Tyrannosaurus rex, Osborn, 1905

Christine Lipkin, Paul C. Sereno & John R. Horner, 2007, The furcula in Suchomimus tenerensis and Tyrannosaurus rex (Dinosauria: Theropoda: Tetanurae), Journal of Paleontology 81, No. 6, pp. 1523-1527 : 1523-1525

Tyrannosaurus rex .—The only undisputed furcula of Tyrannosaurus rex belongs to an articulated postcranial skeleton ( UCRC V1 ) preserved in a large sandstone concretion from the Lance Formation (Maastrichtian) of eastern Wyoming (Lipkin and Sereno, 2004). The bones of the trunk are preserved in articulation with little transverse or dorsoventral distortion. The furcula is preserved in situ between the right and left coracoids with each clavicular ramus laying near the acrominal process of the scapula ( Fig. 3 ). The distal ends of both clavicular rami are broken away at the edge of the concretion.

The furcula is U-shaped or, more precisely, lyre-shaped in anterior view with an intrafurcular angle of 71 ° ( Table 1). Unlike other theropod furculae, there is a transversely oriented central body from which extend the clavicular rami ( Fig. 4.1 ). Dorsal and ventral margins of the central body are rounded with no development of a hypocleideal process. The anterior surface has a shallow transverse fossa, whereas the posterior surface is flat. The laterally deflected epicleideal process is preserved on the right side near the acromion of the right scapula. The internal bone texture of the furcula is dense without any development of marrow or pneumatic cavities. This furcula ( UCRC V1 ) most closely resembles that of Gorgosaurus (Makovicky and Currie, 1998, figs. 1, 2), which also lacks any development of a hypocleideum, A rudimentary hypocleideum, in contrast, was reported in specimens referred to the tyrannosaurids Albertosaurus and Daspletosaurus (Makovicky and Currie, 1998, figs. 1, 2).

Another nearly complete furcula ( MOR 980 ) was recovered with a partial skeleton of T. rex . MOR 980 was collected from the upper Hell Creek Formation (uppermost Maastrichtian), east of Fort Peck Lake in Montana ( Derstler, 2005). This furcula is Ushaped with an intrafurcular angle of 87 ° ( Fig. 4.2 ; Table 1). The central body has a maximum anteroposterior thickness of 19 mm, which is thicker than the others described herein. The unusual thickness is due to pathology on the left posterior-side of the central body. The pathology may have extended all the way up to the dorsal tip of the left epicleideal process. However, the anterior-side of the furcula is missing the outer portion of the clavicular ramus but the epicleideal process is preserved, and it is also pathologic. The right side of the furcula appears to be free of pathology.

Brochu (2003:95, fig. 84) tentatively identified an elongated shaft with a broken end as a ‘‘partial furcula’’ from a well preserved skeleton of T. rex ( FMNH PR2081 ; Fig. 5 ). He noted its similarity to elements from the gastral basket, which we suggest here, is the correct identification. A bone from the same specimen was tentatively regarded as the proximal portion of a posteriormost dorsal rib ( Brochu, 2003, fig. 77 ). The bone, however, does not match the form of more anterior dorsal ribs or the articular facets on the posteriormost dorsal vertebra. We concur with Larson and Rigby (2005, page 253) that this bone ( Fig. 4.3 ) is a partial furcula, which is very similar in size and form to a fourth furcula found in association with another skeleton of T. rex ( CMI 2001.90.1 ; Fig. 4.4 ).

The fifth furcula known for T. rex , comes from a skeleton from northeast Montana ( MOR 1125 ; Fig. 4.5 ). The specimen is approximately three-fourths the size of the large adult FMNH PR2081 (Horner and Padian, 2004), is approximately 18 years old (Horner and Padian, 2004), and is hypothesized to be a female ( Schweitzer et al., 2005). The furcula is broad and U-shaped with an intrafurcular angle of 113 ° ( Table 1). Articular scars are present on the posterior side of the rami. It most clearly resembles the furcula in the tyrannosaurid Daspletosaurus (Makovicky and Currie, 1998, figs. 1, 2). However, unlike Daspletosaurus , there is no development of a hypocleideum.

Figures

FIGURE 3—Photograph (1) and matching line drawing (2) of the furcula and pectoral girdle of Tyrannosaurus rex (UCRC V1) in anterior view. Crosshatching indicates broken bone. Dashed lines indicate missing or hidden bone. Abbreviations: ac, acromion; co, coracoid; f, furcula; hu, humerus; sc, scapula. Scale bar equals 5 cm.

FIGURE 4—Line drawing of the morphologic variation in the five Tyrannosaurus rex furculae. 1, UCRC V1; 2, MOR 980; 3, FMNH PR 2081; 4, CMI 2001.90.1; 5, MOR 1125. Cross-hatching indicates broken bone. Dashed lines indicate missing bone. Abbreviation: p, pit. Scale bar equals 5 cm.

FIGURE 5—Line drawing of the mounted furcula of Tyrannosaurus rex (FMNH PR 2081) in anterior view. Dashed lines indicate missing portions. Scale bar equals 5 cm.

FIGURE 77. FMNH PR2081, Tyrannosaurus rex. Isolated bone possibly representing a rib for the last presacral vertebra (p17). Scale = 5 cm. Photograph by J. Weinstein.