Tarbosaurus (from Latin for “astonishing lizard”) is a genus of tyrannosurid dinosaur that flourished in Asia about 70 million years ago, at the end of the Late Cretaceous period, thought to contain a single known species, Tarbosaurus bataar. Fossils have been recovered in Mongolia, with more fragmentary remains found farther away in parts of China.
| Tarbosaurus | |
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| Skeleton on display at the Maryland Science Center | |
Scientific classification |
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Family: | †Tyrannosauridae |
| Subfamily: | †Tyrannosaurinae |
| Gender: | †Tarbosaurus Maleev, 1955 |
| Type species | |
| †Tyrannosaurus bataar Maleev, 1955
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| Species | |
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| Synonyms | |
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Due to the close similarities between Tarbosaurus and Tyrannosaurus, it was originally called Tyrannosaurus bataar, and some scientists still consider this name preferable to Tarbosaurus bataar, but others advocate that the species be kept in separate genera because of geographical separation: T. rex lived in North America, while T. bataar lived in Asia, and in the Cretaceous period the continents were separated.
Like most known tyrannosaurids, Tarbosaurus was a large bipedal predator, measuring approximately 10 meters in length, 3 meters in height from the hip, and weighing up to 5 tons. It had a unique locking mechanism in its jaw, equipped with about sixty large teeth and the smallest forelimbs relative to the body size of all tyrannosaurids, famous for their disproportionately small, two-toed forelimbs.
Most of the Tarbosaurus specimens have been found in the Nemegt Formation. In its environment, it reigned as a superpredator, coexisting with ankylosaurids such as Saichania, sauropods such as Nemegtosaurus, and hadrosaurids such as Saurolophus, which were possibly its prey. The younger individuals probably competed with smaller predators such as Alioramus and Bagaraatan. It also coexisted with more distinctive theropods such as Therizinosaurus and Deinocheirus, as well as ornithomimosaurs such as Anserimimus and Gallimimus.
In 1946, a joint Soviet-Mongolian expedition to the Gobi Desert in the Mongolian province of Ömnögovi discovered a large theropod skull and some vertebrae in 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 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 termed Tarbosaurus efremovi, a new generic name composed of the Ancient Greek τάρβος (tarbus) (“terror”, “alarm”, “awe” or “reverence”) and σαυρος (saurians) (“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 of the last three 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 different from the North American tyrannosaurus. He created a new combination, Tarbosaurus bataar, to include all of the specimens described in 1955, as well as more recent material. Later authors, including Maleev himself, agreed with Rozhdestvensky’s analysis, although some used the name Tarbosaurus efremovi instead of 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 grouped all specimens in the species Tyrannosaurus bataar, except for the remains that Maleev had named “Gorgosaurus” novojilovi. Carpenter thought that this specimen represented a smaller, separate genus of tyrannosaurid, which he named 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 contemporary genera of the Nemegt Formation. A 1999 study later reclassified Maleevosaurus as a juvenile Tarbosaurus. All the research published since 1999 recognizes only a single species, which is called Tarbosaurus bataar or Tyrannosaurus bataar.
Following the original Russian-Mongol expeditions in the 1940s, joint Polish-Mongolian 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 organized by Canadian paleontologist Phil Currie at the turn of the 21st century, discovered and collected more material from Tarbosaurus. More than 30 specimens are known, including more than 15 skulls and several complete postcranial skeletons.
Smuggled specimens
Main article: United States v. One Tyrannosaurus Bataar Skeleton_officials_return_a_Tarbosaurus_bataar_skeleton_to_the_government_of_Mongolia_during_a_repatriation_ceremony_May_6,_2013,_at_a_Manhattan_hotel_in_New_York_130506-H-ZZ999-002.jpg)
Tarbosaurus fossils are found only in the Gobi Desert, Mongolia, and China, both of which prohibit their export, although some specimens have been looted by private collectors. A $1 million smuggling deal was uncovered when suspicions arose about a catalog released by Heritage Auctions for an event in New York City on May 20, 2012. By Mongol law, any specimen found in the Gobi Desert was to rest in an appropriate Mongol institution and there was little reasonable doubt that the Tarbosaurus bataar advertised in the catalogue was stolen. Mongolia’s president and many paleontologists raised objections to the sale, which led to a last-minute investigation that confirmed it was a specimen that can only be found in the Gobi Desert, which rightfully belongs 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 on display in Sukhbaatar Square, Ulaanbaatar city center. Prokopi sold the dinosaur with a partner and fellow commercial hunter in England, Christopher Moore. The case led to the repatriation of dozens more Mongolian dinosaurs, including several skeletons of Tarbosaurus bataar as well as generating a discussion in the American media about the effect of “dinosaur smuggling”. An emblematic case involved actor Nicolas Cage, who in 2015, returned a Tarbosaurus skull he had bought in 2007. The skull was purchased from a gallery that has sold fossils smuggled by Prokopi.
