| Tarbosaurus | |
|---|---|
 | |
| Skeleton on exhibit inMaryland Science Center | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Superfamily: | †Tyrannosauroidea |
| Family: | †Tyrannosauridae |
| Subfamily: | †Tyrannosaurinae |
| Tribe: | †Tyrannosaurini |
| Genus: | †Tarbosaurus Maleev,1955b |
| Type species | |
| †Tyrannosaurus bataar Maleev, 1955a | |
| Synonyms | |
Genus synonymy Species synonymy
| |
Tarbosaurus (/ˌtɑːrbəˈsɔːrəs/TAR-bə-SOR-əs; meaning "alarming lizard") is agenus of largetyrannosauriddinosaur that lived inAsia during theMaastrichtianage of theLate Cretaceous epoch. It contains thetype and single speciesTarbosaurus bataar, which is known from theNemegt Formation ofMongolia, with more fragmentary remains found further afield in theSubashi Formation ofChina.Tarbosaurus is represented by dozens offossil specimens, including several complete skulls and skeletons. These remains have allowed studies focusing on itsphylogeny, skull mechanics, and brain structure. Further fossil remains have been reported from othergeologic formations of Asia, however, these remains are fragmentary and cannot be confidently assigned toTarbosaurus or the type species.
Like most known tyrannosaurids,Tarbosaurus was a largebipedalpredator, with the type specimen measuring approximately 10 metres (33 ft) long, 3 metres (9.8 ft) tall at the hips, and weighing up to 4.5–5 metric tons (4.4–4.9 long tons; 5.0–5.5 short tons). 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 manyspecies have been named, most modern paleontologists recognize only one species,T. bataar. Some experts see this species as an Asian representative of theNorth American genusTyrannosaurus, which would make the genusTarbosaurus redundant.Tarbosaurus andTyrannosaurus, if notsynonymous, are considered to be very closely related genera.Alioramus, also from Mongolia, has previously been thought by some authorities to be the closest relative ofTarbosaurus, though this has since been disproven with the discovery ofQianzhousaurus and the description of thetyrannosaurine tribeAlioramini.
Tarbosaurus lived in a humidfloodplain dominated by deserts, forests, and plains, and criss-crossed by river channels. It was anapex predator in this environment, preying on other large dinosaurs includingankylosaurids, such asTarchia andSaichania,hadrosaurids, such asSaurolophus andBarsboldia, andsauropods, such asNemegtosaurus andOpisthocoelicaudia.
In 1946, a jointSoviet-Mongolian expedition to theGobi Desert in theÖmnögovi Province turned up a large theropod skull and somevertebrae from theNemegt Formation. In 1955,Evgeny Maleev, a Sovietpaleontologist, made this specimen theholotype (PIN 551–1) of a new species, which he calledTyrannosaurus bataar.[1] The specific name is a misspelling of the Mongolian word баатар/baatar, meaning "hero".[2] 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 namedTarbosaurus efremovi, a new generic name composed of theAncient Greek wordsτάρβος (tarbos), meaning "terror", "alarm", "awe", or "reverence", andσαυρος (sauros), meaning "lizard",[3] and the species was named afterIvan Yefremov, a Russian paleontologist andscience fiction author. The other two, PIN 553-1 and PIN 552–2, were also named as new species and assigned to the North American genusGorgosaurus asG. lancinator andG. novojilovi, respectively. All three of these latter specimens are smaller than the first.[4]
A 1965 paper byA. K. Rozhdestvensky recognized all of Maleev's specimens as differentgrowth stages of the same species, which he believed to be distinct from the North AmericanTyrannosaurus. He created a new combination,Tarbosaurus bataar, to include all the specimens described in 1955 and newer material.[5] Later authors, including Maleev himself,[6] agreed with Rozhdestvensky's analysis, although some used the nameTarbosaurus efremovi instead ofT. bataar.[7]
American paleontologistKen Carpenter re-examined the material in 1992. He concluded that it belonged to the genusTyrannosaurus, as originally published by Maleev, and lumped all the specimens into the speciesTyrannosaurus bataar (except the remains that Maleev had namedGorgosaurus novojilovi). Carpenter thought this specimen represented a separate, smaller genus of tyrannosaurid, which he calledMaleevosaurus novojilovi.[8] George Olshevsky, Tracy Ford and Seiji Yamamoto created the new generic nameJenghizkhan (afterGenghis Khan) forTyrannosaurus bataar in 1995, while also recognizingTarbosaurus efremovi andMaleevosaurus novojilovi, for a total of three distinct, contemporaneous genera from the Nemegt Formation.[9] A 1999 study subsequently reclassifiedMaleevosaurus as a juvenileTarbosaurus.[10] Most research published since 1999 recognizes only a single species, which is either calledTarbosaurus bataar[11][12][13] orTyrannosaurus bataar,[14][15][16] although some scholars suspect that there could be a second species.[17]
