Tyrannosaurus bataar Maleev 1955

SUBGENUS TYRANNOSAURUS (TYRANNOSAURUS) (Osborn, 1906)

TYRANNOSAURUS (TYRANNOSAURUS) BATAAR Maleev, 1955

Synonyms— Tarbosaurus bataar, Tarbosaurus efremovi, Gorgosaurus lancinator, Gorgosaurus novojilovi

type—PIN 551-1

best specimens—type (skull), PIN 551-3, ZPAL MgD-1/3 (juvenile)

time —early to mid-Maastrichtian? of the late Late Cretaceous

horizon and locality—Nemegt Formation of Mongolia

MAIN ANATOMICAL STUDY—Maleev 1974

[table omitted]

[table omitted]

Discovered by the Soviet expedition of 1949, this taxa was at first correctly named Tyrannosaurus bataar by E. Maleev. But then he gave a smaller specimen the fine generic title, Tarbosaurus, and T. bataar is often sunk into the latter. But if the very big type skull had been found in North America it would have been assigned to T. rex: they are that alike! So much so that if they did overlap in time—the exact age of the Nemegt is hard to pin down — T. bataar may have even been an interbreeding, geographical subspecies of T. rex, much as the Eurasian brown bear and American grizzly are subspecies of Ursus arctos. T. bataar’s somewhat smaller size might be due to its living in a harsher, more arid habitat. So Maleev was right the first time in making this Tyrannosaurus. The Mongolian predator does have smaller teeth, a shallower snout and mandible, and somewhat different skull roof bones than T. rex. Also, T. bataar’s orbital horns, both before and behind the eye socket, appear to be the smallest among tyrannosaurs. The biggest complete T. bataar and T. rex skulls are the same length, so these individuals were about equal in size. Their skulls also share the same degree of binocular vision.

A more serious taxonomic problem is that the many good skulls and skeletons may represent more than one species.18 Maleev and Osmolska believe in two or more, Rhozhdestvensky argues for one.19 Initially, I inclined toward the former view. After all, three species of the big-cat genus Panthera are found in India (lion, tiger, and leopard), and there is always more than one tyrannosaur present in North American formations. That the Nemegt had only one seemed wrong. Yet, careful examination of published remains and those I saw in Warsaw leaves me pretty sure that Rhozhdestvensky is right. Whatever the specimen’s size, the teeth of all the specimens are alike in size and design, the orbital horns are the same, and there just is no significant variation in morphology. One small, partial skull (“ Gorgosaurus novojilovi, ” Maleev, 1955 20) has been restored as very long and low, quite different from the others. But the individual bones match other T. bataar skulls, and restored properly they form a normal skull. There have been suggestions that this specimen’s foot bones are unique, but as far as I can tell they are not.

All Nemegt tyrannosaurs may therefore represent a growth series of T. bataar. As the species grew up, the body became more robust, the shank and feet somewhat shorter, and the transverse braincase crest seems to have enlarged, rather like what appears to occur in A. libratus. Unlike the latter, T. bataar teeth show no dramatic alteration in size relative to the skull, but the snout did became longer as they matured.

Note that the larger skeletal restoration (see page 341) is of a fairly complete subadult skull and skeleton. Full adults were even more like T. rex, as shown by the big type skull. The juvenile skeleton is based on a superb individual that lacks only the tail.

Prior to the Nemegt deposition, Mongolia was too arid to support big herbivore populations large enough to feed big tyrannosaurs, so only a few big theropod teeth are known. Even the Nemegt was a dryer, more open, savanna-like habitat than were the heavily forested North American tyrannosaur environments. T. bataar’s prey consisted mainly of armored ankylosaurs, the big duckbill Saurolophus, and one or two of species of medium-sized brontosaurs. There is little doubt that 5-tonne T. bataar could bring down the 5-to-10-tonne brontosaurs in its neighborhood. So, although T. rex never met Brontosaurus itself, the comic books are correct in showing tyrannosaurs preying on its relative. These bulky herbivores may have provided most of T. bataar’s prey biomass. With the possible exception of the rare and possibly herbivorous Deinocherius, T. bataar had no competitors.