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Tarbosaurus (meaning "terrifying lizard") is a genus of tyrannosaurid theropod dinosaur that flourished in Asia between, at the end of the Late Cretaceous period. Fossils have been recovered in Mongolia with more fragmentary remains found further afield in parts of China. Although many species have been named, modern paleontologists recognize only one, T. bataar, as valid. Some experts contend that this species is actually an Asian representative of the North American genus Tyrannosaurus; if true, this would invalidate the genus Tarbosaurus altogether.

Tarbosaurus and Tyrannosaurus are considered closely related genera, even if they are not synonymous. Alioramus, also from Mongolia, is thought by some authorities to be the closest relative of Tarbosaurus. Like most known tyrannosaurids, Tarbosaurus was a large bipedal predator, weighing more than a ton and equipped with dozens of large, sharp teeth. It had a unique locking mechanism in its lower jaw and the smallest forelimbs relative to body size of all tyrannosaurids, renowned for their disproportionately tiny, two-fingered forelimbs.

Tarbosaurus lived in a humid floodplain criss-crossed by river channels. In this environment, it was an apex predator at the top of the food chain, probably preying on other large dinosaurs like the hadrosaur Saurolophus or the sauropod Nemegtosaurus. Tarbosaurus is very well-represented in the fossil record, known from dozens of specimens, including several complete skulls and skeletons. These remains have allowed scientific studies focusing on its phylogeny, skull mechanics, and brain structure.

Description
Although smaller than Tyrannosaurus, Tarbosaurus was one of the largest tyrannosaurids. The largest known individuals were between 10 and 12 meters (33 to 40 ft]]) long, each with a head held up to 5 meters (16.5 ft) above the ground. The mass of a fully grown individual has never been published, although it is considered comparable to or slightly smaller than Tyrannosaurus.

The largest known Tarbosaurus skull is more than 1.3 meters (4 ft) long, larger than all other tyrannosaurids except Tyrannosaurus. The skull was tall, like that of Tyrannosaurus, but not as wide, especially towards the rear. The unexpanded rear of the skull meant that Tarbosaurus eyes did not face directly forwards, suggesting that it lacked the binocular vision of Tyrannosaurus. Large fenestrae (openings) in the skull reduced its weight. Between 60 and 64 teeth lined its jaws, slightly more than in Tyrannosaurus but fewer than in smaller tyrannosaurids like Gorgosaurus and Alioramus. Most of its teeth were oval in cross section, although the teeth of the premaxilla at the tip of the upper jaw had a D-shaped cross section. This heterodonty is characteristic of the family. The longest teeth were in the maxilla (upper jaw bone), with crowns up to 85 millimeters (3.3 in) long. In the lower jaw, a ridge on the outer surface of the angular bone articulated with the rear of the dentary bone, creating a locking mechanism unique to Tarbosaurus and Alioramus. Other tyrannosaurids lacked this ridge and had more flexibility in the lower jaw.

Tyrannosaurids varied little in body form, and Tarbosaurus was no exception. The head was supported by an S-shaped neck, while the rest of the vertebral column, including the long tail, was held horizontally. Tarbosaurus had tiny, two-fingered forelimbs, which were smaller relative to its body size than those of any other member of the family. In contrast, the three-toed hindlimbs were long and thick, supporting the body in a bipedal posture. The long, heavy tail served as a counterweight to the head and torso and placed the center of gravity over the hips.

Classification and systematics
Carr et al. 2005

* Note : Carr et al. use the name Tyrannosaurus bataar

Currie et al. 2003

Tarbosaurus is classified as a theropod in the subfamily Tyrannosaurinae within the family Tyrannosauridae. Other members include Tyrannosaurus and the earlier Daspletosaurus, both from North America, and possibly the Mongolian genus Alioramus. Animals in this subfamily are more closely related to Tyrannosaurus than to Albertosaurus and are known for their robust build with proportionally larger skulls and longer femurs than in the other subfamily, the Albertosaurinae.