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 ofTarbosaurus from the Nemegt Formation.[2] Expeditions involvingJapanese and Mongolian scientists between 1993 and 1998,[18] as well as private expeditions hosted byCanadian paleontologistPhil Currie around the turn of the 21st century, discovered and collected even moreTarbosaurus material.[19][20] More than 30 specimens are known, including more than 15 skulls and several complete postcranial skeletons.[11]
Chinese paleontologists discovered a partial skull and skeleton of a small theropod (IVPP V4878) in theXinjiang Autonomous Region of China in the mid-1960s. In 1977,Dong Zhiming described this specimen, which was recovered from theSubashi Formation inShanshan County, as a new genus and species,Shanshanosaurus huoyanshanensis.[21]Gregory S. Paul recognizedShanshanosaurus as a tyrannosaurid in 1988, referring it to the dubious genusAublysodon.[22] Dong and Currie later re-examined the specimen and deemed it to be a juvenile of a larger species oftyrannosaurine. These authors refrained from assigning it to any particular genus, but suggestedTarbosaurus as a possibility.[23] Subsequent authors have provisionally acceptedS. houyanshanensis as a synonym ofTarbosaurus bataar.[11][24]
Riabinin (1930b) described several fragments from theYuliangze Formation ofHeilongjiang, China asAlbertosaurus periculosus.[25][26] In the first edition ofThe Dinosauria, Molnar et al. (1990) listedA. periculosus as a provisional synonym ofTarbosaurus bataar, as followed by Holtz (2004) in the second edition.[27][11] Olshevsky, Ford, and Yamamoto (1995) tentatively referredA. periculosus toTarbosaurus, listing it asTarbosaurus?periculosus.[9] Alifanov and Bolotsky (2002) referred material from the neighboringUrduchukan Formation ofAmur, Russia, to this taxon, but Bolotsky (2015) later regardedA. periculosus as anomen dubium.[28][29]
Zhai et al. (1978) listed material from the Subashi Formation asTyrannosaurus turpanensis.[30] Since the specific name "turpanensis" was published in a faunal list only and lacks a description, it is anomen nudum according to ICZN Article 13.1.[31] Olshevsky (1991) listed T. "turpanensis" under the combinationTarbosaurus "turpanensis."[32] Several fragments from the Subashi Formation described by Dong (1977) asTarbosaurus sp. may represent this taxon.[21][33][34] As withAlbertosaurus periculosus, Molnar et al. (1990) listedT. "turpanensis" as a provisional synonym ofTarbosaurus bataar without comment, as did Holtz (2004).[27][11]
Dong (1979) described five teeth (IVPP V4733) discovered in theQuipa Formation ofHenan, China as a new species,Tyrannosaurus luanchuanensis.[35] Olshevsky (1991) listed it asTarbosaurus luanchuanensis, and later in 1995, with Ford and Yamamoto, asJenghizkhan luanchuanensis.[32][9] Holtz (2004) listed it as a provisional synonym ofTarbosaurus bataar.[11]
Along withAlbertosaurus periculosus,Tyrannosaurus luanchuanensis, andTyrannosaurus "turpanensis,"Chingkankousaurus fragilis was considered a provisional junior synonym ofTarbosaurus bataar by Holtz (2004), but it has since been assessed as dubious by Brusatte et al. (2013).[11][36]
Named in 1976 bySergei Kurzanov,Alioramus is another genus of tyrannosaurid from slightly older sediments in Mongolia.[37] Several analyses have concludedAlioramus was quite closely related toTarbosaurus.[2][12] It was described as an adult, but its long, low skull is characteristic of a juvenile tyrannosaurid. This led Currie to speculate thatAlioramus might represent a juvenileTarbosaurus, but he noted that the much higher tooth count and row of crests on top of the snout suggested otherwise.[38]
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 toTarbosaurus sp. given some similarities in tooth morphology as well as the general geographic proximity to the Nemegt Formation.[35] In 1993 Tomasz Jerzykiewiczz examined the general outcrops from theBayan Mandahu Formation of China. In this paper, he reported isolated tyrannosaurid premaxillary and maxillary teeth, which he assigned toTarbosaurus without anatomical comparisons.[39] Lev A. Nessov in 1995 reported a partial femur from theBostobe Formation ofKazakhstan and referred it asTarbosaurus sp. without any diagnosis.[40]
In 2004, David B. Weishampel and team listed a dubiousTarbosaurus? sp. as a component of the known dinosaur taxa of theCampanian-agedDjadokhta Formation without further ado.[41] 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 toTarbosaurus or evenT. bataar.[42][39][43][44] 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.[45]
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 ofTarbosaurus 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 toTarbosaurus bataar, cataloged as MPC-D 107/7 within the collections of theMongolian Paleontological Center.[46]
