Tyrannosaurus rex, Osborn, 1905

Kenneth Carpenter & D. Bruce Young, 2002, Late Cretaceous dinosaurs from the Denver Basin, Colorado, Rocky Mountain Geology 37, pp. 237-254 : 245

Family TYRANNOSAURIDAE

Tyrannosaurus rex

Figures 4 , 13 , 14

Material. — DMNH 2827 three teeth, left scapulocoracoid, ribs, distal caudal, partial left ilium, left femur, right partial tibia, right fibula, right astragalus (Denver Formation, Loc. 36) ; DMNH 32825 tooth (Denver Formation, Loc. 20) ; DMNH 32825 tooth (Denver Formation, Loc. 21) ; UCM 36303 tooth (Denver Formation, Loc. 22) ; UCM 38804 tooth (Laramie Formation, Loc. 4) ; and YPM 4192 tooth (Denver Formation, Loc. 32) .

Description and discussion. — As discussed above, a large tooth ( YPM 4192 ) collected from South Table Mountain is one of the first dinosaur specimens recovered in the Denver Basin. Although originally unnamed, the tooth is now referred to Tyrannosaurus rex .

The most important specimen of Tyrannosaurus rex from the Denver Basin is the partial skeleton found by Charlie Fickle at a housing development in Littleton. The skeleton was completely disarticulated and scattered, and unfortunately it was damaged by earthmoving equipment. Several large teeth with their roots were found, indicating that the teeth had slipped from their alveoli. The scapula and coracoid are co-ossified ( Fig. 13 A ), although the suture is traced as a ridge. Their combined length is 106 cm, of which 82 cm is scapula. The distal end of the scapular blade is not expanded dorsally as in the holotype, CMNH 9380 (see Osborn, 1906, fig. 6 B ), but more closely resembles that of FMNH PR 2081 (see Carpenter and Smith, 2001, fig. 9.2). The coracoid is 39.5 cm tall. The ilium is missing all of its pre-acetabular blade but is estimated to have been 185 cm long ( Fig. 13 B, C ). The femur is also damaged, lacking its distal end, but is estimated to have been about 111 cm long ( Fig. 13 D, E ). The shaft has collapsed into the medullary cavity along most of its length so that no minimal circumference can be measured. Almost half of the proximal part of the tibia is missing, but it is estimated to have been about as long as the femur ( Fig. 13F ).

The fibula of Tyrannosaurus rex has not been described before, but it is generally similar to that of Tyrannosaurus haatar ( Maleev, 1974) . DMNH 2827 is 87.2 cm long and has a long, slender, straight shaft that is subtriangular in cross-section at mid-shaft. The proximal end is expanded, especially posteriorly ( Fig. 13 G, H ). The proximal end is crescentic in dorsal view. This dorsal surface is sloped medially to accommodate the distal fibular condyle of the femur. On the medial side of its proximal end is a large oval fossa that has a sharp dorsal rim. The fossa shallows distally and no sharp border is present marking its end. The fossa has a shallow, triangular platform just below its dorso-posterior rim. Most of the medial side of the shaft is rugose for ligaments binding the bone to the tibia. The scar for the M. biceps femoralis on the anterior face just above mid-shaft is very rugose and sub-oval.

The most significant element of DMNH 2827 is an astragalus found associated with the tibia ( Fig. 13 F , 14 ). Welles and Long (1974) described what they thought was the astragalus of Tyrannosaurus rex , but which Carpenter (1992) demonstrated as a right quadrate. The astragalus of Tyrannosaurus rex is illustrated here in detail for the first time ( Fig. 14 ). It measures 28.8 cm across the anterior surface of the distal condyles, and it has a maximum height of 30.8 cm. The ascending process is not as tall proportional to its width as in Gorgosaurus (see Lambe 1917), which has one of the few tyrannosaurid astragali described. The process is almost as tall as it is wide (as measured just above the condyles) and forms an acute, almost right triangle. The process is situated oblique to a vertical plane through the distal condyles. A groove is present along the lateral surface of the process to accommodate the fibula ( Fig. 14 E ). A large fossa is present at the base of the process between the dorsal surfaces of the condyles ( Fig. 14A ). The fossa is rugose and probably housed the robust collateral ligament under which the tendons for the foot extensors passed. The distal condyles are asymmetrical, the medial one being considerably larger than the lateral one ( Fig. 14A ). Furthermore, the medial condyle extends farther anteriorly than the lateral one ( Fig. 14 C ) as in most theropods. Posteriorly, there is a deep groove at the base of the ascending process to accommodate the distal end of the tibia. The posterior part of the astragalus is tall ( Fig. 14B, E ) and extends partially onto the posterior side of the tibia, thus ensuring a tight union. Laterally, there is a fossa to accommodate the calcaneum.

The teeth referred to Tyrannosaurus have a crown basal width that is almost equivalent to crown basal anteroposterior length.

Figures

Fig. 6. B Scapula of Tyrannosaurus. A Scapula of Allosaurus.

Figure 13, below. Postcrania ofLittleton specimen ofTyrannosaurus rex (DMNH 2827). A, scapulo-coracoid; B, left ilium in medial view; C, left ilium in lateral view; D, left femur in anterior view; E, left femur in lateral view; F, astragalus and distal end of tibia; G, left fibula in lateral view; and H, left fibula in medial view. Scale bars = 10 cm.

Figure 4. The first known specimen of what can now be identified as Tyrannosaurus rex is a tooth collected from Denver Formation at South Table Mountain. It was first mentioned and sketched in a letter from Edward Berthoud to O. C. Marsh dated June 20, 1874. Specimen is now at Yale Peabody Museum (YPM 4192).

Figure 14. Astragalus ofthe Littleton specimen of Tyrannosaurus rex (DMNH 2827) in anterior (A); medial (B); ventral (C); posterior (D); and lateral (E) views. Scale bar = 10 cm.