Tarbosaurus bataar was originally described as a species of Tyrannosaurus, an arrangement that has been supported by more recent studies. Others prefer to keep the genera separate, while still recognizing them as sister taxa. A 2003 cladistic analysis based on skull features instead identified Alioramus as the closest known relative of Tarbosaurus, as the two genera share skull characteristics that are related to stress distribution and that are not found in other tyrannosaurines. If proven, this relationship would preclude Tarbosaurus from becoming a synonym for Tyrannosaurus and would suggest that separate tyrannosaurine lineages evolved in Asia and North America. The single known specimen of Alioramus, which shows juvenile characteristics, is not likely a juvenile Tarbosaurus because of its much higher tooth count (76 to 78 teeth) and the unique row of bony bumps along the top of its snout.

Discovery and naming
In 1946, a joint Soviet-Mongolian expedition to the Gobi Desert in the Mongolian Ömnögovi Province turned up a large theropod skull and some vertebrae in the Nemegt Formation. In 1955, Evgeny Maleev, a Russian paleontologist, made this specimen the holotype (PIN 551-1) of a new species, which he called Tyrannosaurus bataar. The species name is a misspelling of the Mongolian баатар/baatar ("hero"). In the same year, Maleev also described and named three new theropod skulls, each associated with skeletal remains discovered by the same expedition in 1948 and 1949. The first of these (PIN 551-2) was named Tarbosaurus efremovi, a new generic name composed of the Ancient Greek ταρβος/tarbos ("terror", "alarm", "awe", or "reverence") and σαυρος/sauros ("lizard"), and the species named after Ivan Yefremov, a Russian paleontologist and science fiction author. The other two (PIN 553-1 and PIN 552-2) were also named as new species and assigned to the North American genus Gorgosaurus (G. lancinator and G. novojilovi, respectively). All three of these latter specimens are smaller than the first. A 1965 paper by A.K. Rozhdestvensky recognized all of Maleev's specimens as different growth stages of the same species, which he believed to be distinct from the North American Tyrannosaurus. He created a new combination, Tarbosaurus bataar, to include all the specimens described in 1955 as well as newer material. Later authors, including Maleev himself, agreed with Rozhdestvensky's analysis, although some used the name Tarbosaurus efremovi rather than T. bataar. American paleontologist Kenneth Carpenter re-examined the material in 1992. He concluded that it belonged to the genus Tyrannosaurus, as originally published by Maleev, and lumped all the specimens into the species Tyrannosaurus bataar except the remains that Maleev had named Gorgosaurus novojilovi. Carpenter thought this specimen represented a separate, smaller genus of tyrannosaurid, which he called Maleevosaurus novojilovi. George Olshevsky created the new generic name Jenghizkhan (after Genghis Khan) for Tyrannosaurus bataar in 1995, while also recognizing Tarbosaurus efremovi and Maleevosaurus novojilovi, for a total of three distinct, contemporaneous genera from the Nemegt Formation. A 1999 study subsequently reclassified Maleevosaurus as a juvenile Tarbosaurus. All research published since 1999 recognizes only a single species, which is either called Tarbosaurus bataar or Tyrannosaurus bataar.

After the original Russian-Mongolian expeditions in the 1940s, Polish-Mongolian joint expeditions to the Gobi Desert began in 1963 and continued until 1971, recovering many new fossils, including new specimens of Tarbosaurus from the Nemegt Formation. Expeditions involving Japanese and Mongolian scientists between 1993 and 1998, as well as private expeditions hosted by Canadian paleontologist Phil Currie around the turn of the 21st century, discovered and collected further Tarbosaurus material. More than 30 specimens are known, including more than 15 skulls and several complete postcranial skeletons.