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.[47] 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 theGobi Desert was to rest at an appropriate Mongolian institution and there was little reasonable doubt that theTarbosaurus 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.[48] During the court case (United States v. OneTyrannosaurus 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 onSukhbaatar Square, the center of the city of Ulaanbaatar.[49] Prokopi had sold the dinosaur with a partner and fellow commercial hunter in England named Christopher Moore.[50] The case led to the repatriation of dozens of Mongolian dinosaurs, including several skeletons ofTarbosaurus bataar.[51]
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 2.4 millimeters (0.094 in) 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.[52]
Phil Currie and colleagues (2003) described two footprints from the Nemegt locality that probably pertain toTarbosaurus. 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 61 centimeters (24 in) in length, thus representing a large individual. The second track, although even larger, was affected byerosion and does not show any detail.[52]
In 1997, Ken Carpenter reported a damagedTarbosaurus skull with impressions of a dewlap or throat pouch beneath the lower jaws, based on a personal communication from Konstantin Mikhailov.[53] Carpenter speculated that the pouch may have been used for display, possibly being brightly colored and inflatable like afrigatebird's.[54] 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 bySergei Kurzanov and that it was Kurzanov himself who had originally interpreted the impressions as a throat structure.[55] This specimen may be the same as one that was purportedly destroyed by poachers in 1992.[56]
Although slightly smaller thanTyrannosaurus,Tarbosaurus was one of the largest tyrannosaurines, with the type specimen PIN 551–1 measuring approximately 10 metres (33 ft) long, 3 metres (9.8 ft) tall at the hips, and weighing up to 4.5–5 metric tons (5.0–5.5 short tons).[57][33][58] Some adult specimens have been estimated at more than 12 metres (39 ft) long.[59] Other adult specimens were smaller, as MPC-D 107/2, ZPAL.MgD-I/4 and PIN 552-1 likely weighed around 2.2 metric tons (2.4 short tons) to 3.4 metric tons (3.7 short tons).[60][61] The largest knownTarbosaurus skull is about 1.35 m (4.4 ft) long,[62] which is larger than that of all other tyrannosaurids, aside fromTyrannosaurus.[11]
The skull was tall, like that ofTyrannosaurus, but not as wide, especially towards the rear. The unexpanded rear of the skull meant thatTarbosaurus's eyes did not face directly forwards, suggesting that it lacked thebinocular vision ofTyrannosaurus. Largefenestrae 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 ofTyrannosaurus, but fewer than in smaller tyrannosaurids, likeGorgosaurus andAlioramus.[2]
Most of its teeth were ovular incross section, although the teeth of thepremaxilla at the tip of the upper jaw had aD-shaped cross section. However, thisheterodonty is characteristic of the family. The longest teeth were in themaxilla (upper jaw bone), withcrowns up to 85 millimeters (3.3 in) long. In thelower jaw, a ridge on the outer surface of theangular bone articulated with the rear of thedentary bone, creating a locking mechanism unique toTarbosaurus andAlioramus. Other tyrannosaurids lacked this ridge and had more flexibility in the lower jaw.[2]
Tyrannosaurids varied little in overall body form andTarbosaurus was no exception. The head was supported by anS-shaped neck, while the rest of thevertebral 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 thatTarbosaurus also has a theropod reduction of fingers IV-I "developed further" than in other tyrannosaurids,[63] as the second metacarpal in theTarbosaurus 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 inTarbosaurus is proportionally shorter than in other tyrannosaurids. In other tyrannosaurids, likeAlbertosaurus andDaspletosaurus, the third metacarpal is often longer than the first metacarpal, while in theTarbosaurus specimens studied by Holtz, the third metacarpal is shorter than the first.[11] 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 acounterweight to the head and torso, while also placing thecenter of gravity directly over the hips.[4]
Tarbosaurus is classified as atheropod in the subfamily Tyrannosaurinae of the familyTyrannosauridae. Other members includeTyrannosaurus and the earlierDaspletosaurus, both from North America,[14] and possibly the Mongolian genusAlioramus.[2][12] Animals in this subfamily are more closely related toTyrannosaurus than toAlbertosaurus and are known for their robust build with proportionally larger skulls and longerfemurs than in the other subfamily, Albertosaurinae.[11]
