Tyrannosaurus rex Osborn, 1905

Lawson, Douglas A., 1976, Tyrannosaurus and Torosaurus, Maestrichtian Dinosaurs From Trans-Pecos, Texas, Journal of Paleontology 50 (1), pp. 158-164 : 159-160

Tyrannosaurus rex Osborn, 1905

Text-fig. 2 a, b

Material.— TMM 41436 -1, left maxilla with teeth. Locality.— Tornillo Flat, Big Bend National Park , Brewster County, Texas. Horizon.— Within the lower third of the Tornillo Group (base of the Group faulted out); Maestrichtian Age.

Description.— The anterior tip, lower edge, and anterior teeth of the specimen have been broken away exposing the aveoli and the roots of the anterior teeth. The maxilla is triangular in lateral outline, with a height of 228 mm measured at the anterior edge of the first antorbital fenestra. The estimated length of the bone from its anterior edge to the posterior blade of the last tooth is 390 mm. The greatest diameter of the second antorbital fenestra is 96 mm and the least diameter is 69 mm. This fenestra lies at the anterior margin of a lateral depression which also contains the first antorbital fenestra and is separated from the latter by a slender bar 24 mm across at its narrowest point. The anteriormost antorbital fossa lies at the anterior edge of the second antorbital fenestra and is not visible in lateral aspect. The transversely rounded anterodorsal edge of the bone indicates that the maxilla took part in forming the external nares. The palatal flange of the maxilla is a triangular prism that runs the length of the maxilla, decreasing in height posteriorly. The dorsal surface of the flange is nearly horizontal, sloping downward toward the inner surface of the maxilla. The last three teeth decrease gradually in length posteriorly and are sharply recurved. The interdental plates are not present (the burial of this specimen in a conglomeratic sandstone, suggests that the plates were lost during stream transport).

Discussion.— The general proportions of the Texas specimen are much like those of Tyrannosaurus rex ( AMNH 973 ; AMNH 5027 : Table 1). The maxilla of Tyrannosaurus is higher than long; in Daspletosaurus it is equidimensional; in Albertosaurus is longer than high ( Text-fig. 3 ). As in Tyrannosaurus , but unlike Daspletosaurus or Albertosaurus , the second antorbital fenestra lies at the anterior edge of the well defined lateral depression which contains the first antorbital fenestra ( Osborn, 1912; Matthew and Brown, 1923). Unlike Albertosaurus and Daspletosaurus , but like Tyrannosaurus , the anteriormost antorbital fossa of the Texas specimen is not visible in lateral aspect, and the first and second antorbital fenestrae are separated by a narrow bar as in Tyrannosaurus rex . The anterodorsal edge of this specimen takes part in the border of the external nares as in Albertosaurus and Tyrannosaurus ( Russell, 1970) .

The small size of TMM 41436-1 and the large diameter of the second antorbital fenestra might have distinguished this specimen from other maxillae of larger members of the species. Text-figure 4 shows the characteristic relationships between fenestra size and tooth row length in three species of carnivorous dinosaurs. In Albertosaurus libratus the second antorbital fenestra appears to increase in diameter more rapidly than the maxilla does in length; in Allosaurus fragilis there appears to be no growth differential; and in Tyrannosaurus rex the length of the maxilla increases faster than that of the second antorbital fenestra. Thus, TMM 41436-1 probably exhibits the characteristics of a young adult Tyrannosaurus rex .

Figures

Text-fig. 2—Tyrannosaurus rex Osborn, TMM 41436-1, left maxilla, a, lateral view, b, medial view. x 1/4.

Text-fig. 3—Maxilla, TMM 41436-1 (stippled) compared with other tyrannosaurs (sensu Russell, 1970). The height of TMM 41436-1 is used as a standard, a, Tyrannosaurus rex, AMNH 5027, from photograph in Osborn (1912). b, Daspletosaurus torosus, NMC 8506, modified from Russell (1970). c, Albertosaurus libratus, FMNH PR 308, from AMNH negative No. 39113.

Text-fig. 4—Growth changes in the maxilla of three species of carnivorous dinosaurs. Acronyms: H, height of the maxilla at the anterior edge of the first antorbital fenestra; LSAF, length of the second antorbital fenestra; TRL, tooth row length, a, size of the second antorbital fenestra relative to the size of the maxilla, b, rectangularity of the maxilla relative to size.