Possible synonyms
Chinese paleontologists discovered a partial skull and skeleton of a small theropod (IVPP V4878) in the Xinjiang Autonomous Region of China in the mid-1960s. In 1977, Dong Zhiming described this specimen, which was recovered from the Subashi Formation, as a new genus and species, Shanshanosaurus huoyanshanensis. Gregory Paul recognized Shanshanosaurus as a tyrannosaurid in 1988, referring it to the now-defunct genus Aublysodon. Dong and Currie later re-examined the specimen and deemed it to be a juvenile of a larger species of tyrannosaurid. These authors refrained from assigning it to any particular genus but suggested Tarbosaurus as a possibility. Over the years, other Chinese localities have produced tyrannosaurid teeth and fragmentary remains, several of which have been given names. Albertosaurus periculosis, Tyrannosaurus luanchuanensis, Tyrannosaurus turpanensis and Chingkankousaurus fragilis are often considered synonyms of Tarbosaurus.

Named in 1976 by Sergei Kurzanov, Alioramus is another genus of tyrannosaurid from slightly older sediments in Mongolia. Several analyses have concluded Alioramus was quite closely related to Tarbosaurus. It was described as an adult, but its long, low skull is characteristic of a juvenile tyrannosaurid. This led Currie to speculate that Alioramus might represent a juvenile Tarbosaurus, but he noted that the much higher tooth count and row of crests on top of the snout suggested otherwise.

Paleobiology
Like several other large tyrannosaurids, Tarbosaurus is known from relatively abundant and well-preserved fossil material. In fact, one quarter of all fossils collected from the Nemegt Formation belong to Tarbosaurus. Although Tarbosaurus has not been studied as thoroughly as the North American tyrannosaurids, the available material has allowed scientists to draw limited conclusions about its biology.

Skull mechanics
The skull of Tarbosaurus was completely described for the first time in 2003. Scientists noted key differences between Tarbosaurus and the North American tyrannosaurids. Many of these differences are related to the handling of stress by the skull bones during a bite. When the upper jaw bit down on an object, force was transmitted up through the maxilla, the primary tooth-bearing bone of the upper jaw, into surrounding skull bones. In North American tyrannosaurids, this force went from the maxilla into the fused nasal bones on top of the snout, which were firmly connected in the rear to the lacrimal bones by bony struts. These struts locked the two bones together, suggesting that force was then transmitted from the nasals to the lacrimals. Tarbosaurus lacked these bony struts, and the connection between the nasals and lacrimals was weak. Instead, a backwards projection of the maxilla was massively developed in Tarbosaurus and fit inside a sheath formed from the lacrimal. This projection was a thin, bony plate in North American tyrannosaurids. The large backwards projection suggests that force was transmitted more directly from the maxilla to the lacrimal in Tarbosaurus. The lacrimal was also more firmly anchored to the frontal and prefrontal bones in Tarbosaurus. The well-developed connections between the maxilla, lacrimal, frontal and prefrontal would have made its entire upper jaw more rigid.

Another major difference between Tarbosaurus and its North American relatives was its more rigid mandible (lower jaw). While many theropods, including North American tyrannosaurids, had some degree of flexibility between the bones in the rear of the mandible and the dentary in the front, Tarbosaurus had a locking mechanism formed from a ridge on the surface of the angular, which articulated with a square process on the rear of the dentary.

Some scientists have hypothesized that the more rigid skull of Tarbosaurus was an adaptation to hunting the massive titanosaurid sauropods found in the Nemegt Formation, which did not exist in most of North America during the Late Cretaceous. The differences in skull mechanics also have an impact on tyrannosaurid phylogeny. Tarbosaurus-like articulations between the skull bones are also seen in Alioramus from Mongolia, suggesting that it, and not Tyrannosaurus, is the closest relative of Tarbosaurus. Similarities between Tarbosaurus and Tyrannosaurus might therefore be related to their large size, independently developed through convergent evolution.