Synonyms
Chinese paleontologists discovered a partial skull and skeleton of a small theropod (IVPP V4878) in China’s Xinjiang Autonomous Region in the mid-1960s. In 1977, Dong Zhiming described this specimen, which was recovered from the Subashino Formation Shanshan County, as a new genus and species, Shanshanosaurus huoyanshanensis. Gregory S. Paul recognized Shanshanosaurus as a tyrannosaurid in 1988, referring it to the now-extinct genus Aublysodon. Dong and Currie later re-examined the specimen and found it to be a juvenile of a larger species of Tyrannosaurus. These authors refrained from attributing it to any particular genus, but suggested Tarbosaurus as a possibility.
Albertosaurus periculosus, Tyrannosaurus luanchuanensis, Tyrannosaurus turpanensis, and Chingkankousaurus fragilis were considered synonyms of Tarbosaurus in the second edition of Dinosauria, but Chingkankousaurus was assessed as doubtful by Brusatte et al. (2013).
Named in 1976 by Sergei Kurzanov, Alioramus is another genus of tyrannosaurid from slightly older Mongolian sediments. Several analyses concluded that Alioramus was closely related to Tarbosaurus. It has been 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 larger tooth count and row of crests at the top of the snout suggested otherwise.
Description
Although slightly smaller than Tyrannosaurus, Tarbosaurus was one of the largest tyrannosaurids. The largest known individuals were between 10 and 12 meters long. The mass of a fully grown Tarbosaurus is considered comparable to or slightly smaller than its North American relative, often estimated to be around 4-5 tons.
Skull
The largest known Tarbosaurus skull is over 1.3 m long, larger than all other tyrannosaurids except Tyrannosaurus. The skull was tall, like that of its American relative, but not as wide, especially at the rear. The unexpanded rear of the skull meant that the Tarbosaurus‘ eyes were not facing directly forward, suggesting that it lacked the binocular vision of the Tyrannosaurus. Large fenestrations (openings) in the skull reduced his weight. Between 58 and 64 teeth lined its jaws, slightly more than in Tyrannosaurus, but fewer than in smaller tyrannosaurids such as Gorgosaurus and Alioramus. Most of its teeth were oval in cross-section, although the premaxillary teeth at the tip of the upper jaw had a D-shaped cross-section. The longest teeth were in the maxilla (upper jaw bone), with crowns up to 85 millimeters long. In the lower jaw, a ridge on the outer surface of the angular bone articulates with the back of the dental bone, creating a locking mechanism unique to Tarbosaurus and Alioramus. Other tyrannosaurids did not have this crest and had more flexibility in the lower jaw.
Postcranial skeleton
Tyrannosaurids varied little in body shape, and Tarbosaurus was no exception. The head was supported by an S-shaped neck, while the rest of the spine, including the long tail, was held horizontally. Tarbosaurus had tiny forelimbs, proportional to body size, the smallest of all family members. The hands had two clawed fingers each, with a third clawless metacarpal found in some specimens, similar to closely related genera. Thomas Holtz suggested that Tarbosaurus also has a “more developed” IV-I finger theropod reduction than in other tyrannosaurids, since the second metacarpal in the Tarbosaurus specimens he studied is less than twice the length of the first metacarpal (other tyrannosaurs have a second metacarpal about twice the length of the first metacarpal). In addition, the third metacarpal in Tarbosaurus is proportionally shorter than in other tyrannosaurids; in other tyrannosaurids (such as 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 forelimbs, the three-toed hind limbs 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
Tarbosaurus is classified as a theropod in the subfamily Tyrannosaurinae within the family Tyrannosauridae. Other members include Tyrannosaurus and Daspletosaurus anterior, 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 proportionately 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 identified Alioramus as the closest known relative of Tarbosaurus, as the two genera share skull features related to stress distribution that are not found in other tyrannosaurs. If proven, this relationship would argue against Tarbosaurus becoming a synonym for Tyrannosaurus and suggest that separate lineages of Tyrannosaurus evolved in Asia and North America. The two known Alioramus specimens, which show juvenile features, are probably not juvenile individuals of Tarbosaurus because of their much higher tooth count (76 to 78 teeth) and their unique row of bony protuberances along the top of their snouts.
