The skull of a newly prepared Tarbosaurus bataar is described bone by bone and compared with a disarticulated skull of Tyrannosaurus rex. Both Tarbosaurus bataar and Tyrannosaurus rex skulls are deep in lateral view. In dorsal view, the skull of T. rex is extremely broad posteriorly but narrows towards the snout; in Ta. bataar the skull is narrower (especially in its ventral part: the premaxilla, maxilla, jugal, and the quadrate complex), and the expansion of the posterior half of the skull is less abrupt. The slender snout of Ta. bataar is reminiscent of more primitive North American tyrannosaurids. The most obvious difference between T. rex and Ta. bataar is the doming of the nasal in Ta. bataar which is high between the lacrimals and is less attached to the other bones of the skull, than in most tyrannosaurids. This is because of a shift in the handling of the crushing bite in Ta. bataar. We propose a paleogeographically based division of the Tyrannosaurinae into the Asiatic forms (Tarbosaurus and possibly Alioramus) and North American forms (Daspletosaurus and Tyrannosaurus). The division is supported by differences in anatomy of the two groups: in Asiatic forms the nasal is excluded from the major series of bones participating in deflecting the impact in the upper jaw and the dentary-angular interlocking makes a more rigid lower jaw.

Key words: Dinosauria, Theropoda, Tyrannosauridae, Tarbosaurus, Tyrannosaurus, skull, anatomy, Mongolia.

Jørn H. Hurum [j.h.hurum@nhm.uio.no], Paleontologisk Museum, Boks 1172 Blindern, N−0318 Oslo, Norway; Karol Sabath [sabath@twarda.pan.pl], Instytut Paleobiologii PAN, ul. Twarda 51/55, PL−00−818 Warszawa, Poland.