Gorgosaurus libratus

W. D. Matthew & Barnum Brown, 1923, Preliminary notices of skeletons and skulls of Deinodontidae from the Cretaceous of Alberta, American Museum Novitates 89, pp. 1-10 : 1-3

1.- Gorgosaurus libratus LAMBE.

Skeleton in position as found. Belly River formation, Alberta. This skeleton, No. 5428 , was obtained by the American Museum Expedition of 1913, in charge of Barnum Brown. The locality is Red Deer River , Alberta, Canada .

It was prepared by Peter Kaisen and placed on exhibition in 1918. It is nearly complete, except for the tail, of which only three distal caudals and the spine of the first caudal are preserved.

The position of the skeleton, with the head thrown backward so that the top of the skull rests against the backbone, and the legs doubled up under the body, shows well in the illustration ( Fig. 1 ). This position is commonly seen in articulated skeletons resting upon the s'de. It is the usual thing with the Belly River dinosaurs, although most strikingly displayed among those with a flexible or slender neck. In the Ceratopsia and ankylosaurs the neck is too short and stiff to permit of a complete reversal of the skull, but in Monoclonius, No. 5351, the skull in its original position was drawn upward and backward as far as the limits of its movement permitted. This - has been partially corrected in the completed mount. In the trachodont skeletons, Procheneosaurus No. 5340, Corythosaurus No. 5240, and others, the same position is clearly seen. In the'long, slender-necked Struthiomimus the head is completely reversed, as it is in this skeleton.

It is usual in preparing a panel mount of a skeleton to correct this distortion by resetting the skull and sometimes the anterior cervicals. In consequence, it is not so common in exhibition specimens as in those that are brought in from the field. But, so far as our experience goes, it is the usual thing in an articulated fossil skeleton. It is quite as common among fossil mammals as among dinosaurs. Skeletons of Stenomylus (the "Gazelle Camel"), Promerycochoerus , etc., show the same thing.

This position has been cited by Moodie as evidence of the animal's death in that peculiar form of spasm "opisthotony," characteristic of lockjaw, and hence as indicating the prevalence of that disease among the Cretaceous dinosaurs. The explanation appears to us untenable.

For, not only is the position characteristic of most articulated fossil skeletons found lying on their side, but it is equally common among the modern skeletons of sheep or cattle that are found lying out on the Western plains. These animals certainly did not die of lockjaw, but of exposure, cold, hunger, or various forms of disease. Nor is it reasonable to suppose that lockjaw was the usual cause of death among extinct mammals and dinosaurs.

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.

In comparison with the fine skeleton of Gorgosaurus libratus in the Ottawa Museum, described in detail by Mr. L. M. Lambe, this specimen has the skull much more complete, but the tail and the forelimbs much less so. The missing parts of the specimen have been painted in outline on the panel block, and are clearly shown in the photograph.

Tail restored except for caudals 17-19. Distal end of ischium restored. Proximal parts of right metapodials restored, also parts of two ribs. Right humerus, radius, and forefoot restored.

Figures

Fig. 1. Gorgosaurus libratus Lambe. Skeleton mounted as found in the rock. The parts restored are painted on the panel. Amer. Mus. No. 5428. Length of 12 feet.