Like other tyrannosaurids,Tyrannosaurus was abipedalcarnivore with a massive skull balanced by a long, heavy tail. Relative to its large and powerful hind limbs, the forelimbs ofTyrannosaurus were short but unusually powerful for their size, and they had two clawed digits. The most complete specimen measures 12.3–12.4 m (40–41 ft) in length, but according to most modern estimates,Tyrannosaurus could have exceeded sizes of 13 m (43 ft) in length, 3.7–4 m (12–13 ft) in hip height, and 8.8 t (8.7 long tons; 9.7 short tons) in mass. Although some other theropods might have rivaled or exceededTyrannosaurus insize, it is still among the largest known land predators, with its estimated bite force being the largest among all terrestrial animals. By far the largest carnivore in its environment,Tyrannosaurus rex was most likely anapex predator, preying uponhadrosaurs, juvenile armored herbivores likeceratopsians andankylosaurs, and possiblysauropods. Some experts have suggested the dinosaur was primarily ascavenger. The question of whetherTyrannosaurus was an apex predator or a pure scavenger was among the longest debates inpaleontology. Most paleontologists today accept thatTyrannosaurus was both a predator and a scavenger.
Somespecimens ofTyrannosaurus rex are nearly complete skeletons.Soft tissue andproteins have been reported in at least one of these specimens. The abundance of fossil material has allowed significant research into many aspects of the animal's biology, including its life history andbiomechanics. The feeding habits,physiology, and potential speed ofTyrannosaurus rex are a few subjects of debate. Itstaxonomy is also controversial, as some scientists considerTarbosaurus bataar from Asia to be a thirdTyrannosaurus species, while others maintainTarbosaurus is a separate genus. Several other genera of North American tyrannosaurids have also beensynonymized withTyrannosaurus. At present, two species ofTyrannosaurus are considered valid: the type species,T. rex, and the earlier in age and more recently discoveredT. mcraeensis.
As the archetypal theropod,Tyrannosaurus has been one of the best-known dinosaurs since the early 20th century and has been featured in film, advertising, postal stamps, and many other media.
In 1892,Edward Drinker Cope found two vertebral fragments of a large dinosaur. Cope believed the fragments belonged to an "agathaumid" (ceratopsid) dinosaur, and named themManospondylus gigas, meaning "giant porous vertebra", in reference to the numerous openings for blood vessels he found in the bone.[2] TheM. gigas remains were, in 1907, identified by Hatcher as those of a theropod rather than a ceratopsid.[3]
Henry Fairfield Osborn recognized the similarity betweenManospondylus gigas andT. rex as early as 1917, by which time the second vertebra had been lost. Owing to the fragmentary nature of theManospondylus vertebrae, Osborn did not synonymize the two genera, instead considering the older genus indeterminate.[4] In June 2000, theBlack Hills Institute found around 10% of aTyrannosaurus skeleton (BHI 6248) at a site that might have been the originalM. gigas locality.[5]
Skeleton discovery and naming
Outdated skeletal restoration byWilliam D. Matthew from 1905, published alongside Osborn's description paper
Barnum Brown, assistant curator of theAmerican Museum of Natural History, found the first partial skeleton ofT. rex in eastern Wyoming in 1900. Brown found another partial skeleton in theHell Creek Formation in Montana in 1902, comprising approximately 34 fossilized bones.[6] Writing at the time Brown said "Quarry No. 1 contains the femur, pubes, humerus, three vertebrae and two undetermined bones of a large Carnivorous Dinosaur not described byMarsh. ... I have never seen anything like it from theCretaceous."[7]Henry Fairfield Osborn, president of theAmerican Museum of Natural History, named the second skeletonT. rex in 1905. The generic name is derived from theGreek wordsτύραννος (tyrannos, meaning "tyrant") andσαῦρος (sauros, meaning "lizard"). Osborn used theLatin wordrex, meaning "king", for the specific name. The fullbinomial therefore translates to "tyrant lizard the king" or "King Tyrant Lizard", emphasizing the animal's size and presumed dominance over other species of the time.[6]
Osborn named the other specimenDynamosaurus imperiosus in a paper in 1905.[6] In 1906, Osborn recognized that the two skeletons were from the same species and selectedTyrannosaurus as the preferred name.[8] In 1941, theT. rex type specimen was sold to theCarnegie Museum of Natural History in Pittsburgh, Pennsylvania, for $7,000.[7] The originalDynamosaurus material now resides in the collections of theNatural History Museum, London.[9]Dynamosaurus would later be honored by the 2018 description of another species of tyrannosaurid by Andrew McDonald and colleagues,Dynamoterror dynastes, whose name was chosen in reference to the 1905 name, as it had been a "childhood favorite" of McDonald's.[10]
From the 1910s through the end of the 1950s, Barnum's discoveries remained the only specimens ofTyrannosaurus, as theGreat Depression and wars kept many paleontologists out of the field.[5]
Beginning in the 1960s, there was renewed interest inTyrannosaurus, resulting in the recovery of 42 skeletons (5–80% complete by bone count) from Western North America.[5] In 1967, Dr. William MacMannis located and recovered the skeleton named "MOR 008", which is 15% complete by bone count and has a reconstructed skull displayed at theMuseum of the Rockies. The 1990s saw numerous discoveries, with nearly twice as many finds as in all previous years, including two of the most complete skeletons found to date:Sue andStan.[5]
Sue Hendrickson, anamateur paleontologist, discovered the most complete (approximately 85%) and largestTyrannosaurus skeleton in theHell Creek Formation on August 12, 1990. The specimen Sue, named after the discoverer, was the object of a legal battle over its ownership. In 1997, the litigation was settled in favor of Maurice Williams, the original land owner. The fossil collection was purchased by theField Museum of Natural History at auction for $7.6 million, making it the most expensive dinosaur skeleton until the sale of Stan for $31.8 million in 2020.[11] From 1998 to 1999, Field Museum of Natural History staff spent over 25,000 hours taking the rock off the bones.[12] The bones were then shipped toNew Jersey where the mount was constructed, then shipped back to Chicago for the final assembly. The mounted skeleton opened to the public on May 17, 2000, in the Field Museum of Natural History. A study of this specimen's fossilized bones showed that Sue reached full size at age 19 and died at the age of 28, the longest estimated life of any tyrannosaur known.[13]