Brain structure
A Tarbosaurus skull found in 1948 by Soviet and Mongolian scientists (PIN 553-1, originally called Gorgosaurus lancinator) included the skull cavity that held the brain. Making a plaster cast, called an endocast, of the inside of this cavity allowed Maleev to make preliminary observations about the shape of a Tarbosaurus brain. A newer polyurethane rubber cast allowed a more detailed study of Tarbosaurus brain structure and function.

Tyrannosaurus rex brain structure has also been analyzed, and Tarbosaurus was similar, differing only in the positions of some cranial nerve roots, including the trigeminal and accessory nerves. Tyrannosaurid brains were more similar to those of crocodilians and other reptiles than to birds. The total brain volume for a 12 meter (40 foot) Tarbosaurus is estimated at only 184 centimeters3 (11.2 in3). The large size of the olfactory bulbs, as well as the terminal and olfactory nerves, suggest that Tarbosaurus had a keen sense of smell, as was also the case with Tyrannosaurus. The vomeronasal bulb is large and differentiated from the olfactory bulb, indicating a well-developed Jacobsen's organ, which was used to detect pheromones. This may imply that Tarbosaurus had complex mating behavior. The auditory nerve was also large, suggesting good hearing, which may have been useful for auditory communication and spatial awareness. The nerve had a well-developed vestibular component as well, which implies a good sense of balance and coordination. In contrast, the nerves and brain structures associated with eyesight were smaller and undeveloped. The midbrain tectum, responsible for visual processing in reptiles, was very small in Tarbosaurus, as were the optic nerve and the oculomotor nerve, which controls eye movement. Unlike Tyrannosaurus, which had forward-facing eyes that provided some degree of binocular vision, Tarbosaurus had a narrower skull more typical of other tyrannosaurids in which the eyes faced primarily sideways. All of this suggests that Tarbosaurus relied more on its senses of smell and hearing than on its eyesight.

Palaeoecology
The vast majority of Tarbosaurus fossils are known from the Nemegt Formation of southern Mongolia. This geologic formation has never been dated radiometrically, but the fauna present in the fossil record indicate it was probably deposited during the Maastrichtian stage, at the end of the Late Cretaceous. The Maastrichtian stage occurred 70 to 65 million years ago. The Subashi Formation, in which Shanshanosaurus remains were discovered, is also Maastrichtian in age.

Nemegt sediments preserve large river channels and soil deposits that indicate a far more humid climate than those suggested by the underlying Barun Goyot and Djadochta Formations. However, caliche deposits indicate at least periodic droughts. Sediment was deposited in the channels and floodplains of large rivers. Occasional mollusc fossils are found, as well as a variety of other aquatic animals like fish and turtles. Crocodilians included several species of Shamosuchus, a genus with teeth adapted for crushing shells. Mammal fossils are exceedingly rare in the Nemegt Formation, but many birds are known, including the enantiornithine Gurilynia and the hesperornithiform Judinornis, as well as Teviornis, an early representative of the still-existing Anseriformes (waterfowl), a bird order. Scientists have described many dinosaurs from the Nemegt, including ankylosaurids such as Tarchia and pachycephalosaurs such as Homalocephale and Prenocephale. By far the largest predator of the time, adult Tarbosaurus most likely preyed upon large hadrosaurs such as Saurolophus and Barsboldia or sauropods such as Nemegtosaurus and Opisthocoelicaudia. Adults would have received little competition from small theropods such as troodontids (Borogovia, Tochisaurus, Saurornithoides), oviraptorosaurs (Elmisaurus, Nemegtomaia, Rinchenia) or Bagaraatan, sometimes considered a basal tyrannosauroid. Other theropods, like the gigantic Therizinosaurus, may have been herbivorous, and ornithomimosaurs such as Anserimimus, Gallimimus and Deinocheirus might have been omnivores that only took small prey and were therefore no competition for Tarbosaurus. However, as in other large tyrannosaurids as well as modern Komodo dragons, juveniles and subadult Tarbosaurus may have filled niches between the massive adults and these smaller theropods.