Tyrannosaurus rex Osborn, 1905

Dalman, Sebastian G., 2013, New Examples of Tyrannosaurus rex from the Lance Formation of Wyoming, United States, Bulletin of the Peabody Museum of Natural History 54, pp. 241-254 : 244-246

ORDER Saurischia Seeley, 1888

SUBORDER Theropoda Marsh, 1881

INTRAORDER Coelurosauria Huene, 1914

SUBFAMILY Tyrannosauroidea Osborn, 1905 (sensu Sereno et al. 2005)

Tyrannosaurinae Osborn, 1905 (sensu Currie et al. 2003)

FAMILY Tyrannosauridae Osborn, 1906

GENUS Tyrannosaurus Osborn, 1905

SPECIES Tyrannosaurus rex Osborn, 1905

Material. YPM VPPU 016516 ,right anterior dentary with teeth. Horizon and locality.Upper Cretaceous,Lance Formation, Big Horn County, SW1/4, SE1/4, Sec.7, T57N, R99W, Wyoming, USA.

Description. Dentary. YPM VPPU 016516 is a well-preserved anterior end of the right dentary and identified here as an adult Tyrannosaurus rex ( Figure 3 A–E).The dentary is massive,mediolaterally thick along the tooth row, and deep. The length of the preserved dentary fragment is 25 cm and its depth is 20 cm. Approximately 3.5 cm of the bone in the dorsal region is missing, including a large portion of the interdental plates and the supradentary, which suggests that the dentary was deeper.In life the length of the entire dentary measured approximately 85 cm when comparing it to known dentaries of T. rex ( AMNH 5027 ; BHI 3033 , 4100 , 4182 ; CM 9380 [formerly AMNH 973] ; LACM 23844 , 7509/150167 ; FMNH PR 2081 [formerly BHI 2033] ; MOR 009 , 555 , 088 , 980 , 1125 ;ROM 1247; RTMP 81.6.1 ; SDSM 12047 ; TCM 2001.90.1 [formerly BHI 4960] ; UMNH 110000) . On the basis of this estimate, the entire length of the animal to which the dentary YPM VPPU 016516 belongs was approximately 12 m. The lateral surface of the dentary under the tooth row is convex and perforated by several large neurovascular foramina. The foramina are arranged in two rows: an alveolar and ventral row. The alveolar row is situated directly ventral to the alveolar margin.There are two foramina at first alveolus,and four at each consecutive alveolus. The ventral row of foramina parallels the alveolar row and the ventral margin of the dentary in a way that is characteristic of T. rex ( Brochu 2003; Currie 2003a; Carr and Williamson 2004). The medial surface is flat, which gives the dentary a D-shaped cross section similar to other tyrannosaurids ( Brusatte et al.2012).Anteriorly, the ventral and anterior margins of the dentary meet at a point ventral to the level of the third alveolus and brace the dentary symphysis medially ( Brusatte et al.2010, 2012).This ventrally projecting rugose process is known as the “chin” or transition point ( Carr et al. 2005); however, it is not as pronounced as in T. rex FMNH PR2081 .In adult examples of Tyrannosaurus ( AMNH FR 5027 , BHI 3033 and FMNH PR2081 ) the transition point is below the third alveolus ( Brusatte et al. 2012). In other derived Asian and North American tyrannosaurids this point is variable:below the third alveolus in Bistahieversor NMMNH P-27469, Daspletosaurus CMN 8506, Gorgosaurus ROM 1247,“ Tyrannosaurus ” NMMNH P-1013-1, the new Ojo Alamo tyrannosaurid taxon ACM 7975, Zhuchengtyrannus ZCDM V0031, below the fourth alveolus in Albertosaurus TMP 2003.45.64, Appalachiosaurus RMM 6670, Daspletosaurus MOR 533S, Teratophoneus BYU 9398, Tyrannosaurus CM 9380 , and below the fourth and fifth alveoli in Alioramus IGM 100/1844.In more primitive taxa such as Bagaraatan ZPAL MgD-I/108 the transition point is below the third alveolus.In the juvenile forms of Tyrannosaurus “ Nanotyrannus ” CMNH 7541 and “ Aublysodon ” LACM 28471 and Tarbosaurus MPC-D 107/7 the transition point is below the fourth alveolus.These variations are most likely ontogenetic and changed during growth.

