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 of China. 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 predation, 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 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 Ankylosauridae, such as Tarchia and Saichania, Hadrosauridae, such as Saurolophus and Barsboldia, and sauropods, such as Nemegtosaurus and Opisthocoelicaudia.
A 1965 paper by A. K. Rozhdestvensky recognized all of Maleev's specimens as different ontogeny 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, Poland-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 and Mongolian scientists between 1993 and 1998, as well as private expeditions hosted by Canadians 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.
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).Brusatte, Hone and Xu, 2013. Phylogenetic revision of Chingkankousaurus fragilis, a forgotten tyrannosauroid from the Late Cretaceous of China. in Parrish, Molnar, Currie and Koppelhus (eds.). Tyrannosaurid Paleobiology. Indiana University Press. 1–13.
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.
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.
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. 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.
Most of its teeth were ovular 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 tooth crown up to long. In the mandible, 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.
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 evolution 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.
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 and . Tyrannosaurid brains were more similar to those of and other nonavian reptiles than they were to . The total brain volume for a long Tarbosaurus is estimated at only .
The large size of the , as well as the terminal nerve and , suggest that Tarbosaurus had a highly keen sense of smell, as was also the case with Tyrannosaurus. The vomeronasal bulb is large and differentiated from the olfactory bulb, which was initially suggested as being indicative of a well-developed Jacobsen's organ, which was used to detect . 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, 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 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.Gallagher W.B., Tumanova T.A., Dodson P., Axel L., 1998, "CT scanning Asian ankylosaurs: paleopathology in a Tarchia skull", Journal of Vertebrate Paleontology 18: 44A-45A
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 and 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 that the more rigid skull of Tarbosaurus was an adaptation to hunting the massive titanosaurid 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.
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 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 of the holotype specimen of the large ornithomimosaur Deinocheirus 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. 'In addition, a wound found on a Tarchia skull may have been inflicted by a Tarbosaurus. The ankylosaur would have survived the attack but eventually succumbed to a pathology in the healing process.
A 2020 study involving stable isotopes found that Tarbosaurus primarily hunted large dinosaurs in its environment, most notably titanosaurs and hadrosaurs.
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 and Djadochta Formations. However, 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 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, ( Borogovia, Tochisaurus, Zanabazar), ( Elmisaurus, Nemegtomaia, Rinchenia) or Bagaraatan, sometimes considered a basal tyrannosauroid. Other theropods, like the gigantic Therizinosaurus, might have been herbivore and , 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 , juveniles and subadult Tarbosaurus would have filled niches between the massive adults and these smaller theropods.
Readable material
Synonyms
Additional specimens
Poached specimens
Skin impressions and footprints
Description
Skull
Postcranial skeleton
Classification
Paleobiology
Ontogeny
Senses
Skull mechanics
Bite force and feeding
Paleoenvironment
Nemegt Formation
Subashi Formation
See also
External links
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