"Scotty", the largest known specimen, exhibited in Japan
AnotherTyrannosaurus, nicknamed Stan (BHI 3033), in honor of amateur paleontologist Stan Sacrison, was recovered from the Hell Creek Formation in 1992. Stan is the second most complete skeleton found, with 199 bones recovered representing 70% of the total.[14] This tyrannosaur also had many bone pathologies, including broken and healed ribs, a broken (and healed) neck, and a substantial hole in the back of its head, about the size of aTyrannosaurus tooth.[15]
In 1998, 20-year-old Bucky Derflinger noticed aT. rex toe exposed above ground, making him the youngest person to discover aTyrannosaurus. The specimen, dubbedBucky in honor of its discoverer, was a young adult, 3.0 metres (10 ft) tall and 11 metres (35 ft) long. Bucky is the firstTyrannosaurus to be found that preserved afurcula (wishbone). Bucky is permanently displayed atThe Children's Museum of Indianapolis.[16]
The specimens "Sue", AMNH 5027, "Stan", and "Jane", to scale with a human.
In the summer of 2000, crews organized byJack Horner discovered fiveTyrannosaurus skeletons near theFort Peck Reservoir.[17] In 2001, a 50% complete skeleton of a juvenileTyrannosaurus was discovered in the Hell Creek Formation by a crew from theBurpee Museum of Natural History. Dubbed Jane (BMRP 2002.4.1), the find was thought to be the first known skeleton of apygmy tyrannosaurid,Nanotyrannus, but subsequent research revealed that it is more likely a juvenileTyrannosaurus, and the most complete juvenile example known;[18] Jane is exhibited at the Burpee Museum of Natural History.[19] In 2002, a skeleton nicknamed "Wyrex", discovered by amateur collectors Dan Wells and Don Wyrick, had 114 bones and was 38% complete. The dig was concluded over 3 weeks in 2004 by theBlack Hills Institute with the first liveonlineTyrannosaurus excavation providing daily reports, photos, and video.[5]
In 2006,Montana State University revealed that it possessed the largestTyrannosaurus skull yet discovered (from a specimen named MOR 008), measuring 5 feet (152 cm) long.[20] Subsequent comparisons indicated that the longest head was 136.5 centimetres (53.7 in) (from specimen LACM 23844) and the widest head was 90.2 centimetres (35.5 in) (from Sue).[21]
Two isolated fossilizedfootprints have been tentatively assigned toT. rex. The first was discovered atPhilmont Scout Ranch, New Mexico, in 1983 by American geologist Charles Pillmore. Originally thought to belong to ahadrosaurid, examination of the footprint revealed a large 'heel' unknown inornithopod dinosaur tracks, and traces of what may have been ahallux, the dewclaw-like fourth digit of the tyrannosaur foot. The footprint was published as theichnogenusTyrannosauripus pillmorei in 1994, byMartin Lockley and Adrian Hunt. Lockley and Hunt suggested that it was very likely the track was made by aT. rex, which would make it the first known footprint from this species. The track was made in what was once a vegetated wetland mudflat. It measures 83 centimeters (33 in) long by 71 centimeters (28 in) wide.[22]
A second footprint that may have been made by aTyrannosaurus was first reported in 2007 by British paleontologist Phil Manning, from theHell Creek Formation of Montana. This second track measures 72 centimeters (28 in) long, shorter than the track described by Lockley and Hunt. Whether or not the track was made byTyrannosaurus is unclear, thoughTyrannosaurus is the only large theropod known to have existed in the Hell Creek Formation.[23][24]
A set of footprints in Glenrock, Wyoming dating to theMaastrichtian stage of the Late Cretaceous and hailing from theLance Formation were described by Scott Persons, Phil Currie and colleagues in 2016, and are believed to belong to either a juvenileT. rex or the dubious tyrannosauridNanotyrannus lancensis. From measurements and based on the positions of the footprints, the animal was believed to be traveling at a walking speed of around 2.8 to 5 miles per hour and was estimated to have a hip height of 1.56 to 2.06 m (5.1 to 6.8 ft).[25][26][27] A follow-up paper appeared in 2017, increasing the speed estimations by 50–80%.[28]
Description
Size
Size (in blue) compared to select giant theropods and a human
T. rex was one of the largest land carnivores of all time. One of its largest and the most complete specimens, nicknamedSue (FMNH PR2081), is located at theField Museum of Natural History in Chicago. Sue measured 12.3–12.4 m (40–41 ft) long,[29][30] was 3.66–3.96 m (12.0–13.0 ft) tall at the hips,[31][32][33] and according to the most recent studies, using a variety of techniques, maximum body masses have been estimated approximately 8.4–8.46 t (8.27–8.33 long tons; 9.26–9.33 short tons).[34][35] A specimen nicknamedScotty (RSM P2523.8), located at theRoyal Saskatchewan Museum, is reported to measure 13 m (43 ft) in length. Using a mass estimation technique that extrapolates from thecircumference of the femur, Scotty was estimated as the largest known specimen at 8.87 t (8.73 long tons; 9.78 short tons) in body mass.[34][36]
Not every adultTyrannosaurus specimen recovered is as big. Historically average adult mass estimates have varied widely over the years, from as low as 4.5 t (4.4 long tons; 5.0 short tons),[37][38] to more than 7.2 t (7.1 long tons; 7.9 short tons),[39] with most modern estimates ranging between 5.4 and 8.0 t (5.3 and 7.9 long tons; 6.0 and 8.8 short tons).[29][40][41][42][43]
A 2024 study found that there was little evidence of size-based sexual dimorphism inT. rex.[44]
Skull
Skull of specimen AMNH 5027 with labelled diagrams in dorsal, lateral, anterior, posterior, and medial (lower jaw) views
