34865) from the Chinle Formation of Petrified Forest National Park (Arizona, USA) includes a left maxilla with a large, mediolaterally compressed tooth, left dentary, right prearticular and a partial postcranium. The character states of P. gracilis (bipedal, ‘sail-less’ and toothed) demonstrate that the evolution of bipedalism, the origin/loss of a dorsal ‘sail’ and the shift to an edentulous beak are complex in poposauroids. P. gracilis is widespread in the Upper Triassic formations in the western USA and is restricted temporally prior to the Adamanian–Revueltian faunal turnover during the Norian.


  1. 23 chapters plus introduction.

  2. Darn, still no Chinleana love for the only other released paper from the basal archosaur volume. We should have titled it: a review of weathered, fragmentary chopped liver.


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