Maleevosaurus

''Tarbosaurus'' ( ; meaning "alarming lizard") is a genus of large tyrannosaurid dinosaur that lived in Asia during the Late Cretaceous epoch, about 70 million years ago (Maastrichtian age). It contains the single type species: ''Tarbosaurus bataar'', which is known from the Nemegt Formation of Mongolia, with more fragmentary remains found further afield in the Subashi Formation. ''Tarbosaurus'' is represented by dozens of fossil specimens, including several complete skulls and skeletons. These remains have allowed studies focusing on its phylogeny, skull mechanics, and brain structure. Further fossil remains have been reported from other geologic formations of Asia, however, these remains are fragmentary and cannot be confidently assigned to ''Tarbosaurus'' or the type species.

Like most known tyrannosaurids, ''Tarbosaurus'' was a large bipedal predator, with the type specimen measuring approximately long, tall at the hips, and weighing up to . It had a unique locking mechanism in its jaw, equipped with about sixty large teeth, and the smallest arms relative to body size of all tyrannosaurids, renowned for their disproportionately tiny, two-fingered hands.

Although many species have been named, modern paleontologists recognize only one species, ''T. bataar''. Some experts see this species as an Asian representative of the North American genus ''Tyrannosaurus'', which would make the genus ''Tarbosaurus'' redundant. ''Tarbosaurus'' and ''Tyrannosaurus'', if not synonymous, are considered to be very closely related genera. ''Alioramus'', also from Mongolia, has previously been thought by some authorities to be the closest relative of ''Tarbosaurus'', though this has since been disproven with the discovery of ''Qianzhousaurus'' and the description of the tyrannosaurine tribe Alioramini.

''Tarbosaurus'' lived in a humid floodplain dominated by deserts, forests and plains, and criss-crossed by river channels. In this environment, it was an apex predator preying on other large dinosaurs, like ankylosaurids, such as ''Tarchia'' and ''Saichania'', hadrosaurids, such as ''Saurolophus'' and ''Barsboldia'', and sauropods, such as ''Nemegtosaurus'' and ''Opisthocoelicaudia''.

Discovery and naming

In 1946, a joint Soviet-Mongolian expedition to the Gobi Desert in the Ömnögovi Province turned up a large theropod skull and some vertebrae from the Nemegt Formation. In 1955, Evgeny Maleev, a Soviet paleontologist, made this specimen the holotype (PIN 551–1) of a new species, which he called ''Tyrannosaurus bataar''. The specific name is a misspelling of the Mongolian word баатар/''baatar'', meaning "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 words ''τάρβος'' ('), meaning "terror", "alarm", "awe", or "reverence", and ('), meaning "lizard", and the species was 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'' as ''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 and newer material. Later authors, including Maleev himself, agreed with Rozhdestvensky's analysis, although some used the name ''Tarbosaurus efremovi'' instead of ''T. bataar''.

American paleontologist Ken 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 Soviet-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 even more ''Tarbosaurus'' material. More than 30 specimens are known, including more than 15 skulls and several complete postcranial skeletons.

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 S. 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 tyrannosaurine. These authors refrained from assigning it to any particular genus, but suggested ''Tarbosaurus'' as a possibility.

''Albertosaurus periculosus'', ''Tyrannosaurus luanchuanensis'', ''Tyrannosaurus turpanensis'', and '' Chingkankousaurus fragilis'' were all considered synonyms of ''Tarbosaurus'' in the second edition of the Dinosauria, but ''Chingkankousaurus'' has been assessed as dubious by Brusatte et al. (2013).

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.

Additional specimens

In 1979 Dong Zhiming described several dinosaur remains from the strata of South China, reporting teeth, a dorsal vertebra, and several fragmentary foot bones of a tyrannosaurid. He tentatively referred these to ''Tarbosaurus'' sp. given some similarities in tooth morphology as well as the general geographic proximity to the Nemegt Formation. In 1993 Tomasz Jerzykiewiczz examined the general outcrops from the Bayan Mandahu Formation of China. In this paper, he reported isolated tyrannosaurid premaxillary and maxillary teeth, which he assigned to ''Tarbosaurus'' without anatomical comparisons. Lev A. Nessov in 1995 reported a partial femur from the Bostobe Formation of Kazakhstan and referred it as ''Tarbosaurus'' sp. without any diagnosis.