The largest knownT. rex skulls measure up to 1.54 m (5.1 ft) in length.[20][31] Largefenestrae (openings) in the skull reduced weight, as in all carnivorous theropods. In other respectsTyrannosaurus's skull was significantly different from those of large non-tyrannosaurid theropods. It was extremely wide at the rear but had a narrow snout, allowing unusually goodbinocular vision.[45][46] The skull bones were massive and thenasals and some other bones were fused, preventing movement between them; but many werepneumatized (contained a "honeycomb" of tiny air spaces) and thus lighter. These and other skull-strengthening features are part of thetyrannosaurid trend towards an increasingly powerful bite, which easily surpassed that of all non-tyrannosaurids.[47][48][49] The tip of the upper jaw was U-shaped (most non-tyrannosauroid carnivores had V-shaped upper jaws), which increased the amount of tissue and bone a tyrannosaur could rip out with one bite, although it also increased the stresses on the front teeth.[50]
The teeth ofT. rex displayed markedheterodonty (differences in shape).[51][52] Thepremaxillary teeth, four per side at the front of the upper jaw, were closely packed,D-shaped in cross-section, had reinforcing ridges on the rear surface, wereincisiform (their tips were chisel-like blades) and curved backwards. TheD-shaped cross-section, reinforcing ridges and backwards curve reduced the risk that the teeth would snap whenTyrannosaurus bit and pulled. The remaining teeth were robust, like "lethal bananas" rather than daggers, more widely spaced and also had reinforcing ridges.[53] Those in the upper jaw, twelve per side in mature individuals,[51] were larger than their counterparts of the lower jaw, except at the rear. The largest found so far is estimated to have been 30.5 cm (12.0 in) long including the root when the animal was alive, making it the largest tooth of any carnivorous dinosaur yet found.[54] The lower jaw was robust. Its frontdentary bone bore thirteen teeth. Behind the tooth row, the lower jaw became notably taller.[51] The upper and lower jaws ofTyrannosaurus, like those of many dinosaurs, possessed numerousforamina, or small holes in the bone. Various functions have been proposed for these foramina, such as a crocodile-like sensory system[55] or evidence ofextra-oral structures such as scales or potentially lips,[56][57][58] with subsequent research on theropod tooth wear patterns supporting such a proposition.[59]
Skeleton
Life restoration showing scaly skin with sparse feathering, and lipped jaws
Skeletal reconstruction of specimen "Sue"
Thevertebral column ofTyrannosaurus consisted of ten neck vertebrae, thirteen back vertebrae and five sacral vertebrae. The number of tail vertebrae is unknown and could well have varied between individuals but probably numbered at least forty. Sue was mounted with forty-seven of such caudal vertebrae.[51] The neck ofT. rex formed a natural S-shaped curve like that of other theropods. Compared to these, it was exceptionally short, deep and muscular to support the massive head. The second vertebra, the axis, was especially short. The remaining neck vertebrae were weakly opisthocoelous, i.e. with a convex front of the vertebral body and a concave rear. The vertebral bodies had single pleurocoels, pneumatic depressions created byair sacs, on their sides.[51] The vertebral bodies of the torso were robust but with a narrow waist. Their undersides were keeled. The front sides were concave with a deep vertical trough. They had large pleurocoels. Their neural spines had very rough front and rear sides for the attachment of strong tendons. The sacral vertebrae were fused to each other, both in their vertebral bodies and neural spines. They were pneumatized. They were connected to the pelvis by transverse processes and sacral ribs. The tail was heavy and moderately long, in order to balance the massive head and torso and to provide space for massivelocomotor muscles that attached to the thighbones. The thirteenth tail vertebra formed the transition point between the deep tail base and the middle tail that was stiffened by a rather long front articulation processes. The underside of the trunk was covered by eighteen or nineteen pairs of segmented belly ribs.[51]
Right forelimb of specimen "Sue"
Theshoulder girdle was longer than the entire forelimb. The shoulder blade had a narrow shaft but was exceptionally expanded at its upper end. It connected via a long forward protrusion to thecoracoid, which was rounded. Both shoulder blades were connected by a smallfurcula. The paired breast bones possibly were made ofcartilage only.[51]
The forelimb or arm was very short. The upper arm bone, the humerus, was short but robust. It had a narrow upper end with an exceptionally rounded head. The lower arm bones, theulna and radius, were straight elements, much shorter than the humerus. The secondmetacarpal was longer and wider than the first, whereas normally in theropods the opposite is true. The forelimbs had only two clawed fingers,[51] along with an additional splint-like small thirdmetacarpal representing the remnant of a third digit.[60]
Pelvic girdle of specimen MOR 555
Thepelvis was a large structure. Its upper bone, theilium, was both very long and high, providing an extensive attachment area for hindlimb muscles. The frontpubic bone ended in an enormous pubic boot, longer than the entire shaft of the element. The rearischium was slender and straight, pointing obliquely to behind and below.[51]
In contrast to the arms, the hindlimbs were among the longest in proportion to body size of any theropod. In the foot, themetatarsus was "arctometatarsalian", meaning that the part of the third metatarsal near the ankle was pinched. The third metatarsal was also exceptionally sinuous.[51] Compensating for the immense bulk of the animal, many bones throughout the skeleton were hollowed, reducing its weight without significant loss of strength.[51]
Classification
Skull casts of differentTyrannosaurus specimens
Tyrannosaurus is thetype genus of the superfamilyTyrannosauroidea, thefamilyTyrannosauridae, and the subfamily Tyrannosaurinae; in other words it is the standard by which paleontologists decide whether to include other species in the same group. Other members of the tyrannosaurine subfamily include the North AmericanDaspletosaurus and theAsianTarbosaurus,[18][61] both of which have occasionally been synonymized withTyrannosaurus.[62]
Tyrannosaurids were once commonly thought to be descendants of earlier large theropods such asmegalosaurs andcarnosaurs, although more recently they were reclassified with the generally smallercoelurosaurs.[50] The earliest tyrannosaur group were the crested proceratosaurids, while later and more derived members belong to thePantyrannosauria. Tyrannosaurs started out as small theropods; however at least some became larger by theEarly Cretaceous.
