Dinosaurs - Tarbosaurus bataar

Introduction

Pronounced Tar-bow-SORE-us baa-taa

Tarbosaurus means 'alarming lizard' in Greek. The species name is a mispelling of baatar, which means 'hero' in Mongolian.

Tarbosaurus bataar was a large carnivorous theropod dinosaur from Central Asia and nearly as large as T. rex. While T. rex can be called the king of North American dinosaurs during the Late Cretaceous, Tarbosaurus was its counterpart in Asia. It had the smallest arms of any large tyrannosaur relative to its body size.

Identification

Like its close North American relative, Tyrannosaurus rex, it had a very large head and powerful jaws. However, its skull was relatively light as many of the bones were hollow, with spaces filled with air pockets. Known as pneumatisation, these air-filled bones were not restricted to the skull, but also seen in limb bones and in vertebrae. This feature appears in other saurischian, or lizard-hipped, dinosaurs - a group that includes the giant sauropods as well as the carnivorous theropods.

The small size ofTarbosaurus bataar's arms is typical of larger, later tyrannosaurs and also found in some other theropod families. This reduction in arm size may have been advantageous in aiding a two-legged animal with a large head to maintain its balance by reducing the weight of its forelimbs.

Habitat

Tarbosaurus bataar lived in a humid floodplain criss-crossed by river channels. In this environment, it was a predator at the top of the food chain, probably preying on other large dinosaurs like the hadrosaur Saurolophus or the sauropod Nemegtosaurus.

Distribution

First found in 1946 and described and named in 1955, Tarbosaurus bataar lived 72-68 million years ago during the Late Cretaceous Period, in what is now southern Mongolia and China. There have been no specimens of T. bataar reported outside of Asia.

Feeding and diet

Tarbosaurus bataar was a carnivore, probably eating carrion as well as hunting prey.

Both Asiatic and North American tyrannosaurines hunted large prey, whilst probably also being opportunistic scavengers. However, the prey available to them was different. T. rex probably specialised in bringing down large dinosaurs, like the ceratopsianTriceratops, but this group was absent outside North America. Mongolian tyrannosaurines were probably feeding on sauropods. In fact, some researchers think that the robust and rigid skull of Tarbosaurus bataar was an adaptation to hunt the massive sauropods found in this region, which did not exist in most of North America during the later Cretaceous.

Recently, an analysis of the fossils of a hadrosaur, Parasaurolophus, revealed numerous Tarbosaurus bite marks. This suggests this tyrannosaur methodically scavenged its victim's already-dead corpse rather than hunting and killing it. This doesn't conclusively settle the debate about whether tyrannosaurs were hunters or scavengers, they probably pursued both strategies, as necessary.

Other behaviours and adaptations

Tarbosaurus bataar had sturdy and quite long legs and its forelimbs were reduced as typical of all advanced tyrannosaurs. The function of the forelimbs is still not clear. Like other tyrannosaurs, it had a huge head with large cutting, serrated teeth. Its brain was tiny in comparison with its huge body.

Fossils description

The first fossils of Tarbosaurus bataar were discovered in the Nemegt Formation of Mongolia by a joint Soviet-Mongolian expedition to the Gobi Desert in 1946. The holotype consists of a partial skull and some vertebrae which Evgeny Maleev originally named Tyrannosaurus bataar in 1955.

This species of tyrannosaur is by far the most common found in the sedementary Nemegt formation. At least five skulls and postcrania belonging to about 30 individuals of Tarbosaurus are known. This estimate is conservative, and may be tripled (the list of catalogued specimens with skull material attributable to Tarbosaurus does not include probable specimens in Chinese collections nor uncatalogued material in Mongolian and Japanese collections).

Known from numerous specimens, its anatomy has been extensively studied. These studies, however, were based mostly on adult or subadult specimens, and juvenile individuals of this dinosaur have rarely been found or described. This is unlike North American tyrannosaurids such as Albertosaurus, Daspletosaurus and Tyrannosaurusfor which immature individuals and even growth series are known.

Evolutionary relationships

Tarbosaurus bataar is a member of the family Tyrannosauridae and subfamily Tyrannosaurinae. Also in this subfamily are the genera Alioramus, Teratophoneus, Daspletosaurus, Zhuchengtyrannus and Tyrannosaurus.

Some scientists think that Tarbosaurus and the very closely related T. rex should both be in the same genus (Tyrannosaurus, which was named before Tarbosaurus). However, compared to T. rex, Tarbosaurus had a longer, more slender skull and more teeth. Studies comparing nerve pathways in skulls of the two species also show significant differences, supporting the alternative idea that Tarbosaurus and Tyrannosaurus are separate genera.

It is possible that Tarbosaurus had evolutionary precedence, and spawned Tyrannosaurus rex when some hardy individuals crossed the Siberian land bridge into North America.

Further reading

The evolution of large-bodied theropod dinosaurs during the Mesozoic in Asia. S. L. Brusatte, R. B. J. Benson, X. Xu.