Field of Science

The Braincase of the Late Triassic Archosauriform Proterochampsa and its Implications for Archosauriform Phylogeny.

Trotteyn, M. J. and J. A. Haro. In Press. The braincase of a specimen of Proterochampsa Reig (Archosauriformes: Proterochampsidae) from the Late Triassic of Argentina. Paläontologische Zeitschrift. Published on-line May 11 2010. DOI 10.1007/s12542-010-0068-7

Abstract: The proterochampsids are a Triassic group of superficially crocodile-like forms belonging to the Archosauriformes. In the present contribution, we present new information regarding the braincase of the proterochampsid Proterochampsa Reig 1959, from the Ischigualasto Formation (Carnian) of Argentina, and discuss its phylogenetic considerations. Some unique neurocranial features of Proterochampsa are described, including: the prominence and thickness of the V-shaped ridge that surrounds the basisphenoidal fossa; the medially concave lateral arms of the same ridge; and the semilunar depression on the parabasisphenoid ventrolaterally exposed. Other features are only shared with likely unrelated archosauriforms, including: the great lateral development of the basipterygoid processes and caudal development of its distal end; an eight-shaped metotic foramen; laterally directed basipterygoid processes; and rostral boundary of the basisphenoidal recess V-shaped. Proterochampsa differs in many other aspects from the archosauriform Chanaresuchus, including: a proportionally shorter basioccipital basal tubera; cultriform process ovoid in cross-section; longitudinal sulcus dorsal to the basipterygoid process; deep basisphenoidal recess; and the absence of a prominent intertuberal plate. In many braincase features, Proterochampsa is more similar to archosaurs than to Euparkeria, erythrosuchids and Proterosuchus. They include a reduced semilunar depression. A ventral border of the basioccipital forming a wide convexity and a dorsoventrally thin paroccipital process likely represents a feature shared with Chanaresuchus, but not with Doswellia and other basal archosauriforms.

Comments: There is currently no clear consensus among Triassic workers on the phylogentic relationships of the archosauriforms. Older (e.g., Sereno 1990, Juul, 1994) analyses recover the proterochampsids as the sister taxon of Archosauria, with Euparkeria, Erythrosuchus, and Proterosuchus as successive outgroups, relationships that were also recovered in a more recent analysis of the Triassic archosauriform Vancleavea campi by Parker and Barton (2008). However, two more recent analyses Dilkes and Sues (2009) and Nesbitt et al. (2009) have challenged this. Dilkes and Sues (2009), in a study of Doswellia, still recovered Proterochampsidae as the sister taxon of Archosauria, but found Erythrosuchus to be more closely related to that clade than Euparkeria. Contrastingly Nesbitt et al. (2009) found Euparkeria more closely related to Archosauria than proterochampsids. Furthermore, analyses based solely on braincase characters (e.g., Gower and Sennikov, 1996) have not included preterochampsids.


The Trotteyn and Haro paper provides a much needed, up to date, description and comparison of a referred braincase of the proterochampsid Proterochampsa barrionuevoi. Interestingly the braincase of Proterochampsa implies a closer relationship with Archosauria, rather than Euparkeria and Erythrosuchus; however, it is important to remember to a recent study by Rauhut (2007) found that braincase characters are not 'conservative' in nature and instead highly prone to homoplasy and dependence on other character suites. Nonetheless, this paper provides important information for workers wishing to incorporate proterochampsid braincase data into their analysis. We are still awaiting a phylogenetic analysis of the Archosauriformes that incorporates all proposed archosauromorph taxa.

REFERENCES

Dilkes, D., and H.-D. Sues. 2009. Redescription and phylogenetic relationships of Doswellia kaltenbachi (Diapsida: Archosauriformes) from the Upper Triassic of Virginia. Journal of Vertebrate Paleontology 29: 58–79.

Gower, D.J., and A.G. Sennikov. 1996. Morphology and phylogenetic informativeness of early archosaur braincases. Palaeontology 39: 883–906.

Juul, L. 1994. The phylogeny of basal archosaurs. Palaeontologia Africana 31: 1–38.

Nesbitt, S. J., Stocker, M. R., Small, B. J. and A. Downs. 2009. The osteology and relationships of Vancleavea campi (Reptilia: Archosauriformes). Zoological Journal of the Linnaean Society 157: 814-864.

Parker, W. G., and B. J. Barton. 2008. New Information on the Upper Triassic Archosauriform Vancleavea campi based on new material from the Chinle Formation of Arizona. Palaeontologia Electronica Vol. 11, Issue 3; 14A: 20p; http://palaeo-electronica.org/2008_3/158/index.html

Rauhut, O. W. M. 2007. The myth of the conservative character: braincase characters in theropod phylogenies. Hallesches Jahrbuch für Geowissenschaften, Beiheft 23: 51-54.

Sereno, P.C., and F.E. Novas. 1994. The skull and neck of the basal theropod Herrerasaurus ischigualastensis. Journal of Vertebrate Paleontology 13: 451–476.

4 comments:

  1. Thanks for posting these, Bill, it's useful to have a one-stop shop for all my Triassic needs.

    Only one suggestion: I always start reading your comments thinking they're the abstract, then have to mentally rewind when I realise what's going on. I'd find it more natural, and I guess others would too, if you started with the reference and abstract, then added your comments.

    Anyway, however you choose to do it, it's a great service. Thanks again.

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  2. Thanks Mike. I've realized the same thing, but I guess it took someone pointing it out to get me to fix it.

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  3. Great to see papers like this coming out of nowhere. This needed to be done. It's a shame some other stuff I've worked on is too far out to possibly take advantage of this new information.

    Nick

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  4. Dear William: thanks for your nice comments. Your opinion acourage us to follow studying more proterochampsid braincases, as we are doing by now. Today we study the braincase of a new proterochampsid from Ischigualasto Formation (Upper Triassic).
    Best regards
    Jimena Trotteyn & Augusto Haro.

    ReplyDelete

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