Located on the medial surface of the anterior end of the dentary is the interdentary symphysis.It is an elongated, elliptically-shaped and rugose surface terminating posteroventrally below the third alveolus. As in other tyrannosaurids ( Brochu 2003; Currie 2003a, 2003b) the interdentary symphysis in YPM VPPU 016516 was weak in life.A small foramen intramandibularis oralis is located in the vicinity of the posteroventral tip of the interdentary symphysis and at the level of fourth alveolus as in other Tyrannosaurus (e.g. AMNH 5027 and BHI 3033 ) ( Figure 4 A–D).

The dentary YPM VPPU 016516 preserves five alveoli with partially erupted and broken off teeth.The first alveolus is small and laterally compressed. It contains a partially erupted tooth that is missing the apex. The second alveolus is slightly larger, rounded and empty. The third and fourth alveoli are significantly larger than the first and second.The fifth alveolus is only partially preserved; however, it is significantly larger than the other alveoli.The third alveolus contains a well-preserved tooth crown that is partially erupted,whereas the fourth and fifth alveoli are empty. The preserved tooth d3 is laterally compressed, but not as much as in other non-avian theropods such as Allosaurus and Carcharodontosaurus and basal tyrannosauroids such as Sinotyrannus, Xiongguanlong and Yutyrannus. The tooth is slightly re-curved distally and serrated.The carinae are mesiolingually and distolabially oriented and both are contiguous over the tip of the crown ( Smith 2005; Reichel 2012).

The specimen YPM VPPU 016516 preserves a small remnant of the interdental plates. As in other tyrannosaurids, the interdental plates in YPM VPPU 016516 are not fully fused; they flare out medially and are separated dorsally by a groove for the dental lamina ventral to the tooth row.As in other nonavian theropods including tyrannosaurids, in life, the interdental plates were covered by the ossified supradentarycoronoid ( Molnar 1991; Brochu 2003; Currie 2003a; Hurum and Sabath 2003; Brusatte et al. 2012). However, in YPM VPPU 016516 the supradentary is not preserved. The lingual bar (“medial bone” of Currie 2003a; Brusatte et al. 2012), which underlies the interdental plates and overlies the Meckelian groove, is flat and flared as it approaches the posterior margin of the interdentary symphysis. As in other tyrannosaurids, including Tyrannosaurus rex , the anterior end of the lingual bar in YPM VPPU 016516 covers the first and second alveolus. Below the lingual bar is a small portion of the Meckelian groove. It is a shallow and elongated structure, which is located in the middle of the medial surface and is approximately 1.5 cm above the foramen intermandibularis oralis. The same condition is also observed in other adult Tyrannosaurus dentaries ( Brochu 2003).

FAMILY Tyrannosauridae Osborn, 1906

GENUS Tyrannosaurus Osborn, 1905

Tyrannosaurus rex Osborn, 1905

Material. YPM VP 057488(A) and YPM VP 057488B, two isolated digital pedal phalanges II-2 and III-2 of the left foot.

Horizon and locality. Upper Cretaceous Lance Formation, Buck Creek, 37 km north of Lusk , Wyoming, USA.

Description. In this description the two isolated digital pedal phalanges are referred to as II-2 YPM VP 057488(A) and III-2 YPM VP 057488(B) .

Pedal phalanx II-2 YPM VP 057488(A) ( Figure 5 A–F) is missing a small portion of the distal articular region.The entire length of the phalanx is 16.5 cm. The specimen YPM VP 057488(A) compares well with pedal phalanx II-2 of other examples of Tyrannosaurus rex (e.g., FMNH PR 2081 ; Figure 6 A–C). In all other tyrannosaurids, including T. rex , the pes phalanx II-2 is constricted at the middle and has expanded proximal and distal articular regions ( Brochu 2003), which is the same in YPM VP 057488(A) .The long axis of phalanx II-2 YPM VP 057488(A) is not parallel to the sagittal plane and slopes medially ( Brochu 2003; Carrano 2007). The dorsal surface of the phalanx is narrower than the ventral surface, which is a common characteristic of tyrannosaurid theropods, including T. rex ( Brochu 2003) .

The proximal articular region of phalanx II-2 YPM VP 057488(A) is concave and oval as in other tyrannosaurids from the coeval Hell Creek, Willow Creek and Scollard formations of the northwestern United States and southwestern Canada. As in other tyrannosaurids, phalanx II-2 YPM VP 057488(A) has ventral rugosities that are adjacent to the proximal articular surface ( Brochu 2003). These rugosities are well developed within the ventral surface and ventrolateral grooves ( Brochu 2003).