Tyrannosauroids are characterized by their fused nasals and dental arrangement. Pantyrannosaurs are characterized by unique features in their hips as well as an enlarged foramen in the quadrate, a broad postorbital and hourglass shaped nasals. Some of the more derived pantyrannosaurs lack nasal pneumaticity and have a lower humerus to femur ratio with their arms starting to see some reduction. Some pantyrannosaurs started developing an arctometatarsus. Eutyrannosaurs have a rough texture on their nasal bones and their mandibular fenestra is reduced externally. Tyrannosaurids lack kinetic skulls or special crests on their nasal bones, and have a lacrimal with a distinctive process on it. Tyrannosaurids also have an interfenestral strut that is less than half as big as the maxillary fenestra.[63]
It is quite likely that tyrannosauroids rose to prominence after the decline in allosauroid and megalosauroid diversity seen during the early stages of the Late Cretaceous. Below is a simple cladogram of general tyrannosauroid relationships that was found after an analysis conducted by Li and colleagues in 2009.[64]
Manyphylogenetic analyses have foundTarbosaurus bataar to be thesister taxon ofT. rex.[61] The discovery of the tyrannosauridLythronax further indicates thatTarbosaurus andTyrannosaurus are closely related, forming a clade with fellow Asian tyrannosauridZhuchengtyrannus, withLythronax being their sister taxon.[65][66] A further study from 2016 by Steve Brusatte, Thomas Carr and colleagues, also indicates thatTyrannosaurus may have been an immigrant from Asia, as well as a possible descendant ofTarbosaurus.[67]
Below is the cladogram of Tyrannosauridae based on thephylogenetic analysis conducted by Loewen and colleagues in 2013.[65]
In their 2024 description ofTyrannosaurus mcraeensis, Dalman et al. recovered similar results to previous analyses, withTyrannosaurus as the sister taxon to the clade formed byTarbosaurus andZhuchengtyrannus, called the Tyrannosaurini. They also found support for amonophyletic clade containingDaspletosaurus andThanatotheristes, typically referred to as theDaspletosaurini.[68][69]
Diagram showing the differences between a generalizedTarbosaurus (A) andTyrannosaurus rex (B) skull
In 1955, SovietpaleontologistEvgeny Maleev named a new species,Tyrannosaurus bataar, fromMongolia.[70] By 1965, this species was renamed as a distinct genus,Tarbosaurus bataar.[71] While most palaeontologists continue to maintain the two as distinct genera, some authors such asThomas Holtz,Kenneth Carpenter, andThomas Carr argue that the two species are similar enough to be considered members of the same genus, restoring the Mongoliantaxon's originalbinomial name.[50][72][55]
VGI, no. 231/3, a largephalanx bone, assigned toTyrannosaurus sp. by Yarkov in 2000, was found in the Lower Maastrichtian ofBereslavka, Russia. In 2004, Averianov and Yarkov reinterpreted it as ametacarpal I ormetatarsal I that possibly belongs toceratosaur.[76] In their 2023 overview, Averianov and Lopatin mention this specimen as well as a single tooth from the same site only as Theropoda indet.[77]
In 2001, various tyrannosaurid teeth and a metatarsal unearthed in a quarry nearZhucheng, China were assigned by Chinese paleontologistHu Chengzhi to the newly erected speciesTyrannosaurus zhuchengensis. However, in a nearby site, a right maxilla and left jawbone were assigned to the newly erected tyrannosaurid genusZhuchengtyrannus in 2011. It is possible thatT. zhuchengensis issynonymous withZhuchengtyrannus. In any case,T. zhuchengensis is considered to be anomen dubium as the holotype lacksdiagnostic features below the level Tyrannosaurinae.[78]
In 2006, a fragmentary tyrannosaurid lacrimal (CM 9401) from theJudith River Formation of Fergus County, Montana was described as ?Tyrannosaurus sp. This isolated right lacrimal was originally collected alongside the holotype specimen ofDeinosuchus rugosus, a giant crocodylian, and remained undescribed until its re-identification as belonging to a tyrannosaurid theropod in the 1980s by paleontologistDale Russell. The lacrimal closely resembles those ofTyrannosaurus rex in both size and morphology. Notably, it lacks the "lacrimal horn" typically present in earlier tyrannosaurids likeAlbertosaurus andGorgosaurus, instead exhibiting a distinct rugosity along the dorsal surface—consistent withT. rex and its Asian relativeTarbosaurus. The specimen's considerable size places it within the range of knownT. rex individuals, suggesting the presence of large tyrannosaurids during the Campanian stage (~75 million years ago), a temporal range earlier than the established Maastrichtian age (~68–66 Ma) forTyrannosaurus rex. However, the exact age and provenance of CM 9401 remain uncertain due to a lack of detailed field documentation.[79]
In 2018, a paper describing tyrannosaurid teeth from the Two Medicine Formation noted a premaxillary tooth (YPM VPPU 023469) had a strong resemblance to the teeth ofSue to the exclusion of any Campanian tyrannosaurid. Additionally, the authors of this paper suggested that CM 9401 also comes from the Two Medicine Formation, as there were preservational similarities between its locality and the Willow Creek anticline, which is where the tooth was found.[80] Notably, this would place both specimens in the Flag Butte Member of the Two Medicine Formation, which dates from 77 to 76.3 Ma, far younger than any otherTyrannosaurus specimen, and directly contemporaneous withDaspletosaurus. In 2025, these specimens, with their old geologic age, were used as evidence by Charlie Scherer to suggest that the Tyrannosaurini did not evolve directly fromDaspletosaurus.[81]