In 2004, David B. Weishampel and team listed a dubious ''Tarbosaurus''? sp. as a component of the known dinosaur taxa of the Campanian-aged Djadokhta Formation without further ado. Despite this referral, scattered, and very sparse tyrannosaurid remains are occasionally found in the strata of the Djadokhta Formation of Mongolia and none of these remains have ever been diagnostically assigned to ''Tarbosaurus'' or even ''T. bataar''. In addition, the extreme conditions of the Djadokhta Formation suggest that the remains of tyrannosaurids and other large-bodied dinosaurs from this unit represent passing by taxa foreign to the region.

During a large fossil prospection led by the Hayashibara Museum of Natural Sciences-Mongolian Paleontological Center Joint Expedition, a juvenile tyrannosaurid was discovered in 2006 from the highly fossiliferous Bügiin Tsav locality where adult specimens of ''Tarbosaurus'' have been recovered. The specimen was found preserving a partial skeleton with a fairly complete skull. In 2011 this juvenile was formally described and referred to ''Tarbosaurus bataar'', catalogued as MPC-D 107/7 within the collections of the Mongolian Paleontological Center.

Poached specimens

''Tarbosaurus'' fossils are only found around the Gobi Desert of Mongolia and China, both of which ban their export, though some specimens have been looted by private collectors. A $1 million smuggling deal was uncovered when suspicions were raised about a catalog put out by Heritage Auctions for an event in New York City on May 20, 2012. By Mongolian law, any specimen found in the Gobi Desert was to rest at an appropriate Mongolian institution and there was little reasonable doubt that the ''Tarbosaurus bataar'' advertised on the catalog was a stolen one. The president of Mongolia and many paleontologists raised objections to the sale, which led to a last-minute investigation that confirmed that it was a specimen that can only be found in the Gobi Desert, rightfully belonging to Mongolia. During the court case ( United States v. One ''Tyrannosaurus Bataar'' Skeleton), Eric Prokopi, the smuggler, pleaded guilty to illegal smuggling and the dinosaur was returned to Mongolia in 2013, where it is temporarily displayed on Sukhbaatar Square, the center of the city of Ulaanbaatar. Prokopi had sold the dinosaur with a partner and fellow commercial hunter in England named Christopher Moore. The case led to the repatriation of dozens of Mongolian dinosaurs, including several skeletons of ''Tarbosaurus bataar''.

Skin impressions and footprints

Skin impressions were recovered from a large skeleton at the Bugiin Tsav locality that was subsequently destroyed by poachers. These impressions show non-overlapping scales with an average diameter of and pertain to the thoracic region of the individual, although the exact position can no longer be assessed due to the destruction of the skeleton.

Phil Currie and colleagues (2003) described two footprints from the Nemegt locality that probably pertain to ''Tarbosaurus''. These tracks represent natural casts, which means that only the sandy infill of the tracks, not the tracks themselves, are preserved. The better-preserved track features skin impressions over large areas on and behind the toe impressions that are similar to those discovered in Bugiin Tsav. It also features vertical parallel slide marks that were left by scales when the foot was pushed into the ground. The track measures in length, thus representing a large individual. The second track, although even larger, was affected by erosion and does not show any detail.

In 1997, Ken Carpenter reported a damaged ''Tarbosaurus'' skull with impressions of a dewlap or throat pouch beneath the lower jaws, based on a personal communication from Konstantin Mikhailov. Carpenter speculated that the pouch may have been used for display, possibly being brightly colored and inflatable like a frigatebird's. In a 2019 communication to Mickey Mortimer, Mikhailov confirmed that this specimen had not been collected because it was on a heavy stone slab. He revealed that it had been discovered by Sergei Kurzanov and that it was Kurzanov himself who had originally interpreted the impressions as a throat structure. This specimen may be the same as one that was purportedly destroyed by poachers in 1992.

Description

Although slightly smaller than ''Tyrannosaurus'', ''Tarbosaurus'' was one of the largest tyrannosaurines, with the type specimen PIN 551–1 measuring approximately long, tall at the hips, and weighing up to . Other adult specimens were a bit smaller, as MPC-D 107/2, ZPAL.MgD-I/4 and PIN 552-1 likely weighed around to . The largest known ''Tarbosaurus'' skull is about long, which is larger than that of all other tyrannosaurids, aside from ''Tyrannosaurus''.