Tarbosaurus bataar was originally described as a species ofTyrannosaurus,[1] an arrangement that has been supported by some more recent studies.[14][8] Others prefer to keep the genera separate, while still recognizing them assister taxa.[11] A 2003cladistic analysis based on skull features instead identifiedAlioramus as the closest known relative ofTarbosaurus, 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 againstTarbosaurus being a synonym ofTyrannosaurus and would suggest that separate tyrannosaurine lineagesevolved in Asia and North America.[2][12] The two known specimens ofAlioramus, which show juvenile characteristics, are not likely juvenile individuals ofTarbosaurus because of their much higher tooth count (76 to 78 teeth) and their unique row of bony bumps along the top of their snouts.[38]
The discovery ofLythronax argestes, a much earlier tyrannosaurine, further reveals the close relationship betweenTyrannosaurus andTarbosaurus. It was also discovered thatLythronax is a sister taxon to a clade consisting of Campanian genusZhuchengtyrannus, and the Maastrichtian generaTyrannosaurus andTarbosaurus. Further studies ofLythronax also suggest that the Asian tyrannosauroids were part of one evolutionary radiation.[64]
Below is the cladogram of Tyrannosaurinae based on thephylogenetic analysis conducted by Voris and team in 2020.[65]
Most specimens ofTarbosaurus 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, 290-millimeter (0.95 ft) 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 juvenileTarbosaurus suggests they may also have been crepuscular or nocturnal hunters. Whether or not the adultTarbosaurus were also nocturnal is currently unknown due to the lack of fossil evidence to suggest so.[46] Ontogenetic changes identified in thefrontal bones suggests that the allometric growth ofTarbosaurus was similar to that of North American tyrannosaurids.[66]
ATarbosaurus skull found in 1948 by Soviet and Mongolian scientists (PIN 553–1, originally calledGorgosaurus lancinator) included the skull cavity that held the brain. Making a plastercast, called anendocast, of the inside of this cavity allowed Maleev to make preliminary observations about the shape of aTarbosaurus brain.[67] A newerpolyurethane rubber cast allowed a more detailed study ofTarbosaurus brain structure and function.[68]
The endocranial structure ofTarbosaurus was similar to that ofTyrannosaurus,[69] differing only in the positions of somecranial nerve roots, including thetrigeminal andaccessory nerves. Tyrannosaurid brains were more similar to those ofcrocodilians and other nonavian reptiles than they were tobirds. The total brain volume for a 12 meters (39 ft) longTarbosaurus is estimated at only 184 cubic centimeters (11.2 cu in).[68]
The large size of theolfactory bulbs, as well as theterminal andolfactory nerves, suggest thatTarbosaurus had a highly keensense of smell, as was also the case withTyrannosaurus. Thevomeronasal bulb is large and differentiated from the olfactory bulb, which was initially suggested as being indicative of a well-developedJacobsen's organ, which was used to detectpheromones. This may imply thatTarbosaurus had complex mating behavior.[68] However, the identification of the vomeronasal bulb has been challenged by other researchers because they are not present in any living archosaurs.[70]
Theauditory nerve was also large, suggesting goodhearing, which may have been useful for auditorycommunication and spatial awareness. The nerve had a well-developedvestibular component as well, which implies a good sense of balance and coordination. In contrast, the nerves and brain structures associated witheyesight were smaller and undeveloped. Themidbrain tectum, responsible for visual processing in reptiles, was very small inTarbosaurus, as were theoptic nerve and theoculomotor nerve, which controls eye movement. UnlikeTyrannosaurus, 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 thatTarbosaurus relied more on its senses of smell and hearing than on its eyesight.[68] It has been suggested that the lack of binocular vision in Asian tyrannosaurs, likeTarbosaurus, 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.[71] However, this is contradicted by numerous lines of evidence indicatingTarbosaurus was actively preying on hadrosaurs, titanosaur sauropods, and other large bodied herbivores in its ecosystem.[72][73][74][75]
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The skull ofTarbosaurus was completely described for the first time in 2003. Scientists noted key differences betweenTarbosaurus 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 fusednasal bones on top of the snout, which were firmly connected in the rear to thelacrimal bones by bony struts. These struts locked the two bones together, suggesting that force was then transmitted from the nasals to the lacrimals.[2]
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 inTarbosaurus 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 inTarbosaurus. The lacrimal was also more firmly anchored to thefrontal andprefrontal bones inTarbosaurus. The well-developed connections between the maxilla, lacrimal, frontal, and prefrontal would have made its entire upper jaw much more rigid.[2]
