Gorgosaurus libratus, Lambe, 1914

Lambe, LM, 1914, On a new genus and species of carnivorous dinosaur from the Belly River Formation of Alberta, with a description of the skull of Stephanosaurus marginatus from the same horizon, The Ottawa Naturalist 28, pp. 13-21 : 13-16

Gorgosaurus libratus , gen. et sp. nov.

Carnivorous dinosaur of large size, reaching a length of about twenty-nine feet; head narrow and moderately elongate; trunk compact; fore-limbs minute; hind-limbs long and robust; tail nearly half the total length of the animal, tapering, and with only a slight lateral compression. In the skull there is a large antorbital vacuity, preceded by a very small opening in the centre of a depressed area. No triangular alveolar plates on the inner sides of the jaws. A foramen present in the surangular, far back and near its upper border. No presplenial. Teeth trenchant, powerful, 4 premaxillary, 13 maxillary and 14 dentary. First tooth of the maxilla similar in shape and size to those of the premaxilla. Vertebrae slightly amphicoelous, concave on the sides and beneath; 2 cervico-dorsals, 11 dorsals, 5 sacrals, and about 34 caudals. Neural spines short throughout the vertebral column. Chevron bones short, beginning with the first caudal. Transverse processes of the caudal vertebrae decreasing in size to and ending with the 14th vertebra. Anterior zygapophyses of the posterior caudals greatly lengthened. Scapula longer than the fore-limb. Humerus twice the length of the ulna. Two digits, Nos. II and III, to the manus, of which the phalangeal formula is 2 II, 3 III, the terminal phalanges being claw bones. Metacarpal IV represented by a proximal vestigial bone. Ilium elongate, plate-like, with a flat upper outline and rounded ends. Preacetabular part shorter than the hinder portion, of which both are strengthened on the outer surface by a prominent, overhanging flange running horizontally at midheight. Ischium terminating narrowly below. Pubis ending in a horizontally expanded foot, of which the posterior extension is the greater. Femur about the same length as the tibia. Metatarsals II, III and IV elongate, of which III, the longest, is nearly two-thirds the length of the femur. Metatarsal I represented distally by a short vestigial bone, and metatarsal V represented in a similar manner proximally. Four clawed digits to the pes, viz.: Nos. I, II, III and IV, of which the phalangeal formula is 2 I, 3II, 4 III and 5 IV. Ventral ribs composite, sixteen in number, overlapping at the longitudinal mid line of the body, and bearing distally slender, closely applied supplementaries.

Gorgosaurus libratus , apart from its dentition, is remarkable for the extreme shortness of the fore-legs and the great length of the hind ones. The long, narrow ilium rises slightly above the short sacral spines, and, in addition to the horizontal flanges, already mentioned, there are two small strengthening buttresses running upward from-the centre of the acetabular border. The length of the metatarsals is surprising. The close application of the vestigial distal end of metatarsal I to metatarsal II is indicated by a slightly concave surface on the latter bone, which gives digit I a forwardly rather than a backwardly directed position in the foot, The vestigial proximal end of metatarsal V is in place in each leg, recalling to mind a similarly reduced bone in Ornithomimus altus , Lambe, also from the Belly River formation of Alberta.

Each abdominal rib consists of two well ossified, flattened lengths, which overlap at their inner ends. Outwardly, each lateral half is slightly grooved on its front margin for the reception of a slender rod like bone (supplementary), which lies closely against the rib and projects but slightly beyond its outer end.

The four premaxillary teeth are remarkably long and slender, With a keel on each side of a slightly convex inner or lingual surface. They are latterly compressed to a slight extent, evenly rounded in front, with their fore and aft diameter a little greater than their breadth. The first or anterior tooth of the maxilla is similar to the prem axillary teeth, in which respect Gorgosaurus differs from other known genera of Cretaceous carnivorous dinosaurs. The other maxillary teeth are long and powerful, of the Megalosauroid type, with two serrated keels, one along the front edge, the other behind. In the second maxillary tooth the anterior keel in descending passes slightly toward the inner side of the crown, and this is seen in a lessening degree in the next two or three succeeding teeth. A similar slight variation is seen also in the more anterior teeth of the dentary.

The chevron bones are intervertebral, but with a greater surface of attachment to the front vertebra of the two, The more anterior ones are bent slightly backward from their midlength. This angulation in succeeding ones becomes more pronounced until the lower edge of the distal half is parallel to the longitudinal axis of the tail. By a gradually increased development and prolongation forward of the anterior angulation at the mid length of the bone, a “ meat-chopper ” shape is attained and adhered to with a gradual diminution in size, more apparent in the depth of the bone than in the length of its “ foot. ”

The long and slender anterior teeth (premaxillary and first maxillary) of Gorgosaurus are very different in shape from the robust supposed anterior teeth of Deinodon horridus of Leidy. In all the large Cretaceous carnivorous dinosaurs, the majority of the teeth, apart from the more anterior ones, are remarkably similar in the different genera and do not afford data for generic distinctions.

Another large form of carnivorous dinosaur, having supporting alveolar plates on the inner sides of the jaws, occurs in the Belly River formation of Alberta and is represented in the collection of 1913 .