Skull

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''‘s eyes did not face directly forwards, suggesting that it lacked the binocular vision of ''Tyrannosaurus''. Large fenestrae in the skull reduced its overall weight and served as attachment points for muscles. Between 58 and 64 teeth lined its jaws. This tooth count is slightly more than that of ''Tyrannosaurus'', but fewer than in smaller tyrannosaurids, like ''Gorgosaurus'' and ''Alioramus''.

Most of its teeth were ovalur in cross section, although the teeth of the premaxilla at the tip of the upper jaw had a ''D''-shaped cross section. However, this heterodonty is characteristic of the family. The longest teeth were in the maxilla (upper jaw bone), with crowns up to 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.

Postcranial skeleton

Tyrannosaurids varied little in overall 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 tail, was held horizontally. ''Tarbosaurus'' had tiny arms, proportionably to body size the smallest of all members in the family. The hands had two clawed digits each, with an additional unclawed third metacarpal found in some specimens, similar to those of closely related genera. Holtz has suggested that ''Tarbosaurus'' also has a theropod reduction of fingers IV-I "developed further" than in other tyrannosaurids,Carpenter K, Tanke D.H. & Skrepnick M.W. (2001), ''Mesozoic Vertebrate Life'' (Indiana University Press, ), p. 71. as the second metacarpal in the ''Tarbosaurus'' specimens he studied is less than twice the length of the first metacarpal. Other tyrannosaurids have a second metacarpal about twice the length of the first metacarpal. Also, the third metacarpal in ''Tarbosaurus'' is proportionally shorter than in other tyrannosaurids. In other tyrannosaurids, like ''Albertosaurus'' and ''Daspletosaurus'', the third metacarpal is often longer than the first metacarpal, while in the ''Tarbosaurus'' specimens studied by Holtz, the third metacarpal is shorter than the first. In contrast to the arms, the three-toed legs were long, thick, and muscular to support the body in a bipedal posture. The long, heavy tail served as a counterweight to the head and torso, while also placing the center of gravity directly over the hips.

Classification

''Tarbosaurus'' is classified as a theropod in the subfamily Tyrannosaurinae of 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, Albertosaurinae.

''Tarbosaurus bataar'' was originally described as a species of ''Tyrannosaurus'', an arrangement that has been supported by some 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 are not found in other tyrannosaurines. If proven, this relationship would argue against ''Tarbosaurus'' being a synonym of ''Tyrannosaurus'' and would suggest that separate tyrannosaurine lineages evolved in Asia and North America. The two known specimens of ''Alioramus'', which show juvenile characteristics, are not likely juvenile individuals of ''Tarbosaurus'' because of their much higher tooth count (76 to 78 teeth) and their unique row of bony bumps along the top of their snouts.

The discovery of ''Lythronax argestes'', a much earlier tyrannosaurine, further reveals the close relationship between ''Tyrannosaurus'' and ''Tarbosaurus''. It was also discovered that ''Lythronax'' is a sister taxon to a clade consisting of Campanian genus ''Zhuchengtyrannus'', and the Maastrichtian genera ''Tyrannosaurus'' and ''Tarbosaurus''. Further studies of ''Lythronax'' also suggest that the Asian tyrannosauroids were part of one evolutionary radiation.

Below is the cladogram of Tyrannosaurinae based on the phylogenetic analysis conducted by Voris and team in 2020.

Paleobiology

Ontogeny

Most specimens of ''Tarbosaurus'' represent adult or subadult individuals, while juveniles remain very rare. Nevertheless, the 2006 discovery of a juvenile individual (MPC-D 107/7) with a complete, long skull was reported and described in 2011, providing information on the life history of this dinosaur. This individual was probably 2 to 3 years old at the time of death. Compared to adult skulls, the juvenile skull was weakly constructed and the teeth were thin, indicating different food preferences in juveniles and adults that reduced competition between different age groups. Examination of the sclerotic rings in this juvenile ''Tarbosaurus'' suggests they may also have been crepuscular or nocturnal hunters. Whether or not the adult ''Tarbosaurus'' were also nocturnal is currently unknown due to the lack of fossil evidence to suggest so. Ontogenetic changes identified in the frontal bones suggests that the allometric growth of ''Tarbosaurus'' was similar to that of North American tyrannosaurids.