In a 2022 study, Gregory S. Paul and colleagues argued thatTyrannosaurus rex, as traditionally understood, actually represents three species: the type speciesTyrannosaurus rex, and two new species:T. imperator (meaning "tyrant lizard emperor") andT. regina (meaning "tyrant lizard queen"). The holotype of the former (T. imperator) is theSue specimen, and the holotype of the latter (T. regina) isWankel rex. The division into multiple species was primarily based on the observation of a very high degree of variation in the proportions and robusticity of thefemur (and other skeletal elements) across cataloguedT. rex specimens, more so than that observed in other theropods recognized as one species. Differences of general body proportions representing robust and gracile morphotypes were also used as a line of evidence, in addition to the number of small, slender incisiform teeth in the dentary, as based on tooth sockets. Specifically, the paper'sT. rex was distinguished by robust anatomy, a moderate ratio of femur length vs circumference, and the possession of a singular slender incisiform dentary tooth;T. imperator was considered to be robust with a small femur length to circumference ratio and two of the slender teeth; andT. regina was a gracile form with a high femur ratio and one of the slender teeth. It was observed that variation in proportions and robustness became more extreme higher up in the sample,stratigraphically. This was interpreted as a single earlier population,T. imperator, speciating into more than one taxon,T. rex andT. regina.[82]
However, several other leading paleontologists, includingStephen Brusatte,Thomas Carr,Thomas Holtz, David Hone,Jingmai O'Connor, andLindsay Zanno, criticized the study or expressed skepticism of its conclusions when approached by various media outlets for comment.[83][84][85] Their criticism was subsequently published in a technical paper.[86] Holtz and Zanno both remarked that it was plausible that more than one species ofTyrannosaurus existed, but felt the new study was insufficient to support the species it proposed. Holtz remarked that, even ifTyrannosaurus imperator represented a distinct species fromTyrannosaurus rex, it may represent the same species asNanotyrannus lancensis and would need to be calledTyrannosaurus lancensis. O'Connor, a curator at the Field Museum, where theT. imperator holotype Sue is displayed, regarded the new species as too poorly-supported to justify modifying the exhibit signs. Brusatte, Carr, and O'Connor viewed the distinguishing features proposed between the species as reflecting natural variation within a species. Both Carr and O'Connor expressed concerns about the study's inability to determine which of the proposed species several well-preserved specimens belonged to. Another paleontologist,Philip J. Currie, originally co-authored the study but withdrew from it as he did not want to be involved in naming the new species.[83] Paul still rejected the objections raised by critics, insisting that they are unwilling to consider thatTyrannosaurus might represent more than one species.[87]
Lehman and Carpenter (1990) suggested that NMMNH P-3698 belonged to a new tyrannosaurid genus,[90] while Carr and Williamson (2000) disagreed with their claim.[91] Sullivan and Lucas (2015) argued that there is little evidence to support NMMNH P-3698 as a specimen ofTyrannosaurus rex, so they tentatively classified it as cf.Tyrannosaurus sp.; they also considered that the McRae tyrannosaur lived before theLancian (before 67 million years ago) based on its coexistence withAlamosaurus.[92]
Dalman et al. (2024) proposed the new nameTyrannosaurus mcraeensis for the holotype (NMMNH P-3698), referencing the McRae Group, the rock layers to which the Hall Lake Formation belongs. The holotype ofT. mcraeensis is found in the strata that are around a few million years older than the accepted range ofT. rex, which existed at the end of the Maastrichtian. The rock layers were initially estimated to date to between 72.7 and 70.9 Ma, correlating to the latest Campanian or earliest Maastrichtian.[68] However, in a 2024conferenceabstracts, the specific stratigraphic layer which yieldedT. mcraeensis was estimated to be younger than 69.0 ± 0.4 Ma and older than 66.0 Ma based on the sandstone from the same fossil locality,[93] which would suggest that the age ofT. mcraeensis is much closer toT. rex than previously thought.[81]T. mcraeensis was estimated at 12 metres (39 ft) long, which is similar to the size of an adultT. rex. The two are distinguished by characters of the skull. Amongst these, the dentary ofT. mcraeensis is proportionately longer and possesses a less prominent chin, and the lower jaw shallower than that ofT. rex, suggesting a weaker bite. The teeth are likewise blunter and more laterally compressed, while the post orbital crests are less prominent. Likewise, the skeletal anatomy showcases shared characteristics withTarbosaurus andZhuchengtyrannus.[68][94]
Nanotyrannus
Cast of CMNH 7541, theholotype ofNanotyrannus lancensis, sometimes interpreted as a juvenileTyrannosaurus.
