A contorted Gorgosaurus

In 1913, an American Museum of Natural History expedition led by Barnum Brown (with P.C. Kaisen and George Sternberg as assistants) searched the Cretaceous Belly River Formation in Alberta, Canada for dinosaurs. Although there had been an expedition to the same area the year before, the 1913 trip yielded "more exhibition material," including the articulated skeleton of Gorgosaurus*. When it arrived in New York it was prepared by Kaisen and put on display in 1918, although it later changed hands and is now labeled USNM 12814 (which, I think, puts it at the National Museum of Natural History).

*There still seems to be some controversy over whether Gorgosaurus is a distinct genus or whether it should be folded into Albertosaurus as a distinct species. Since the sources used in creating this post retain the use of the name Gorgosaurus, I will follow suit.

In describing the skeleton, Matthew and Brown note that it was exhibited in the position in which it was found (head thrown over the back and the legs beneath the body), a position that in their experience was the most common for articulated dinosaur skeletons. The majority of the tail was not preserved, however, and it seems that it was placed under the body as dinosaurs were thought to be tail-draggers at the time. In fact, if the tail was preserved it probably would have been still and bent upwards as in one of the most famous skeletons of Albertosaurus, although it is impossible to know this for certain. Why this particular position was chosen for the Gorgosaurus mount is unknown, however, and could have been for artistic as well as scientific reasons.

This little detail of the missing tail might not seem significant, but it does have an important impact on a paper published last year about the opisthotonic posture of fossil amniotes by Faux and Padian (2007). To briefly summarize, in 1918 Roy Moodie recognized that many fossil vertebrates were preserved in a posture with their head thrown over their backs and their tails curved. Moodie proposed that these positions were due to disease or the cause of death being poisoning, although Matthew and Brown rejected this view in their 1923 paper. Citing that this position is commonly seen among dead modern animals like cattle and sheep as a result of any number of causes, they hypothesized that the position had more to do with the shrinking of ligaments and muscles as a carcass decays;

The explanation probably lies in the shrinkage of ligaments along the dorsal side of the neck and backbone after death. In the course of decay of the fleshy parts the connection of the under side of the neck with the shoulder girdle is rotted away, while the more resistant ligaments on the dorsal side of the spine, less deeply buried in flesh, tend more to desiccation and shrinkage. While these relations will be modified in each individual instance by the circumstances of burial, they hold sufficiently true in general to account for the observed facts.

Faux and Padian reject this explanation, however, and make something of a return to Moodie's view that disease & poisoning are more likely causes. I'm skeptical of this interpretation and think more taphonomic observations need to be carried out in the field, but my specific objections will be withheld for now and potentially used as fodder for another post. Getting back to the Gorgosaurus, Faux and Padian use it in their paper to illustrate the opisthotonic posture seen in some dinosaurs, but the tail is restored in their paper as if it had actually been found! In a caption to the illustration they write;

Note that the neck is reflexed dorsally whereas the tail is reflexed ventrally, with no disarticulation. The position of skeletons like this one and the others in this figure cannot be explained by current flow.

If the tail really were swept under the body as illustrated it would certainly be difficult to explain, but only three caudal vertebrae from the specimen were actually preserved. There's no reason to think that the tail of this dinosaur actually took up such a position, and Kaisen's reconstruction tacked on a tail that wasn't actually found. Matthew & Brown probably put it beneath the body because that's what fit with their view of dinosaurs at the time, but with what we know now the tail would be more likely to orient straight backwards and possibly tilt up. Either way, this makes the comment about current flow irrelevant; the tail only takes the position it does because it was purposefully reconstructed that way.

If what Matthew and Brown state is correct, however, there were many other specimens from the Red Deer River area that were found in similar postures. Many of these may not have been collected (you can only bring back so many hadrosaurs) and those that were subsequently been exhibited in more "life-like" positions (the 1923 notice specifically mentions a Monoclonius in which the opisthotonic position of the head was corrected, which may have been this skeleton). Given so many articulated and opisthotonic skeletons in one area, it would have been interesting to try see if the skeletons were of the same age or died of similar causes, but I have no idea if this is possible (Brown is notorious for his lack of organized note-taking about fossil locations). Still, even if no good records were kept or have been lost many dinosaurs and mammals exhibit this contorted posture in death, and why that should be so is still a very interesting question.

References;

Faux, C.M.; Padian, K. (2007) "The opisthotonic posture of vertebrate skeletons: postmortem contraction or death throes?" Paleobiology, Vol. 33, No. 2, pp. 201-226

Matthew, W.D.; Brown, B. (1923) "Preliminary Notices of Skeletons and Skulls of Deinodontidae from the Cretaceous of Alberta." American Museum Novitates. No. 89, pp. 1-10

Moodie, R.L. (1918) "Studies in Paleopathology. III. Opisthotonus and Allied Phenomena Among Fossil Vertebrates." The American Naturalist, Vol. 52, No. 620/621., pp. 384-394.