Reassessment of the Early Neotheropod Camposaurus arizonensis and some Comments on the Stratigraphic Position of the Placerias Quarry

Ezcurra, M. D., and S. L. Brusatte. 2011. Taxonomic and phylogenetic reassessment of the early neotheropod dinosaur Camposaurus arizonensis from the Late Triassic of North America. Palaeontology 54:763-772. DOI: 10.1111/j.1475-4983.2011.01069.x

Abstract - Camposaurus arizonensis, a small theropod dinosaur from the early–middle Norian of Arizona (USA), is widely considered the oldest known neotheropod. However, despite its importance, Camposaurus is the subject of taxonomic and phylogenetic uncertainty and is often considered a nomen dubium, largely because of a fragmentary holotype. We here reassess the holotype of Camposaurus and identify two autapomorphies: the posterior edge of the tibial articular surface for the fibula offset as a sharp and prominent ridge and the absence of an anteriorly expanded medial condyle of the astragalus. We therefore consider Camposaurus to be a valid and diagnostic taxon of basal theropod dinosaur. For the first time, we include Camposaurus in a phylogenetic analysis, which confirms its neotheropod placement and recovers it as a close relative of Coelophysis rhodesiensis within Coelophysoidea sensu stricto. The position of Camposaurus as the oldest neotheropod provides an important calibration point, but necessitates long ghost lineages, indicating that our knowledge of the early evolutionary history of theropod dinosaurs is still patchy. Furthermore, our phylogenetic analysis recovers a polytomy at the base of Neotheropoda, as most parsimonious trees disagree in recovering a monophyletic or paraphyletic ‘traditional’ Coelophysoidea. This suggests that basal theropod phylogeny remains in a state of flux, and the monophyly of ‘traditional’ Coelophysoidea remains an open question.


This paper provides a redescription of the neotheropod Camposaurus arizonensis Hunt et al. 1998 from the Upper Triassic Placerias Quarry (Chinle Formation) of Arizona. Furthermore it argues that the taxon is valid and diagnosable based on two autapomorphies and provides a phylogenetic analysis that recovers Camposaurus within the clade Coelophysis as the sister taxon to C. rhodesiensis. However, the authors refrain from assigning this taxon to Coelophysis until better material is found (the holotype consists of only the right and left distal ends of the fibulae and tibiae fused to their corresponding astragalocalcanea.

One point I'd like to address the the stratigraphic position listed for the holotype and for the Placerias Quarry. This is actually very important because as stated by the authors Camposaurus represents the stratigraphically lowest occurrence of a neotheropod dinosaur in the Chinle Formation and in part the current confusion is a result of a decision I made back in 2005. Recent papers such as Ezcurra and Brusatte (2011) and others (e.g., Irmis, 2005; Nesbitt et al., 2007; Langer et al., 2010) list the Placerias Quarry as situated in the Mesa Redondo Member of the Chinle Formation. This is mainly because this is the unit I placed the quarry in in my 2005 review of major localities from the Chinle Formation (Parker, 2005). Prior to this the quarry had been placed in the lower Petrified Forest Member (e.g., Jacobs and Murry, 1980) or more recently in the Bluewater Creek Member (e.g., Lucas et al., 1997). Heckert and Lucas (2003) also argued for the latter placement, but in the same paper argued that the Mesa Redondo and Bluewater Creek Members were synonymous. Unlike those authors who prefered the usage of the name Bluewater Creek, I (Parker, 2005) argued that the name Mesa Redondo had priority as it was the older of the two names. This usage was followed by Irmis (2005), Nesbitt et al. (2007) and most subsequent authors including the new paper by Ezcurra and Brusatte.

Interestingly, my recent stratigraphical investigations of the Chinle Formation in conjunction with Jeff Martz have determined that these units are not synonymous and actually quite distinct (the Mesa Redondo Member underlies the Bluewater Creek Member) and even more interestingly the Placerias Quarry does not occur in either of them!  Instead it is above the Bluewater Creek Member in the Blue Mesa Member (Parker and Martz, 2011; Irmis et al., in press).  This makes for an extremely confusing situation with at least four straigraphic unit names listed in the recent literature for the quarry, but hopefully this will stabilize in the future. Furthermore, this revised positioning implies that Camposaurus is younger than previously believed (Hunt et al., 1998; Irmis et al., in press).

REFERENCES

Heckert, A.B., Lucas, S.G., 2003. Stratigraphy and paleontology of the lower Chinle Group (Adamanian - latest Carnian) in the vicinity of St. Johns, Arizona. New Mexico Geological Society Guidebook 54, 281-288.

Hunt, A.P., Lucas, S.G., Heckert, A.B., Sullivan, R.M., Lockley, M.G., 1998. Late Triassic dinosaurs from the western United States. Geobios 31, 511-531.

Irmis, R.B., 2005. The vertebrate fauna of the Upper Triassic Chinle Formation in northern Arizona. Mesa Southwest Museum Bulletin 9, 63-88.

Irmis, R. B., Mundil, R., Martz, J. W., and W. G. Parker. in press.New high-resolution U/Pb ages from the Late Triassic Chinle Formation (New Mexico, USA) support a diachronous rise of dinosaurs. Earth and Planetary Science Letters
Jacobs, L. L., and P. A Murry. 1980. The vertebrate community of the Triassic Chinle Formation near St. Johns, Arizona, p. 99-123. In L. L. Jacobs (ed.), Aspects of Vertebrate History: Essays in honor of Edwin Colbert Harris. Museum of Northern Arizona, Flagstaff, AZ.

Langer, M.C., Ezcurra, M.D., Bittencourt, J.S., Novas, F.E., 2010. The origin and early evolution of dinosaurs. Biological Reviews 85, 55-110.

Lucas, S.G., Heckert, A.B., Hunt, A.P., 1997. Stratigraphy and biochronological significance of the Late Triassic Placerias Quarry, eastern Arizona (U.S.A.). Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen 203, 23-46.

Nesbitt, S.J., Irmis, R.B., Parker, W.G., 2007. A critical re-evaluation of the Late Triassic dinosaur taxa of North America. Journal of Systematic Palaeontology 5, 209-243.

Parker, W.G., 2005. Faunal review of the Upper Triassic Chinle Formation of Arizona. Mesa Southwest Museum Bulletin 11, 34-54.

Parker, W.G., Martz, J.W., 2011. The Late Triassic (Norian) Adamanian-Revueltian tetrapod faunal transition in the Chinle Formation of Petrified Forest National Park, Arizona. Earth & Environmental Science Transactions of the Royal Society of Edinburgh 101, 231-260.