Senses

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 Casting, cast, called an ''endocranial cast, 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.

The endocranial structure of ''Tarbosaurus'' was similar to that of ''Tyrannosaurus'', differing only in the positions of some cranial nerve roots, including the trigeminal nerve, trigeminal and accessory nerves. Tyrannosaurid brains were more similar to those of crocodilians and other nonavian reptiles than they were to birds. The total brain volume for a long ''Tarbosaurus'' is estimated at only .

The large size of the olfactory bulbs, as well as the terminal nerve, terminal and olfactory nerves, suggest that ''Tarbosaurus'' had a highly keen sense of smell, as was also the case with ''Tyrannosaurus''. The Olfactory bulb#Accessory olfactory bulb, vomeronasal bulb is large and differentiated from the olfactory bulb, which was initially suggested as being indicative of a well-developed Vomeronasal organ, Jacobsen's organ, which was used to detect pheromones. This may imply that ''Tarbosaurus'' had complex mating behavior. However, the identification of the vomeronasal bulb has been challenged by other researchers because they are not present in any living archosaurs.

The auditory nerve was also large, suggesting good Hearing (sense), hearing, which may have been useful for auditory animal communication, communication and spatial awareness. The nerve had a well-developed Vestibular system, 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 accurate 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. It has been suggested that the lack of binocular vision in Asian tyrannosaurs, like ''Tarbosaurus'', might have been correlated with a greater amount of scavenging resources provided by sauropod carcasses, which might have afforded them a less active predatory lifestyle when compared to the North American forms, meaning they would need less predatory adaptations. However, this is contradicted by numerous lines of evidence indicating ''Tarbosaurus'' was actively preying on hadrosaurs, titanosaur sauropods, and other large bodied herbivores in its ecosystem.

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 bone, frontal and prefrontal bones in ''Tarbosaurus''. The well-developed connections between the maxilla, lacrimal, frontal, and prefrontal would have made its entire upper jaw much more rigid.

Another major difference between ''Tarbosaurus'' and its North American relatives was its more rigid mandible. 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 hypothesis, 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 affect tyrannosaurid phylogeny. ''Tarbosaurus''-like articulations between the skull bones are also seen in ''Alioramus'' from Mongolia, suggesting that it is the closest relative of ''Tarbosaurus''. Similarities between ''Tarbosaurus'' and ''Tyrannosaurus'' might be related to their large size, independently developed through convergent evolution.

Bite force and feeding

In 2001, Bruce Rothschild and others published a study examining evidence for stress fractures and Avulsion fracture, tendon avulsions in Theropoda, theropod dinosaurs and the implications for their behavior. Since stress fractures are caused by repeated trauma rather than singular events, they are more likely to be caused by regular behavior than other types of injuries. None of the eighteen ''Tarbosaurus'' foot bones examined in the study were found to have a stress fracture, but one of the ten examined hand bones was found to have one. Stress fractures in the hands have special behavioral significance compared to those found in the feet, since stress fractures there can be obtained while running or during migration. Hand injuries, by contrast, are more likely to be obtained while in contact with struggling prey. The presence of stress fractures and tendon avulsions, in general, provide evidence for a "very active" predation-based diet instead of obligate scavenging.Rothschild, B., Tanke, D. H., and Ford, T. L., 2001, Theropod stress fractures and tendon avulsions as a clue to activity: In: Mesozoic Vertebrate Life, edited by Tanke, D. H., and Carpenter, K., Indiana University Press, p. 331-336.

As for its bite force, it was revealed in 2005 that ''Tarbosaurus'' had a bite force of around 8,000 to 10,000 pounds per square inch of force, meaning that it could crush bones just like its North American relative, ''Tyrannosaurus''.

