Tyrannosaurus, Osborn, 1905

Paul, G. S., 1988, Predatory Dinosaurs of the World, Predatory Dinosaurs of the World, New York: Touchstone Books, pp. 323-349 : 337-338

GENUS TYRANNOSAURUS Osborn, 1905

synonyms— Daspletosaurus , Dynamosaurus , Gorgosaurus , Tarbosaurus

Traditionally, Tyrannosaurus is considered to consist of only one species, T. rex . However, tyrannosaurs are so like one another that all the usual genera cannot be justified; they are oversplit. In particular, Daspletosaurus and Tarbosaurus share most of the key characters that characterize T. rex : a stocky, heavy-boned build, relatively short lower hind limbs and large arms, a short snout, smaller preorbital horns, nasal bones that are tightly constricted between the preorbital bones, deep lower jaws, and long yet stout teeth that point a little more forward than they do in Albertosaurus . These three species form their own clade, and the amount of variation between them is less than that seen in some well-established modern or recent genera such as Canis (wolves and jackals) and even our own genus Homo. Daspletosaurus and Tarbosaurus are, therefore, junior synonyms of Tyrannosaurus . However, Daspletosaurus is different enough from the other two species to warrant its own subgenus.

To a fair extent the Tyrannosaurus species are the tyrannosaur’s tyrannosaurs; they have taken to an extreme the development of skull size, strength, and power. This and the larger, more forward-pointing mid-upper-jaw teeth suggest a more potent wounding ability than the albertosaur’s. The stoutness of Tyrannosaurus relative to albertosaurs is readily apparent in the skeletal restorations. They are not as graceful, but they have a well-proportioned, majestic attractiveness of their own.

Because Tyrannosaurus is shorter and stockier-limbed than Albertosaurus , it is tempting to ascribe slower speeds to it. However, the proportional differences are not great, while the morphology is almost identical. Perhaps Tyrannosaurus used the power of its stouter limbs to equal the running performance of Albertosaurus . Or perhaps the former were better sprinters and the latter better long distance runners. The very size of T. rex may have made it the fastest tyrannosaur; there is no way to be certain. Stout Tyrannosaurus was well built for ceratopsian killing. To safely and successfully hunt ceratopsians, tyrannosaurs probably had to surprise them, or panic them into a run in which they could be approached from the rear. Otherwise the powerful horned dinosaurs may have reared like enraged bears to try and intimidate the tyrannosaurs. If that failed, a running charge was the horned dinosaur’s answer, and then the tyrannosaur often did the fleeing!

Unlike Albertosaurus , which remained pretty much the same size and became a little more gracile over ten or so million years, Tyrannosaurus became much larger and stouter during this same time. Tyrannosaurus may have evolved from unknown tyrannosaurs, but the fact that T. torosus and A. libratus were long confused suggests that a form of Albertosaurus may have been its ancestor. Because T. bataar and T. rex are so similar, they must have shared a recent, common ancestor, if they were not geographic subspecies of one another.

There is more variation in this genus than in Albertosaurus , and the lightly built skull of T. torosus makes it a subgenus separate from T. bataar and T. rex . In his 1970 study, Dale Russell reported a juvenile T. torosus -type skeleton from the Horseshoe Canyon Formation.