Other tyrannosaurid fossils found in the same formations asT. rex were originally classified as separate taxa, includingAublysodon andAlbertosaurus megagracilis,[62] the latter being namedDinotyrannus megagracilis in 1995.[95] These fossils are now universally considered to belong to juvenileT. rex.[96] A small but nearly complete skull from Montana, 60 centimeters (2.0 ft) long, might be an exception. This skull,CMNH 7541, was originally classified as a species ofGorgosaurus (G. lancensis) byCharles W. Gilmore in 1946.[97] In 1988, the specimen was re-described byRobert T. Bakker,Phil Currie, and Michael Williams, then the curator of paleontology at the Cleveland Museum of Natural History, where the original specimen was housed and is now on display. Their initial research indicated that the skull bones were fused, and that it therefore represented an adult specimen. In light of this, Bakker and colleagues assigned the skull to a new genus namedNanotyrannus (meaning "dwarf tyrant", for its apparently small adult size). The specimen is estimated to have been around 5.2 meters (17 ft) long when it died.[98] However, In 1999, a detailed analysis byThomas Carr revealed the specimen to be a juvenile, leading Carr and many other paleontologists to consider it a juvenileT. rex individual.[99][100]
In 2001, a more complete juvenile tyrannosaur (nicknamed "Jane", catalog number BMRP 2002.4.1), belonging to the same species as the originalNanotyrannus specimen, was uncovered. This discovery prompted a conference on tyrannosaurs focused on the issues ofNanotyrannus validity at theBurpee Museum of Natural History in 2005. Several paleontologists who had previously published opinions thatN. lancensis was a valid species, including Currie and Williams, saw the discovery of "Jane" as a confirmation thatNanotyrannus was, in fact, a juvenileT. rex.[101][102][103]Peter Larson continued to support the hypothesis thatN.lancensis was a separate but closely related species, based on skull features such as two more teeth in both jaws thanT. rex; as well as proportionately larger hands with phalanges on the third metacarpal and differentwishbone anatomy in an undescribed specimen. He also argued thatStygivenator, generally considered to be a juvenileT. rex, may be a youngerNanotyrannus specimen.[104][105] Later research revealed that other tyrannosaurids such asGorgosaurus also experienced reduction in tooth count during growth,[99] and given the disparity in tooth count between individuals of the same age group in this genus andTyrannosaurus, this feature may also be due toindividual variation.[100] In 2013, Carr noted that all of the differences claimed to supportNanotyrannus have turned out to be individually or ontogenetically variable features or products ofdistortion of the bones.[106]
In 2016, analysis of limb proportions by Persons and Currie suggestedNanotyrannus specimens to have differing cursoriality levels, potentially separating it fromT. rex.[107] However, paleontologist Manabu Sakomoto has commented that this conclusion may be impacted by lowsample size, and the discrepancy does not necessarily reflect taxonomic distinction.[108] In 2016, Joshua Schmerge argued forNanotyrannus' validity based on skull features, including a dentary groove in BMRP 2002.4.1's skull. According to Schmerge, as that feature is absent inT. rex and found only inDryptosaurus and albertosaurines, this suggestsNanotyrannus is a distinct taxon within the Albertosaurinae.[109] The same year, Carr and colleagues noted that this was insufficient to clarifyNanotyrannus' validity or classification, being a common and ontogenetically variable feature among tyrannosauroids.[110]
A 2020 study by Holly Woodward and colleagues showed the specimens referred toNanotyrannus were all ontogenetically immature and found it probable that these specimens belonged toT. rex.[111] The same year, Carr published a paper onT.rex's growth history, finding that CMNH 7541 fit within the expected ontogenetic variation of the taxon and displayed juvenile characteristics found in other specimens. It was classified as a juvenile, under 13 years old with a skull less than 80 cm (31 in). No significant sexual or phylogenetic variation was discernible among any of the 44 specimens studied, with Carr stating that characters of potential phylogenetic importance decrease throughout age at the same rate as growth occurs.[112] Discussing the paper's results, Carr described how allNanotyrannus specimens formed a continual growth transition between the smallest juveniles and the subadults, unlike what would be expected if it were a distinct taxon where the specimens would group to the exclusion ofTyrannosaurus. Carr concluded that "the 'nanomorphs' are not all that similar to each other and instead form an important bridge in the growth series ofT. rex that captures the beginnings of the profound change from the shallow skull of juveniles to the deep skull that is seen in fully-developed adults."[113]
However, a 2024 paper published by Nick Longrich and Evan Thomas Saitta reexamined the holotype and referred specimens ofNanotyrannus. Based on several factors, including differences in morphology, ontogeny, and phylogeny, Longrich and Saitta suggest thatNanotyrannus is a distinct taxon which may fall outside ofTyrannosauridae, based on some of their phylogenetic analyses.[114]
Paleobiology
Life history
Illustration of a juvenileTyrannosaurus rex
The identification of several specimens as juvenileT. rex has allowed scientists to documentontogenetic changes in the species, estimate the lifespan, and determine how quickly the animals would have grown. The smallest known individual (LACM 28471, the "Jordan theropod") is estimated to have weighed only 30 kg (66 lb), while the largest adults, such asFMNH PR2081 (Sue) most likely weighed about 5,650 kg (12,460 lb).Histologic analysis ofT. rex bones showed LACM 28471 had aged only 2 years when it died, while Sue was 28 years old, an age which may have been close to the maximum for the species.[40]
A graph showing the hypothesized growth curve, body mass versus age (drawn in black, with other tyrannosaurids for comparison). Based on Erickson and colleagues 2004
