Tyrannosaurus rex

Scott D. Sampson & Mark A. Loewen, 2005, Tyrannosaurus rex from the Upper Cretaceous (Maastrichtian) North Horn Formation of Utah: biogeographic and paleoecologic implications, Journal of Vertebrate Paleontology 25, No. 2, pp. 469-472 : 469

TYRANNOSAURUS REX Osborn, 1905

DESCRIPTION

UMNH 11000 consists of approximately 17% of a tyrannosaurid skeleton, including cranial and postcranial materials ( Fig. 1 ). The cranial elements consist of a nearly complete right postorbital and squamosal ( Fig. 1 ). The axial column appears to be represented by cervical vertebrae 3 and 4, sacral vertebrae 2, 3, and 4, and a series of mid-caudal vertebrae, perhaps calO, 12, 13, 14, 15 and 16. Chevrons cranial to each of ca6, 13, 14, 15, 16 and 17 are present, along with a single mid-dorsal rib. The specimen includes several incomplete pelvic girdle elements, including part of the distal blade of the right ilium, the left pubic peduncle, and the proximal end of the left ischium. Hindlimb elements consist of left tibia, fibula, and astragalus. Elements of UMNH 11000 are within the size range of other large, more complete Tyrannosaurus specimens (e.g., MOR 555 , BHI 3033 , FMNH PR 2081 ). Some evidence of pathologic modification of bones is present in the fibula and four other unidentified elements.

Despite the incomplete nature of UMNH 11000 , a combination of diagnostic features on its postorbital and squamosal provide strong evidence that this specimen represents Tyrannosaurus rex . The right postorbital is nearly complete, although the distal tip of the squamosal process was displaced and remained in articulation with the squamosal. As in other specimens of Tyrannosaurus (e.g., MOR 555 , TMP 91.36.500, BHI 3033 ; Brochu, 2003), the caudal margin of the ventral ramus is nearly straight, forming an approximate right angle with the squamosal process. This condition contrasts with that of other tyrannosaurids (e.g., TMP 81.10.1, Albertosaurus’, TMP 94.143.1, Daspletosaurus', ZPAL MgD-1/4, Tarbosaurus ), in which the ventral ramus tends to be more convex and angled rostrally. Similarly, UMNH 11000 and specimens of Tyrannosaurus share the presence of a subcircular postorbital process or “horn”—sometimes associated with a capping accessory ossification, here termed the “cornual boss” ( BHI3303 , AMNH 5027 )—positioned caudodorsal to the orbit ( Fig. 1 ). A presumably homologous postorbital projection occurs in other tyrannosaurids, but tends to be more elongate and complex in shape ( Currie, 2003a).

The squamosal of UMNH 11000 is also indistinguishable from that of Tyrannosaurus , possessing a relatively broad, elongate quadratojugal process (distalmost tip missing in UMNH 11000 ), a deeply incised contact surface for the postorbital, and a broad pneumatic exacavation of the squamosal body. Daspletosaurus also possesses a pneumatic excavation of the body, but this feature lacks the remarkable depth, or “inflated” condition, characteristic of Tyrannosaurus . Pneumatic excavation of the squamosal is also present in Tarbosaurus , but with a more restricted, circular foramen for passage of the air sac. The quadratojugal process is significantly broader and less curved than those of Gorgosaurus , Albertosaurus and Daspletosaurus , whereas it is longer than in Tarbosaurus ( Currie, 2003a; Hurum and Sabath, 2003).

Figures

FIGURE 1. A. Skeletal restoration of UMNH 11000, Tyrannosaurus rex, with preserved elements highlighted. Right postorbital and squamosal of UMNH 11000 are shown separately in right lateral view. Abbreviations: co, cornual ossification; ltf, lateral temporal fenestra; o, orbit; PO, postorbital; qjp, quadratojugal process of squamosal; sp, suborbital process; SQ, squamosal. Scale bar equals 10 cm. B. Postorbitals and squamosals of various tyrannosaurids viewed in right lateral view: Gorgosaurus, TMP 91.36.500 (reflected and modified after Currie, 2003a); Albertosaurus, TMP 81.10.1 (reflected and modified after Currie 2003b); Daspletosaurus, combination of NMC 8506 and TMP 2001.36.1 (reflected and modified after Currie 2003b); Tarbosaurus, ZPAL MgD-1/4 (reflected and modified after Hurum and Sabath, 2003); Tyrannosaurus, FMNH PR 2081 (reflected and modified after Brochu, 2003) and UMNH VP 11000.