The discovery of Lythronax argestes, a much earlier tyrannosaurus, further reveals the close relationship between Tyrannosaurus and Tarbosaurus, and Lythronax was found to be a sister taxon to a clade composed of the Campanian genus Zhuchengtyrannus and the Maastrichtian genera Tyrannosaurus and Tarbosaurus. Other studies of Lythronax also suggest that Asian tyrannosaurs were part of an evolutionary radiation.
Below is the cladogram of Tyrannosauridae based on the phylogenetic analysis performed by Loewen et al. in 2013.
Paleobiology
Ontogeny
Most specimens of Tarbosaurus represent adult or subadult individuals; Juveniles remain very rare. However, the discovery in 2006 of a juvenile individual (MPC-D 107/7) including a complete 290-millimeter skull was reported and described in 2011 and provides insight into the life history of this dinosaur. This individual was likely 2 to 3 years old at the time of death. Compared with adult skulls, the juvenile skull was of weak construction and the teeth were thin, indicating different dietary preferences in juveniles and adults that reduced competition between different age groups. Examination of the sclerotic rings in this juvenile specimen suggests that they may also have been crepuscular or nocturnal hunters. Whether the adult limbs were also nocturnal is currently unknown due to a lack of fossil evidence.
Senses
A Tarbosaurus skull found in 1948 by Soviet and Mongolian scientists (PIN 553-1, originally called “Gorgosaurus” lancinator) included the skull cavity that contained the brain. By making a plaster cast of the inside of this cavity, Maleev was able to make preliminary observations about the shape of a Tarbosaurus‘ brain. A new polyurethane rubber mold allowed for a more detailed study of the structure and function of the animal’s brain.
The endocranial structure of Tarbosaurus was similar to that of Tyrannosaurus, differing only in the positions of some cranial nerve roots, including the trigeminal and accessory nerves. The brains of tyrannosaurids were more similar to those of crocodilians and other non-avian reptiles than to those of birds. The total brain volume for a 12-meter Tarbosaurus is estimated to be just 184 cubic centimeters.
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, which was initially suggested as indicative of a well-developed Jacobson’s organ used to detect pheromones. This may imply that Tarbosaurus had complex mating behavior. However, the identification of the vomeronasal bulb has been disputed by other researchers, since they are not present in any living archosaurs.
The auditory nerve was also large, suggesting good hearing, which may have been helpful for auditory communication and spatial awareness. The nerve also had a well-developed vestibular component, implying a good sense of balance and coordination. In contrast, the nerves and brain structures associated with vision were smaller and poorly developed. The midbrain teat, which is 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 tyrannosaurs, in which the eyes were mostly sideways. All of this suggests that Tarbosaurus was a more olfactory and auditory animal in its perception of reality than a visual being.
Skull Mechanics
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The skull of Tarbosaurus was fully described for the first time in 2003. Scientists have observed important differences between this and North American tyrannosaurids. Many of these differences are related to the handling of stress by the bones of the skull during a bite. When the upper jaw bit an object, the force was transmitted through the maxilla, the main bone of the upper jaw, to the surrounding bones of the skull. In North American tyrannosaurids, this force ran from the maxilla to the fused nasal bones at the top of the snout, which were firmly connected at the back to the tear bones by bony supports. These supports locked the two bones together, suggesting that force was then transmitted from the nasals to the lacrimals.
Tarbosaurus lacked these bone supports, and the connection between the nasals and lacrimal was weak. Instead, a backward projection of the maxilla was massively developed in the Tarbosaurus and fitted within a sheath formed from the lacrimal. This projection was a thin bony plate in North American tyrannosaurids. The large backward projection suggests that the force was transmitted more directly from the maxilla to the lacrimal in the Tarbosaurus. The lacrimal was also more firmly anchored to the frontal and prefrontal bones in the Tarbosaurus. The well-developed connections between the maxilla, lacrimal, frontal, and prefrontal would have made the entire upper jaw more rigid.
Another major difference between Tarbosaurus and its North American relatives was its stiffer jaw (lower jaw). While many theropods, including North American tyrannosaurids, had some degree of flexibility between the bones at the back of the jaw and the dent at the front, Tarbosaurus had a locking mechanism formed from a ridge on the surface of the angle, which articulated with a square process at the back of the dentary.
Some scientists have hypothesized that the stiffer skull of Tarbosaurus was an adaptation for hunting the massive sauropod titanosaurids found in the Nemegt Formation, which did not exist in most of North America during the Late Cretaceous. Differences in skull mechanics also affect the phylogeny of tyrannosaurids. Tarbosaurus-like joints between the skull bones are also seen in Mongolian Alioramus, suggesting that it, and not Tyrannosaurus, is the closest relative of Tarbosaurus. The similarities between Tarbosaurus and Tyrannosaurus may therefore be related to their large size, developed independently through convergent evolution.
