Field of Science

Latest Literature - August 2008

The latest issue of Palaeontology contains two papers on Late Triassic fossils. The first by Stein et al. examines the aerodymanics of the British kuehneosaurs (gliding reptiles - reconstruction below) and contains a brief discussion of their taxonomy. To determine gliding angle and speeds, the team constructed several plastic and aluminum models and set them in a wind tunnel. Results suggest that the elongate "winged" Kuehneosuchus was an effenctive glider and that the short "winged" Kuehneosaurus was a parachutist. Also of interest is discussion that the taxonomic status of these two genera is unclear. One possibility is that they represent sexual dimorphs of the same species. According to this hypothesis the long "winged" individuals would be males given that the more elongate "wings" provide a strong display apparatus. Accordingly the "wings" would have had an original function and were secondary adapted for gliding.

The second paper by Heckert et al. is a description of a new sphenodontid from the Upper Triassic Ghost Ranch Coelophysis Quarry in northcentral New Mexico. Sphenodontid (represented by the extant tuatara - photo to left) fossils from the Chinle Formation are rare and usually incomplete. This specimen is important because of its more complete nature (both dentaries, possible maxilla, and impressions of a portion of the palate, all toothbearing). The new taxon is named Whitakersaurus bermani in honor of the late George Whitaker who originally discovered the quarry in 1947, and Dr. David Berman of the Carnegie Museum who conducted the last major excavations of the quarry in the 1980s. The Ghost Ranch quarry contains an extensive assemblage of Late Triassic vertebrates including 100s of well preserved specimens of the theropod Coelophysis bauri (Colbert, 1989).

Despite the extensive discussion in this paper by Heckert et al. regarding the stratigraphic position of this spectactular fossil assemblage it should be noted that this is still hotly debated and the subject of current research. The quarry is in reddish and green mottled siltstones (Schwartz and Gillette, 1994) which Colbert (1989) assigned to the Petrified Forest Member (Chinle Formation). Interestingly Colbert considered the quarry to be lower stratigraphically than an occurrence of Coelophysis from Petrified Forest National Park that was described by Padian (1986). Contrary to this all other workers have considered the quarry to be high stratigraphically in either the "siltstone" member (e.g., Stewart et al., 1972), the Owl Rock Member (e.g., Dubiel, 1989), or the Rock Point Member (e.g., Hunt and Lucas, 1993) (Column to right from Parker, 2005). Stewart et al. (1972) were unsure of the correlation of the siltstone member with either the Rock Point or the Owl Rock Members. Dubiel (1989) argued that the siltstone member represents a lateral facies of the Owl Rock Member, which is absent in north central New Mexico. Lucas and colleagues have countered this in numerous papers since 1992, arguing that the strata are equivalent to the Rock Point Member both lithologically and bio-stratigraphically. In fact, they have been unwavering in this hypothesis with the section in Heckert et al. (2008) being almost verbatim to that of Hunt and Lucas (1993).

However, this may need to change. A detailed paleomagnetism study of these strata, which is part of a recently completed PhD dissertation by Kate Zeigler, argues that the siltstone member and Rock Point Member possess differing paleomagnetic polarity signals (Zeigler, 2008). Even more striking, they also have different paleo-pole positions. Even more significant is that the siltstone member has a different paleo-pole from the Chinle Formation as a whole and one that is identical to that of the stratigraphically higher Moenave Formation (once believed to be Jurassic, but now probably Upper Triassic). This would not only be a significant deviation from previous hypotheses stratigraphically but also biostratigraphically. The Coelophysis Quarry fauna is dominated by the theropod Coelophysis bauri, but also contains phytosaurs, poposaurs, "rauisuchians", "sphenosuchians", and an enigmatic archosauriform which has tentatively been referred to Vancleavea campi, all of which are common in the stratigraphically lower members of the Chinle Formation. This would significantly extend the stratigraphic ranges of all of these taxa and demonstrate the survival of the majority of pseudosuchian lineages right up to the Triassic/Jurassic boundary. What is needed now is some independent verification of this hypothesis by isotopic dates.

REFERENCES

Colbert, E.H. 1989. The Triassic dinosaur Coelophysis. Museum of Northern Arizona Bulletin 57:1-160.

Dubiel, R.F. 1989. Depositional and paleoclimatic setting of the Upper Triassic Chinle Formation, Colorado Plateau; pp. 171-187 in Lucas, S.G., and A.P. Hunt (eds.), Dawn of the Age of Dinosaurs in the American Southwest. New Mexico Museum of Natural History, Albuquerque.

Heckert, A.B., Lucas, S.G., Rinehart, L.F., and A.P. Hunt. 2008. A new genus and species of sphenodontian from the Ghost Ranch Coelophysis Quarry (Upper Triassic: Apachean), Rock Point Formation, New Mexico, USA. Palaeontology 51:827-845.

Hunt, A.P., and S.G. Lucas. 1993. Stratigraphy and vertebrate paleontology of the Chinle Group (Upper Triassic), Chama Basin, north-central New Mexico. New Mexico Museum of Natural History and Science Bulletin 2:61-69.

Padian, K. 1986. On the type material of Coelophysis Cope (Saurischia: Theropoda) and a new specimen from the Petrified Forest of Arizona (Late Triassic: Chinle Formation); pp. 45-60 in Padian, K. (ed.), The Beginning of the Age of Dinosaurs. Faunal change across the Triassic-Jurassic Boundary. Cambridge University Press, Cambridge.

Schwartz, H., and D.D. Gillette. 1994. Geology and taphonomy of the Coelophysis quarry, Upper Triassic Chinle Formation, Ghost Ranch, New Mexico. Journal of Paleontology 68:1118-1130.

Stein, K., Palmer, C., Gill, P.G., and M.J. Benton. 2008. The aerodynamics of the British Late Triassic Kuehneosauridae. Palaeontology 51:967-981.

Stewart, J.H., Poole, F.G, and R.F. Wilson. 1972. Stratigraphy and origin of the Chinle Formation and related Upper Triassic strata in the Colorado Plateau Region. U.S. Geological Survey Professional Paper 690:1-336.

Zeigler, K.E. 2008. Stratigraphy, paleomagnetism, and magnetostratigraphy of the Upper Triassic Chinle Group, North-central New Mexico and preliminary magnetostratigraphy of the Lower Cretaceous Cedar Mountain Formation, Eastern Utah. Unpublished PhD dissertation, University of New Mexico, 224p.

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