Another major difference betweenTarbosaurus 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.[2]
Some scientists havehypothesized that the more rigid skull ofTarbosaurus was an adaptation to hunting the massivetitanosauridsauropods 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 inAlioramus from Mongolia, suggesting that it is the closest relative ofTarbosaurus. Similarities betweenTarbosaurus andTyrannosaurus might be related to their large size, independently developed throughconvergent evolution.[2]
In 2001,Bruce Rothschild and others published a study examining evidence forstress fractures andtendon avulsions intheropod 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 eighteenTarbosaurus 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.[76]
As for its bite force, it was revealed in 2005 thatTarbosaurus 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.[77]
David W. E. Hone and Mahito Watabe in 2011 reported the left humerus of a nearly completeSaurolophus skeleton (MPC-D 100/764) from the Bügiin Tsav locality of the Nemegt Formation, which washeavily damaged from bite marks attributed toTarbosaurus. As suggested by the lack of damage to the rest of the skeleton (such as large wounds in skeletal remains indicative ofpredation), this tyrannosaurid was likelyscavenging an already deadSaurolophus. It is unlikely that a large-bodied predator, such asTarbosaurus, 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 thisTarbosaurus was actively selecting which biting style to employ so it could scavenge the bone.[78]
In 2012, bite marks on two fragmentarygastralia of the holotype specimen of the large ornithomimosaurDeinocheirus mirificus were reported. The size and shape of the bite marks match the teeth ofTarbosaurus, 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.[73] In addition, a wound found on aTarchia skull may have been inflicted by aTarbosaurus. The ankylosaur would have survived the attack but eventually succumbed to a pathology in the healing process.[75]
A 2020 study involving stable isotopes found thatTarbosaurus primarily hunted large dinosaurs in its environment, most notably titanosaurs and hadrosaurs.[72]
The vast majority of knownTarbosaurus fossils were recovered from theNemegt Formation in theGobi Desert of southern Mongolia. Thisgeologic formation has never beendated radiometrically, but thefauna present in the fossil record indicate it was probably deposited during the earlyMaastrichtian stage at the near end of theLate Cretaceous,[79] about 70 million years ago.[80][81]
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 underlyingBarun Goyot andDjadochta Formations. However,caliche deposits indicate at least periodic droughts. Sediment was deposited in the channels and floodplains of large rivers. The rockfacies 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.[82]
The area would have been semi-arid during certain times of the year.[72] The environment was likely dominated byAraucarian conifer forests,[72] which also containedginkgos, reed grasses,fagalean trees, cycad-like plants,sycamores (plane trees),bald cypresses, katsura relatives,pondweeds, tupelos, duckweeds, lotuses, and sedges.[83]
Occasionalmollusk fossils are found, as well as a variety of other aquatic animals, such as fish and turtles.[79] Crocodilians included several species ofParalligator, a genus with teeth adapted for crushing shells.[84]Mammal fossils are exceedingly rare in the Nemegt Formation, but many birds have been found, including theenantiornithineGurilynia and thehesperornithiformJudinornis, as well asTeviornis, an early representative of the still-existingAnseriformes. Scientists have described many dinosaurs from the Nemegt Formation, including theankylosauridsTarchia andSaichania and thepachycephalosaurPrenocephale.[79]
By far the largest predator known from the formation, adultTarbosaurus most likely preyed upon large hadrosaurs, such asSaurolophus andBarsboldia, or sauropods, such asNemegtosaurus andOpisthocoelicaudia.[2] Adults would have received little competition from small theropods, such as the smalltyrannosauridAlioramus,troodontids (Borogovia,Tochisaurus,Zanabazar),oviraptorosaurs (Elmisaurus,Nemegtomaia,Rinchenia) orBagaraatan, sometimes considered abasaltyrannosauroid. Other theropods, like the giganticTherizinosaurus, might have beenherbivorous andornithomimosaurs, such asAnserimimus,Gallimimus, and giganticDeinocheirus might have been omnivores that only took small prey and were therefore no competition forTarbosaurus. However, as in other large tyrannosaurids, as well as modernKomodo dragons, juveniles and subadultTarbosaurus would have filled niches between the massive adults and these smaller theropods.[11]
TheSubashi Formation, in whichShanshanosaurus remains were discovered, dates to both the Campanian and Maastrichtian stages.[85][86]
Winter 2011 Appendix
Images and photos
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