Bite force and feeding
In 2001, Bruce Rothschild and others published a study examining evidence of stress fractures and tendon avulsions in theropod dinosaurs and the implications for their behavior. Because stress fractures are caused by repeated trauma rather than singular events, they are more likely to be caused by regular behavior than by other types of injuries. None of the eighteen foot bones of Tarbosaurus examined in the study had a stress fracture, but one of the ten hand bones examined had one. Stress fractures in the hands have a special behavioral significance compared to those found in the feet, as stress fractures can be obtained while running or during migration. Hand injuries, on the other hand, are more likely to be obtained during contact with fighting prey. The presence of stress fractures and tendon avulsions, in general, provide evidence of a diet based on “very active” predation rather than an obligatory necrophagy.
As for its bite force, it was revealed in 2005 that Tarbosaurus had a bite force of about 8,000 to 10,000 pounds per force, meaning it could crush bones 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 heavily damaged by 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 eliminating an already dead Saurolophus. It is unlikely that a large predator, such as Tarbosaurus, would have left sparse feeding trails on a single humerus while having an entire carcass to feed on. The humerus shows three distinct methods of feeding, interpreted as punches, drag marks, and bite and drag marks. Hone and Watabe noticed that the bite marks were primarily located on the deltopectoral crest, suggesting that this Tarbosaurus was actively selecting which bite style to employ to clean the bone.
In 2012, bite marks were reported on two fragmentary gastralia of the holotype specimen of the large ornithomimaur Deinocheirus mirificus. The size and shape of the bite marks correspond to the teeth of Tarbosaurus, the largest known predator of the Nemegt Formation. Several types of feeding traits have been identified; perforations, grooves, striations, fragmentary teeth, and combinations of the above marks. The bite marks likely represent feeding behavior rather than aggression between species, and the fact that bite marks were not found elsewhere on the body indicates that the predator focused on internal organs. Bite marks of Tarbosaurus have also been identified in 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 hunted mostly large dinosaurs in its environment, most notably titanosaurs and hadrosaurs.
Paleoenvironment
The vast majority of known Tarbosaurus fossils have been recovered from the Nemegt Formation in the Gobi Desert in southern Mongolia. This geological formation has never been radiometrically dated, but the fauna present in the fossil record indicates that it was probably deposited during the early Maastrichtian, at the end of the Late Cretaceous about 70 million years ago. The Subashi Formation, in which the remains of Shanshanosaurus were discovered, is also of Maastrichtian age.
Tarbosaurus is found primarily in the Nemegt Formation, whose sediments preserve large river channels and soil deposits that indicate a much wetter climate than those suggested by the underlying Barun Goyot and Djadochta Formations. However, caliche deposits indicate at least periodic droughts. The sediments were deposited in the channels and floodplains of large rivers. The rocky facies of this formation suggest the presence of mudflats and shallow lakes. The sediments also indicate that a rich habitat existed, offering diverse food in abundant quantities that could sustain massive Cretaceous dinosaurs. Fossils of an unidentified tyrannosaurus from the ancient Djadochta Formation, which resemble those of the Tarbosaurus, may indicate that it also lived at an earlier time and in a more arid ecosystem than the Nemegt.
Fossils of occasional mollusks are found, as are a variety of other aquatic animals such as fish and turtles. Crocodilians included several species of Paralligator, a genus with teeth adapted to crush shells. Mammal fossils are extremely rare in the Nemegt Formation, but many birds have been found, including the enantiornithine Gurilynia and the hesperornithiform Judinornis, as well as Teviornis, one of the earliest representatives of the still-extant Anseriformes (waterfowl). Scientists have described many dinosaurs from the Nemegt Formation, including theropods such as the tyrannosaurid Alioramus, the tyrannosauroid Bagaraatan, troodontids (Borogovia, Tochisaurus, Zanabazar), oviraptorosaurs such as Elmisaurus, Nemegtomaia and Rinchenia). Among the herbivores, which Tarbosaurus probably preyed on, were the ankylosaurid Saichania, the pachycephalosaurus Prenocephale, the large hadrosaurs Saurolophus and Barsboldia and the sauropods Nemegtosaurus and Opisthocoelicaudia. Other theropods, such as the gigantic Therizinosaurus, may have been herbivores, and ornithomimosaurs, such as Anserimimus, Gallimimus, and the gigantic Deinocheirus, may have been omnivores that only caught small prey and therefore did not compete directly with Tarbosaurus. However, as in other large tyrannosaurids, as well as modern Komodo dragons, juveniles and subadults Tarbosaurus would have filled niches between the massive adults and these smaller theropods.