YPM VP 057488(A) preserves the lateral and medial distal condyles, which contain deep ligament pits. In large tyrannosaurids like Tyrannosaurus rex the distal condyles of the proximal pedal phalanges are cleft and asymmetrical ( Carr and Williamson 2000; Brochu 2003). This characteristic morphology is also evident in YPM VP 057488(A) .

Pedal phalanx III-2 YPM VP 057488(B) ( Figure 7 A–F) is missing a portion of the dorsoanterior surface. The entire length of the phalanx is 12. 5 cm. YPM VP 057488(A) compares well with pedal phalanx III-2 of other examples of Tyrannosaurus rex (e.g., FMNH PR 2081 ; Figure 8 A–C). The phalanx is shorter distally and constricted at the middle, with expanded proximal and distal articular regions ( Brochu 2003). The proximal articular region for the articulation with the corresponding pedal phalanx is concave and oval. The ventrolateral surface of the proximal articular region is rugose as in other tyrannosaurids. These rugosities are large and situated within the ventral and ventrolateral grooves of the phalanx ( Brochu 2003). In dorsal view the lateral and medial condyles of the phalanx have linear margins, which are symmetrical. The ligament pits are circular, deep and are visible in dorsal view. The distal articular region of the phalanx is trapezoidal, as in other tyrannosaurids including T. rex .

Figures

FIGURE 3. Tyrannosaurus rex (YPM VPPU 016516) right anterior dentary fragment from the Bighorn basin of Wyoming,USA.A, lateral.B, medial.C, dorsal.D, ventral. E, anterior. A1–E1, interpretive drawings of the dentary fragment. Abbreviations: ab, convex articular brace “chin” on the dentary symphysis; alv, alveolus; alvr, alveolar row of foramina; gdl, groove for dental lamina; idp, interdental plates; fio, foramen intermandibularis oralis; ip, inflection point where anterior and ventral margins of dentary meet; lb, lingual bar; mg, Meckelian groove; rim, sinuous rim continuing anterodorsally from articular brace of symphysis; ids, interdentary symphysis; rs, rugose surface; t, tooth; ventr, ventral row of foramina.

FIGURE 4. Examples of an adult Tyrannosaurus rex dentaries from the Hell Creek Formation Upper Cretaceous (Maastrichtian) of North America. AMNH FR 5027 from Dry Creek, McCone County, Montana. BHI 3033 from the Niemi Ranch, near Buffalo, Harding County, South Dakota. A, lateral. B, medial. All photographs by Lawrence Witmer; used with permission.

FIGURE 5.Pedal phalanx II-2 YPM VP 057488(A) of Tyrannosaurus rex from the Upper Cretaceous (Maastrichtian) Buck Creek of Wyoming, USA. A, lateral view. B, medial view. C, dorsal view. D, ventral view. E, posterior view. F, anterior view. G, position of pedal phalanx II-2 in the foot (left foot). Abbreviation: lp, lateral ligament pit.

FIGURE 6. Examples of the pedal phalanx II-2 of adult Tyrannosaurus rex (FMNH PR 2081) from the Hell Creek Formation Upper Cretaceous (Maastrichtian) of South Dakota, USA. A, dorsal view. B, ventral view. C, lateral view. Abbreviation: lp, lateral ligament pit. Pedal phalanx II-2 redrawn from Brochu (2003).

FIGURE 7. Pedal phalanx III-2 YPM VP 057488(B) of Tyrannosaurus rex from the Upper Cretaceous (Maastrichtian) Buck Creek of Wyoming, USA.A, lateral view.B, medialview.C, dorsal view.D, ventral view.E, posterior view. F, anterior view. G, position of pedal phalanx III-2 in the foot (left foot).Abbreviation: lp, lateral ligament pit.

FIGURE 8. Examples of the pedal phalanx III-2 of adult Tyrannosaurus rex (FMNH PR 2081) from the Hell Creek Formation Upper Cretaceous (Maastrichtian) of South Dakota, USA. A, lateral view. B, dorsal view. C, ventral view. Abbreviation: lp, lateral ligament. Pedal phalanx III-2 redrawn from Brochu (2003).