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 : 3-6

2.- Gorgosaurus libratus LAMBE.

Skeleton in running pose. Belly River formation, Alberta. This skeleton, No. 5458 , was obtained by the American Museum Expedition of 1914, in charge of Barnum Brown. The locality is Red Deer River , Alberta, Canada .

It was prepared and mounted by Peter Kaisen and placed on exhibition in May 1921. The mount is a large panel, 23 x 14 ft., made in eight sections which are easily separable for convenience in removal, as the panel is too large and unwieldy to be handled easily as a single block. Each section consists of a wooden framework in which the individual bones or blocks of matrix containing several bones left in the original rock have been articulated in position and clamped in place with steel straps and braces. The front of the framework between the bones of the skeleton is covered by a galvanized wire mesh faced with tinted plaster of paris, the surface of the plaster being chipped to give the effect of a chipped stone block. The thickness of the netting and plaster facing is only about an inch, and the sections are by no means heavy to handle, except for the weight of the petrified bones themselves. When set up, the sections are bolted in position, and the back of the framework is covered by panels of compo-board. The skull and jaws are supported on steel brackets and are removable without disturbing the remainder; the left forelimb and shoulder girdle are also separately removable.

The design of this mount embodies certain practical advantages in that, in spite of its gigantic size, it can readily be taken down, removed, and re-erected elsewhere without damage or loss of unity, save for the slight chipping at the edges of the sections, which can easily be touched out with a little plaster after the specimen has been set up where desired. The skull and forelimb are supported by steel brackets free of the background and can be removed for study when desired.

The pose adopted ( Fig. 2 ) is that of a running dinosaur, and was studied from photographs of running lizards and from the dinosaur footprints of the Counecticut Triassic sandstones. In addition to the classic photographs by Saville-Kent, we used photographs of the Western Tiger Lizard recently taken by Mr. G. K. Noble.

Many western lizards run on their hind legs when in haste, as may be seen from a study of their tracks on the sand. The photographs show that the animal has the fore part of the body well raised from the ground and the tail projects backward as a balance to the weight of the body. The tiger lizard, however, does not swing the legs directly under the body, as a bird does when running, but flings the leg outward to one side in the middle of the step. This is conditioned by the shortness of the leg and the articulation of the femur outward from the side of the body instead of beneath it as in birds or mammals. This relation is clearly seen in the characters of the lizard femur. The tibial condyles are wholly beneath (posterior) instead of extending partly distal. The head of. the femur is almost wholly proximal, instead of partly lateral. In these features the dinosaur femur differs from that of ordinary reptiles and approaches the type characteristic of birds and mammals. This is correlated with the greater relative size of limb to body, which is characteristic of mammals and birds, as compared with other reptiles. It is concluded that the bipedal dinosaurs walked with a comparatively straight step, swinging the hind limb well under the body and with the foot near to the median line of movement of the animal. In the quadrupedal dinosaurs the position of the forelimb, secondarily readapted to the support Qf the body, appears to have been with the elbow everted to a varying degree. The carnivorous dinosaurs, however, are fully bipedal even in the Jurassic, and in Gorgosaurus the forelimb is so small as to have no practical influence even in balancing the weight. The animal appears to have walked and run much like a gigantic bird, save that the long tail served to balance the weight of the large and heavy head and shoulders. The balance is, of course, incomplete, the pitching forward of the body being as essential to maintaining the speed of the step as it is in a man running.

The length of the stride shown in the mount is not nearly so extreme as in a swiftly running lizard; but an animal of such size and weight could not take so long a stride as a smaller and lighter creature. Comparison of the stride'of a running elephant with that of a dog or cat running clearly brings out this difference, which would be inferred from the laws of mechanics in their relation to the size of any animal.

The 4th to 19th caudals are restored, and all beyond the 30th. Distal half of left femur, left tibia and most of left fibula restored. Distal ends of ischium and pubis restored. Right ribs restored and some parts of left ribs. Parts of forelimbs.

Figures

Fig. 2. Gorgosaurus libratus Lambe. Skeleton mounted in running pose. The missing parts are painted on the panel, except for parts of the right fore and hind limbs, restored in plaster. Amer. Mus. No. 5458. Length of panel 24 feet.