Histology has also allowed the age of other specimens to be determined. Growth curves can be developed when the ages of different specimens are plotted on a graph along with their mass. AT. rex growth curve is S-shaped, with juveniles remaining under 1,800 kg (4,000 lb) until approximately 14 years of age, when body size began to increase dramatically. During this rapid growth phase, a youngT. rex would gain an average of 600 kg (1,300 lb) a year for the next four years. At 18 years of age, the curve plateaus again, indicating that growth slowed dramatically. For example, only 600 kg (1,300 lb) separated the 28-year-old Sue from a 22-year-old Canadian specimen (RTMP 81.12.1).[40] A 2004 histological study performed by different workers corroborates these results, finding that rapid growth began to slow at around 16 years of age.[115]
A study by Hutchinson and colleagues in 2011 corroborated the previous estimation methods in general, but their estimation of peak growth rates is significantly higher; it found that the "maximum growth rates for T. rex during the exponential stage are 1790 kg/year".[29] Although these results were much higher than previous estimations, the authors noted that these results significantly lowered the great difference between its actual growth rate and the one which would be expected of an animal of its size.[29] The sudden change in growth rate at the end of the growth spurt may indicate physical maturity, a hypothesis which is supported by the discovery of medullary tissue in thefemur of a 16 to 20-year-oldT. rex from Montana (MOR 1125, also known asB-rex). Medullary tissue is found only in female birds during ovulation, indicating that B-rex was of reproductive age.[116] Further study indicates an age of 18 for this specimen.[117] In 2016, it was finally confirmed by Mary Higby Schweitzer and Lindsay Zanno and colleagues that the soft tissue within the femur of MOR 1125 was medullary tissue. This also confirmed the identity of the specimen as a female. The discovery of medullary bone tissue withinTyrannosaurus may prove valuable in determining the sex of other dinosaur species in future examinations, as the chemical makeup of medullary tissue is unmistakable.[118] Other tyrannosaurids exhibit extremely similar growth curves, although with lower growth rates corresponding to their lower adult sizes.[119]
Diagram showing growth stages
An additional study published in 2020 by Woodward and colleagues, for the journalScience Advances indicates that during their growth from juvenile to adult,Tyrannosaurus was capable of slowing down its growth to counter environmental factors such as lack of food. The study, focusing on two juvenile specimens between 13 and 15 years old housed at the Burpee Museum in Illinois, indicates that the rate of maturation forTyrannosaurus was dependent on resource abundance. This study also indicates that in such changing environments,Tyrannosaurus was particularly well-suited to an environment that shifted yearly in regards to resource abundance, hinting that other midsize predators might have had difficulty surviving in such harsh conditions and explaining the niche partitioning between juvenile and adult tyrannosaurs. The study further indicates thatTyrannosaurus and the dubious genusNanotyrannus are synonymous, due to analysis of the growth rings in the bones of the two specimens studied.[120][121]
Over half of the knownT. rex specimens appear to have died within six years of reaching sexual maturity, a pattern which is also seen in other tyrannosaurs and in some large, long-lived birds and mammals today. These species are characterized by high infant mortality rates, followed by relatively low mortality among juveniles. Mortality increases again following sexual maturity, partly due to the stresses of reproduction. One study suggests that the rarity of juvenileT. rex fossils is due in part to low juvenile mortality rates; the animals were not dying in large numbers at these ages, and thus were not often fossilized. This rarity may also be due to the incompleteness of thefossil record or to the bias of fossil collectors towards larger, more spectacular specimens.[119] In a 2013 lecture, Thomas Holtz Jr. suggested that dinosaurs "lived fast and died young" because they reproduced quickly whereas mammals have long lifespans because they take longer to reproduce.[122] Gregory S. Paul also writes thatTyrannosaurus reproduced quickly and died young but attributes their short lifespans to the dangerous lives they lived.[123]
The discovery offeathered dinosaurs led to debate regarding whether, and to what extent,Tyrannosaurus might have been feathered.[124][125] Filamentous structures, which are commonly recognized as the precursors offeathers, have been reported in the small-bodied, basal tyrannosauroidDilong paradoxus from the Early CretaceousYixian Formation of China in 2004.[126] Becauseintegumentary impressions of larger tyrannosauroids known at that time showed evidence ofscales, the researchers who studiedDilong speculated that insulating feathers might have been lost by larger species due to their smaller surface-to-volume ratio.[126] The subsequent discovery of the giant speciesYutyrannus huali, also from the Yixian, showed that even some large tyrannosauroids had feathers covering much of their bodies, casting doubt on the hypothesis that they were a size-related feature.[127] A 2017 study reviewed known skin impressions of tyrannosaurids, including those of aTyrannosaurus specimen nicknamed "Wyrex" (HMNS 2006.1743.01, formerly known as BHI 6230) which preserves patches of mosaic scales on the tail, hip, and neck.[124] The study concluded that feather covering of large tyrannosaurids such asTyrannosaurus was, if present, limited to the upper side of the trunk.[124]
A conference abstract published in 2016 posited that theropods such asTyrannosaurus had their upper teeth covered in lips, instead of bare teeth as seen incrocodilians. This was based on the presence ofenamel, which according to the study needs to remain hydrated, an issue not faced by aquatic animals like crocodilians.[57] However, there has been criticism where it favors the idea for lips, with the 2017 analytical study proposing that tyrannosaurids had large, flat scales on their snouts instead of lips, as modern crocodiles do.[55][128] But crocodiles possess rather cracked keratinized skin, not flat scales; by observing the hummocky rugosity of tyrannosaurids, and comparing it to extant lizards, researchers have found that tyrannosaurids had squamose scales rather than a crocodillian-like skin.[129][130]
