The Paleobiology of Coelophysis Part I: Introduction
10:26 pm
The Triassic dinosaur Coelophysis bauri is well-known from hundreds of fossils unearthed at Ghost Ranch in northern New Mexico, at a famous quarry first discovered by George Whitaker in 1947. Excavations at the quarry—initially by the American Museum of Natural History in the 1940s & 1950s, and subsequently by the Carnegie Museum in the 1980s—sent about thirty large blocks, each containing dozens of fossils, to museums across North America. Although it has been more than 25 years since the last major excavation, new discoveries continue to be made (e.g.: Nesbitt & Norell 2006, Heckert et al. 2008, Nesbitt et al. 2009) as fossils from the quarry are slowly prepared and interpreted.
One block, excavated by the Carnegie-led team in the 1980s, was given to the then-nascent New Mexico Museum of Natural History. In 2008, it was put on permanent exhibit. Larry Rinehart, who prepared the block for display, invited me to assist in the illustration and interpretation of some of the specimens it contained, as well as to reconstruct some of the different sizes and morphologies that were being uncovered through the analysis of dozens of specimens in this and other blocks from the quarry. The results of these investigations have been published in the past few weeks as a New Mexico Museum of Natural History and Science Bulletin (Rinehart et al. 2009).
In future posts, I hope to showcase some of the specimens, interpretations, and conclusions we came to as a result of these investigations. Note that everything I post here will be based on my own understanding and interpretation of the work, much of which was performed by the other authors. I don’t intend to misrepresent any of the procedures or findings, but if it happens, errors in reporting should be considered mine alone.
With that caveat in mind, I’d like to introduce the main object of the study: the NMMNH Coelophysis block.
The NMMNH Coelophysis block (Quarry # C-8-82) after preparation
The NMMNH block is a two ton wedge of reddish Chinle siltstone, trimmed down considerably from the original 12,000 pounds pulled out of the quarry in 1982. Across its exposed surface lie more than two dozen specimens referrable to Coelophysis bauri, plus a couple non-dinosaur bits & pieces. Several jackets removed from the block contain many more fossils of Coelophysis, along with an impressive sampling of Triassic fish and reptiles.
Schematic drawing of selected Coelophysis specimens on the exposed surface of the NMMNH block
Up from the Bottom
or, Lessons from a Triassic Turnover.
When looking at the prepared surface of the NMMNH block, everything you see is upside down—the block was flipped over and prepared from the bottom up. This was done to facilitate access to the primary bone layer by avoiding the reworked, poorer-quality bones found above this layer in other blocks. It also revealed new information about the environment where the Coelophysis were buried. Beneath the main Coelophysis beds were found layers containing rip-up clasts running in the same direction as most of the Coelophysis bones—evidence of rushing water that tore up chunks of the underlying mud as it swept the dinosaur bodies in.
A previous study of the taphonomy of the quarry (Schwartz & Gillette 1994) also concluded that water transported the Coelophysis to their burial site. Based on several lines of evidence, Schwartz and Gillette proposed that the animals died during a prolonged drought and were subsequently washed downstream by a flood, where their carcasses clogged a narrow channel prior to burial. But some tiny fossils near the bottom of the NMMNH block suggested a slightly different scenario to us.
Beneath the rip-ups were the fossils of tiny invertebrates—conchostracans and ostracods—often found in temporary bodies of water. They led us to consider that the quarry was once the site of an ephemeral pond—a topographic depression where water might collect during a wet season, followed by dry periods where the water would disappear. An unfortunate flock of Coelophysis, swept up in the flooding of a nearby river, might have been washed into this low spot in the landscape and buried.
The death/burial poses of the seven most complete Coelophysis specimens on the NMMNH block
As for the cause of death, there seemed little reason to suspect any agent beyond the flood that buried them. In fact, one observation previously cited as evidence for post mortem desiccation—the opisthotonic posture where neck and tail are flexed sharply upward and curled over the back—now seems more likely to indicate the death throes of animals suffering the final effects of disease, poison, or (in this case) lack of oxygen due to burial or drowning (Faux & Padian, 2007).
If the bonebed at the Whitaker/Coelophysis quarry is the end result of a single catastrophe, then it preserves an excellent sampling of a population of early dinosaurs. It may possibly be the largest (in terms of numbers of individuals) Mesozoic dinosaur population we know of. This provides an unparalleled opportunity to study growth and variation within a single dinosaur species. Before getting to that, however, I’d like to spend a post reviewing some of the other, non-Coelophysis fossils found within this and other quarry blocks.
Next time: It’s your Lagerstätte, I’m just buried in it
or, Token Diversity in a Dinosaur Graveyard.
- Main Reference: Rinehart, Larry F., Lucas, Spencer G., Heckert, Andrew B., Spielmann, Justin A. and Celeskey, Matthew D., 2009. The Paleobiology of Coelophysis bauri (Cope) from the Upper Triassic (Apachean) Whitaker quarry, New Mexico, with detailed analysis of a single quarry block. New Mexico Museum of Natural History and Science Bulletin 45, 260pp. Abstract [Rich text file]
- Additional References: Faux, Cynthia M. and Padian, Kevin, 2007. The opisthotonic posture of vertebrate skeletons: postmortem contraction or death throes? Paleobiology, vol. 33 (2), pp. 201–226. doi: 0.1666/06015.1
- Heckert, Andrew B., Lucas, Spencer G., Rinehart, Larry F. and Hunt, Adrian P., 2008. A new genus and species of sphenodontian from the Ghost Ranch Coelophysis quarry (Upper Triassic: Apachean), Rock Point Formation, New Mexico, USA. Palaeontology, vol. 51, pp. 827–845. doi: 10.1111/j.1475-4983.2008.00786.x [PDF]
- Nesbitt, Sterling J. and Norell, Mark A., 2006. Extreme convergence in the body plans of an early suchian (Archosauria) and ornithomimid dinosaurs (Theropoda). Proceedings of the Royal Society B, vol. 273, no. 1590, pp. 1045–1048. doi: 10.1098/rspb.2005.3426
- Nesbitt, Sterling J., Stocker, Michelle R., Small, Brian J. and Downs, Alex, 2009. The osteology and relationships of Vancleavea campi Zoological Journal of the Linnean Society, vol. 157 (4), pp. 814–864. doi: 0.1111/j.1096-3642.2009.00530.x
- Schwartz, Hilde L. and Gillette, David D., 1994. Geology and Taphonomy of the Coelophysis Quarry, Upper Triassic Chinle Formation, Ghost Ranch, New Mexico. Journal of Paleontology, vol. 68 (5), pp. 1118–1130.
