Tyrannosaurus rex Osborn 1905

TYRANNOSAURUS REX Osborn 1905

Figs. 1-9

Material— NMMNH P-1013-1. a nearly complete left dentary. an incomplete articular, and a right prearticular. incomplete isolated teeth. and one nearly complete chevron.

Description—The dentary (Figs. 1 —4) has a total length of 89.5 cm and is 22.3 cm deep at a position immediately behind the last tooth. Thirteen alveoli are present with roots of the lst. 6th. 7th, 10th. llth. and 12th teeth. Tooth 4 is emergent and lacks only the apical tip. Tooth 9 is also emergent and complete. The 7th tooth in the row is the largest. The alveoli are separated by alveoloar septae which expand internally into rugose interdental plates or rugosae (Figs. 1, 2, 6). The interdental

plates are most strongly developed anteriorly. The interdental plates are arranged in a staggered fashion so that the anterior half of a plate surrounds the posterior portion of the tooth in front of it. and the posterior half of a plate surrounds the anterior portion of the tooth behind it.

A shallow longitudinal groove at the base of the posterior interdental plates provided the path for the dental artery, which issued side branches from a point low between the interdental plates that were directed toward the lower portions of the teeth. The symphyseal surface (Fig. 2) bears four longitudinal. anteroposteriorly directed ridges that are less than 40 mm long and are raısed slightly above the symphyseal surface. The lingual surface of the dentary is traversed by a well-defined Meckelian groove approximately 7 mm deep and 3 mm wide. Beneath the 3 rd and 4th alveoli, the Meckelian groove flattens to the contour of the dentary surface. A prominent foramen penetrates the Meckelian groove below the 10th tooth.

The buccal surface of the dentary (Fig. 3) is convex and pitted by more than 50 foramina which are concentrated anteriorly. More than 30 foramina occur below the first five teeth. The foramina emarginate the dorsal, ventral, and anterior edges of the buccal surface of the dentary. A less well-developed series of foramina in turn emargınates the main series of foramina more medially (Fig. 1).

The posterior portion of the dentary consists of a vertical sheet of bone less than 1 mm thick that becomes shallower and broader anteriorly to form the stout. tooth-bearing ramus. The posterior margin of the dentary is not preserved.

The teeth (Figs. 4-6) are laterally compressed and recurved. Both anterior and posterior margins are serrated (Figs. 5, 6). The anterior serrations lie on the antero-posterior tooth planes. and the posterior servations are deflected labially (Fig. 6). A prominent dorso-ventral groove marks the buccal surface of the teeth at their base. All of the isolated teeth except one can be fitted to the left dentary. Based on the position of the medial groove and posterior serrations. one isolated. incomplete tooth must have come from the right dentary or left maxilla (Fig. 7). Serration counts for the known teeth range from 6 to 10.5 per 5 mm. This is in agreement with data on serration counts in other tyrannosaurids (Ostrom 1969). and it is lower than in other carnosaurs (Tab. 1).

The right prearticular (Fig. 8) is complete except for broken anterior and posterior ends and a damaged angular articulation (Fig. 8). lt is approximately 43 cm long. rather curved. and markedly concave dorsally. Both the anterior and posterior ends are laterally compressed. but the medial portion broadens to support three ridges and two furrows on the external surface that form the articulation for the angular. The prominent groove along the entire buccal surface of the prearticular is probably the Meckelian groove The articulation of the prearticular with the articular is broken and the coronoid articulation is missing.

The chevron (Fig 9) is stoutly constructed but incomplete. It is broken at the origin of the dorsal processes and has a damaged anteroventral margin.