or, An International Corpse of Mystery
Quick note: New readers might want to look at previous installments of the Paleobiology of Coelophysis (Parts 1 & 2) series before diving into this post.
In order to collect data from other specimens of Coelophysis bauri, some members of the research team (notably Larry Rinehart and Andy Heckert) visited several other museums to study the Whitaker quarry blocks in their care. After a trip to the Royal Tyrrell Museum of Paleontology in Alberta, Larry shared observations and photographs from one specimen that seemed a bit out of place compared to the other Coelophysis we had seen.
TMP 84-63-33, highlighted against the rest of the Tyrrell Museum block
Original photograph courtesy of Larry Rinehart
Specimen TMP 84-63-33 is the most easily seen skeleton on the Tyrrell Museum block. Although the front and back ends of the animal are missing, most of the middle is well-preserved, particularly the two hindlimbs. At first glance, it looks much like any other Coelophysis from the quarry.
Some details, however, led us to wonder about this. For instance, while Coelophysis has five sacral vertebrae connected to its hip, TMP 84-63-33 appears to have only four. Several features on the proximal end of the femur (that is, the part of the thigh bone that connects to the hip bones) also seemed notably different from what we saw on Coelophysis, and these features suggested an alternative identification.
Focus on the Femora
or, A Discomfiting Object Inserted in the Acetabulum
The proximal end of the left femur of TMP 84-63-33
articulated within the acetabulum (hip socket)
Photograph courtesy of Larry Rinehart
Some of the odd femoral features include:
- A well-defined groove on top of the head of the femur.
- The head of the femur is offset but appears to be completely rounded off. There is no sign of the hook-like prong seen in Coelophysis and other theropods.
- A distinct crest-like trochanter (or bump of bone) on the front (anterior) side of the femur toward the outside (lateral) edge, which has not been reported in Coelophysis.
Of these features, the first and (particularly) the third were a close match for features seen on NMMNH P-22298, the holotype specimen of Eucoelophysis baldwini. Eucoelophysis (“True Coelophysis” or “True Hollow Form”) also lacks a hook-like prong on the head of its femur, although it is quite different in overall shape from the rounded, offset femur head of TMP 84-63-33. This might be a real difference, or it might be due to the badly weathered condition of the Eucoelophysis holotype. In the end, based primarily on the similarities noted here, we assigned TMP 84-63-33 to Eucoelophysis sp.
Comparison of the femoral heads of Eucoelophysis baldwini, TMP 84-63-33, and Coelophysis bauri. Click for larger view. This figure is not from Rinehart et al. 2009, but drawn up as a visual aid to this post.
That’s where we left things for publication—a genus-level assignment based on some key characters that TMP 84-63-33 and NMMNH P-22298 have in common. Things get more interesting when you add a little background and some other fossils, so I’ll take the opportunity to explore some of those tangents here.
Eucoelophysis vs. Coelophysis: An Example of Interspecific Digression
or, Reflections on a Hollow Form of Truth
When Eucoelophysis was first described, it was considered to be a theropod dinosaur closely related to Coelophysis (Sullivan and Lucas 1999). However, a pair of later studies (Ezcurra 2006, Nesbitt et al. 2007) concluded that Eucoelophysis was a “non-dinosaurian dinosauriform”—not only was it not particularly close to Coelophysis, it lacked the requisite anatomical features needed to be included in the Dinosauria proper. Its closest companion in these dinosaur hinterlands appeared to be Silesaurus, a beaked, herbivorous reptile known from excellent skull and skeletal material from the Late Triassic of Poland (Dzik 2003).
The idea that Eucoelophysis might be a Silesaurus-style dinosauriform has received support from new and newly-recognized discoveries of other Silesaurus-like fossils in Late Triassic rocks from Arizona and New Mexico (Parker et al. 2006, Irmis et al. 2007). These include some blocky, angled femur heads whose overall shape is similar to that of both Silesaurus and the shape preserved in the Eucoelophysis holotype. And at least one of these femur heads (PEFO 34357) appears to have a Eucoelophysis-style anterolateral trochanter (=the dorsolateral trochanter noted by Nesbitt et al. 2007).
