Field of Science

Koilamasuchus gonzalezdiazi, a New Basal Archosauriform from the Lower Triassic of Argentina

Ezcurra, M. D. , Lecuona, A., and A. Martinelii. 2010. A new basal archosauriform diapsid from the Lower Triassic of Argentina, Journal of Vertebrate Paleontology 30:1433-1450, doi: 10.1080/02724634.2010.501446

Abstract - The best-known South American Early Triassic archosauriform belongs to a putative proterosuchid briefly reported by Jose Bonaparte in 1981, collected from the Quebrada de los Fosiles Formation (Puesto Viejo Group, Argentina). This specimen consists of well-preserved natural external molds of a partial postcranium that preserve dorsal vertebrae, osteoderms, a dorsal rib, a possible gastralium, a chevron, a humerus, an ilium, two metapodials, and an ungual. We re-describe this specimen and identify autapomorphies that allow us to recognize Koilamasuchus gonzalezdiazi, gen. et sp. nov. The presence of an iliac blade with a slightly convex dorsal margin and with a maximum length more than 3 times its maximum height places Koilamasuchus within Archosauriformes. A cladistic analysis of basal Archosauriformes positions Koilamasuchus more crownwards than Proterosuchus, Sarmatosuchus, Fugusuchus, and Osmolskina, as the sister taxon of the clade that includes Erythrosuchidae and Archosauria. Proterosuchidae is found to be paraphyletic. The presence of an iliac preacetabular process, a pubic peduncle that forms an angle lower than 45◦ to the longitudinal axis of the ilium, and dorsal body osteoderms positions Koilamasuchus in Archosauriformes more crownwards than proterosuchids. Koilamasuchus is more basal than erythrosuchids within Archosauriformes because of the presence of dorsal ribs with a poorly developed proximal end. Koilamasuchus importantly increases the diversity of Archosauriformes during the biotic recovery following the Permo-Triassic mass extinction.

This is an interesting paper from the new issue of the Journal of Vertebrate Paleontology describing a new archosauriform from the Early Triassic of South America.  Along with recent descriptions of the enigmatic form Vancleavea (Parker and Barton 2008; Nesbitt et al. 2009) as well as a modern description of Doswellia (Dilkes and Sues 2009), this new taxon helps flesh out relationships in this part of the tree.  I've only briefly skimmed the paper so far, but noticed some interesting conclusions from the study:

1) Proterosuchidae is paraphyletic - the monophyly of the group is rarely tested as most recent studies just use Proterosuchus as a terminal taxon in phylogenetic analyses.

2) Vancleavea and Doswellia form a clade which is closer to Archosauria than Euparkeria and Chanaresuchus are - as stated in the paper this is closer to the results that I obtained in my 2008 paper on Vancleavea (Parker and Barton 2008); however, the results of our phylogenetic analysis weren't very robust.

3) Turfanosuchus is a pseudosuchian - I've always hypothesized this as the calcaneum of Turfanosuchus is extremely similar to that of aetosaurs and the femur is almost identical to that of Revueltosaurus in overall morphology and in possessing a massive fourth trochanter (Parker et al. 2005).

One unfortunate preservational aspect of this material is the lack of the femur, which I feel is a key element in determining whether the taxon would be closer to Proterosuchus and Erythrosuchus with their primitive morphology, or to the more derived forms Vancleavea and Euparkeria (see the discussion in Parker and Barton 2008). Another key aspect that could affect the phylogenetic analysis (and possibly did in this paper) is the coding of osteoderms as present in Erythrosuchus, which could potentially place erythrosuchids in a more crownward position.  Despite the recovery of a couple of possible osteoderms with material of Erythrosuchus (Gower 2003) their actual presence in this taxon is ambiguous and should not be coded as present (Parker and Barton 2008; Nesbitt et al. 2009). This coding may explain why Koilamasuchus is recovered as more basal to erythrosuchids in this analysis.

REFERENCES

Dilkes, D.W., 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. 2003. Osteology of the early archosaurian reptile Erythrosuchus africanus Broom. Annals of the South African Museum 110:1–84.

Nesbitt, S. J., Stocker, M. R., Small, B. J., and Downs, A. 2009. The osteology and relationships of Vancleavea campi (Reptilia: Archosauriformes). Zoological Journal of the Linnean 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. Paleontologia Electronica 11.3.14A:1–20.

Parker, W. G., R. B. Irmis, S. J. Nesbitt, J. W. Martz, and L. S. Browne. 2005. The Late Triassic pseudosuchian Revueltosaurus callenderi and its implications for the diversity of early ornithischian dinosaurs. Proceedings of the Royal Society of London, Series B: Biological Sciences 272:963–969.

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