David W. E. Hone and Mahito Watabe in 2011 reported the left humerus of a nearly complete ''Saurolophus'' skeleton (MPC-D 100/764) from the Bügiin Tsav locality of the Nemegt Formation, which was Paleopathology, heavily damaged from bite marks attributed to ''Tarbosaurus''. As suggested by the lack of damage to the rest of the skeleton (such as large wounds in skeletal remains indicative of predation), this tyrannosaurid was likely scavenging an already dead ''Saurolophus''. It is unlikely that a large-bodied predator, such as ''Tarbosaurus'', would have left sparse feeding traces on a single humerus when having an entire carcass to feed on. The humerus shows three distinctive feeding methods, interpreted as punctures, drag marks, and bite−and−drag marks. Hone and Watabe noted that bite marks were mostly located at the deltopectoral crest, suggesting that this ''Tarbosaurus'' was actively selecting which biting style to employ so it could scavenge the bone.

In 2012, bite marks on two fragmentary Gastralium, gastralia of the holotype specimen of the large ornithomimosaur ''Deinocheirus, Deinocheirus mirificus'' were reported. The size and shape of the bite marks match the teeth of ''Tarbosaurus'', the largest known predator from the Nemegt Formation. Various types of feeding traces were identified. These include punctures, gouges, striae, fragmentary teeth, and combinations of the above marks. The bite marks probably represent feeding behavior instead of aggression between the species and the fact that bite marks were not found elsewhere on the body indicates the predator focused on internal organs. ''Tarbosaurus'' bite marks have also been identified on hadrosaur and sauropod fossils, but theropod bite marks on bones of other theropods are very rare in the fossil record.

A 2020 study involving stable isotopes found that ''Tarbosaurus'' primarily hunted large dinosaurs in its environment, most notably titanosaurs and hadrosaurs.

Paleoenvironment

Nemegt Formation

The vast majority of known ''Tarbosaurus'' fossils were recovered from the Nemegt Formation in the Gobi Desert of southern Mongolia. This geologic formation has never been radiometric dating, dated radiometrically, but the fauna present in the fossil record indicate it was probably deposited during the early Maastrichtian stage at the near end of the Late Cretaceous, about 70 million years ago.Sulliban, R.M. (2006). "A taxonomic review of the Pachycephalosauridae (Dinosauria: Ornithischia)." Pp. 347-366 in Lucas, S.G. and Sullivan, R.M. (eds.), ''Late Cretaceous vertebrates from the Western Interior'.' New Mexico Museum of Natural History and Science Bulletin 3.

''Tarbosaurus'' is found chiefly in the Nemegt Formation, whose sediments preserve large river channels and soil deposits that indicate a far more humid climate than those suggested by the underlying Barun Goyot Formation, Barun Goyot and Djadochta Formations. However, Caliche (mineral), caliche deposits indicate at least periodic droughts. Sediment was deposited in the channels and floodplains of large rivers. The rock facies of this formation suggest the presence of mudflats and shallow lakes. Sediments also indicate that there existed a rich habitat, offering diverse food in abundant amounts that could sustain massive Cretaceous dinosaurs.Novacek, M. (1996). Dinosaurs of the Flaming Cliffs. Bantam Doubleday Dell Publishing Group Inc. New York, New York.

Occasional mollusk fossils are found, as well as a variety of other aquatic animals, such as fish and turtles. Crocodilians included several species of ''Paralligator'', a genus with teeth adapted for crushing shells. Mammal fossils are exceedingly rare in the Nemegt Formation, but many birds have been found, including the enantiornithine ''Gurilynia'' and the Hesperornithes, hesperornithiform ''Judinornis'', as well as ''Teviornis'', an early representative of the still-existing Anseriformes. Scientists have described many dinosaurs from the Nemegt Formation, including the ankylosaurids ''Tarchia'' and ''Saichania'' and the pachycephalosaur ''Prenocephale''.

By far the largest predator known from the formation, 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 the small tyrannosaurid ''Alioramus'', troodontids (''Borogovia'', ''Tochisaurus'', ''Zanabazar (dinosaur), Zanabazar''), oviraptorosaurs (''Elmisaurus'', ''Nemegtomaia'', ''Rinchenia'') or ''Bagaraatan'', sometimes considered a basal (phylogenetics), basal tyrannosauroid. Other theropods, like the gigantic ''Therizinosaurus'', might have been herbivore, herbivorous and ornithomimosaurs, such as ''Anserimimus'', ''Gallimimus'', and gigantic ''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'' would have filled niches between the massive adults and these smaller theropods.

Subashi Formation

The Subashi Formation

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