TMP 84-63-33, on the other hand, looks a lot more like Coelophysis than Silesaurus in many observable parts of its anatomy, especially the bones of its pelvis and feet. I wouldn’t say that our assignment of TMP 84-63-33 to Eucoelophysis reaffirms close relationship between Eucoelophysis and Coelophysis. But if this identification holds, then it doesn’t appear to do much for a Eucoelophysis-Silesaurus connection, either.
One last osteological nubbin of interest: the lesser trochanter (also referred to as the cranial or anterior trochanter) is a prong of bone that, in the animals we’re discussing, sits just below the head of the femur on the front-facing side. This trochanter is slender and crest-like on both Eucoelophysis and TMP 84-63-33. On observed and reported specimens of Coelophysis bauri, the lesser trochanter is thick, blocky, and connected to a well-developed shelf of bone that wraps around the outside of the femur. In other coelophysoids, both forms of lesser trochanter have been found within the same species—such as in the African species Coelophysis rhodesiensis, where the two different shapes may represent a difference between males and females (Raath 1990).
Comparison of the proximal left femurs of specimens mentioned in this post, in anterior (front) view, resized to similar widths. Inset shows silhouettes to scale. Redrawn from various sources. Click for larger view.
When a couple more femur heads are added to the previous figure, I begin to see a gradation of forms between the block-headed, slender-trochantered dinosauriform femora through to the hooked femur heads and robust trochanters of Coelophysis bauri. Note that I do not suggest that this shows any sort of evolutionary sequence. Instead, the continuum of shapes and features is probably due to a mix of phylogenetic differences, sexual dimorphism, age- and size-related changes, individual variation, and preservation quality.
The trick is to figure out what sort of meaningful divisions might be found within this femoral spectrum. In Rinehart et al. 2009, we made one division based on similarities between the femora TMP 84-63-33 and Eucoelophysis. I suspect that the wealth of fossils from the Whitaker quarry will have more to reveal on the topic, both from close evaluation of femur variation in the large Coelophysis population, and from comparing those variations with data gleaned from other parts of the skeleton.
Next time: Data Gleaned from other Parts of the Skeleton
With far less talk of femur variation, and perhaps even some actual paleobiology!
Previously:
Part I: Introduction
Part II: Other Critters in the Quarry
- 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: Dzik, Jerzy, 2003. A beaked herbivorous archosaur with dinosaur affinities from the Early Late Triassic of Poland. Journal of Vertebrate Paleontology, vol. 23 (3), pp. 556–574.
- Ezcurra, Martín D., 2006. A review of the systematic position of the dinosauriform archosaur Eucoelophysis baldwini Sullivan & Lucas, 1999 from the Upper Triassic of New Mexico, USA. Geodiversitas, vol. 28 (4), pp. 649–684. [PDF]
- Irmis, Randall B., Nesbitt, Sterling J., Padian, Kevin, Smith, Nathan D., Turner, Alan H., Woody, Daniel and Downs, Alex, 2007. A Late Triassic dinosauromorph assemblage from New Mexico and the rise of dinosaurs. Science, vol. 317, no. , pp. 358–361. doi: 10.1126/science.1143325
- Nesbitt, Sterling J., Irmis, Randall B. and Parker, William G., 2007. A critical re-evaluation of the Late Triassic dinosaur taxa of North America. Journal of Systematic Palaeontology, vol. 5 (2), pp. 209–243. doi: 10.1017/S1477201907002040
- Parker, William G., Irmis, Randall B. and Nesbitt, Sterling J., 2006. Review of the Late Triassic dinosaur record from Petrified Forest National Park, Arizona. Museum of Northern Arizona Bulletin 62, pp. 160–161.
- Raath, Michael A. 1990. Morphological variation in small theropods and its meaning in systematics: evidence from Syntarsus rhodesiensis in Dinosaur Systematics: Perspectives and Approaches, Kenneth Carpenter and Philip J. Currie, eds. Cambridge University Press. pp. 91–105.
- Sullivan, Robert M. and Lucas, Spencer G., 1999. Eucoelophysis baldwini, a new theropod dinosaur from the Upper Triassic of New Mexico, and the status of the original types of Coelophysis. Journal of Vertebrate Paleontology, vol. 19 (1), pp. 81–90.
—Matt Celeskey.
File under: Dinosaurs, Reptiles, Triassic.
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