In 2023, Cullen and colleagues supported the idea that theropods like tyrannosaurids had lips based on anatomical patterns, such as those of the foramina on their face and jaws, more similar to those of modernsquamates such asmonitor lizards ormarine iguanas than those of moderncrocodilians likealligators. Comparison of the teeth ofDaspletosaurus andAmerican alligators shows that the enamel of tyrannosaurids had no significant wear and that the teeth of modern crocodilians were eroded on the labial side and were substantially worn. This suggests that it is likely that theropod teeth were kept wet by lips. On the basis of the relationship between hydration and wear resistance, the authors argued that it is unlikely that the teeth of theropods, including tyrannosaurids, would have remained unworn when exposed for a long time, because it would have been hard to maintain hydration. The authors also performed regression analyses to demonstrate the relationship between tooth height and skull length, and found thatvaranids like thecrocodile monitor had substantially greater ratios of tooth height to skull length than those ofTyrannosaurus, indicating that the teeth of theropods were not too big to be covered by extraoral tissues when the mouth was closed.[59]
As the number of known specimens increased, scientists began to analyze the variation between individuals and discovered what appeared to be two distinct body types, ormorphs, similar to some other theropod species. As one of these morphs was more solidly built, it was termed the 'robust' morph while the other was termed 'gracile'. Severalmorphological differences associated with the two morphs were used to analyzesexual dimorphism inT. rex, with the 'robust' morph usually suggested to be female. For example, thepelvis of several 'robust' specimens seemed to be wider, perhaps to allow the passage ofeggs.[131] It was also thought that the 'robust' morphology correlated with a reducedchevron on the first tail vertebra, also ostensibly to allow eggs to pass out of thereproductive tract, as had been erroneously reported forcrocodiles.[132]
In recent years, evidence for sexual dimorphism has been weakened. A 2005 study reported that previous claims of sexual dimorphism in crocodile chevron anatomy were in error, casting doubt on the existence of similar dimorphism betweenT. rex sexes.[133] A full-sized chevron was discovered on the first tail vertebra of Sue, an extremely robust individual, indicating that this feature could not be used to differentiate the two morphs anyway. AsT. rex specimens have been found fromSaskatchewan toNew Mexico, differences between individuals may be indicative of geographic variation rather than sexual dimorphism. The differences could also be age-related, with 'robust' individuals being older animals.[51]
Only a singleTyrannosaurus specimen has been conclusively shown to belong to a specific sex. Examination of B-rex demonstrated the preservation ofsoft tissue within several bones. Some of this tissue has been identified as a medullary tissue, a specialized tissue grown only in modern birds as a source ofcalcium for the production ofeggshell duringovulation. As only female birds lay eggs, medullary tissue is only found naturally in females, although males are capable of producing it when injected with female reproductivehormones likeestrogen. This strongly suggests that B-rex was female and that she died during ovulation.[116] Recent research has shown that medullary tissue is never found in crocodiles, which are thought to be the closest living relatives of dinosaurs. The shared presence of medullary tissue in birds and other theropod dinosaurs is further evidence of the closeevolutionary relationship between the two.[134]
Like manybipedal dinosaurs,T. rex was historically depicted as a 'living tripod', with the body at 45 degrees or less from the vertical and the tail dragging along the ground, similar to akangaroo. This concept dates fromJoseph Leidy's 1865 reconstruction ofHadrosaurus, the first to depict a dinosaur in a bipedal posture.[135] In 1915, convinced that the creature stood upright,Henry Fairfield Osborn, former president of the American Museum of Natural History, further reinforced the notion in unveiling the first completeT. rex skeleton arranged this way. It stood in an upright pose for 77 years, until it was dismantled in 1992.[136]
By 1970, scientists realized this pose was incorrect and could not have been maintained by a living animal, as it would have resulted in thedislocation or weakening of severaljoints, including the hips and the articulation between the head and thespinal column.[137] The inaccurate AMNH mount inspired similar depictions in many films and paintings (such asRudolph Zallinger's famous muralThe Age of Reptiles inYale University'sPeabody Museum of Natural History)[138] until the 1990s, when films such asJurassic Park introduced a more accurate posture to the general public.[139] Modern representations in museums, art, and film showT. rex with its body approximately parallel to the ground with the tail extended behind the body to balance the head.[140]
To sit down,Tyrannosaurus may have settled its weight backwards and rested its weight on a pubic boot, the wide expansion at the end of the pubis in some dinosaurs. With its weight rested on the pelvis, it may have been free to move the hindlimbs. Getting back up again might have involved some stabilization from the diminutive forelimbs.[141][137] The latter known as Newman's pushup theory has been debated. Nonetheless,Tyrannosaurus was probably able to get up if it fell, which only would have required placing the limbs below the center of gravity, with the tail as an effective counterbalance. Healed stress fractures in the forelimbs have been put forward both as evidence that the arms cannot have been very useful[142][143] and as evidence that they were indeed used and acquired wounds,[144] like the rest of the body.
Arms
The forelimbs might have been used to helpT. rex rise from a resting pose, as seen in this cast (Bucky specimen)
WhenT. rex was first discovered, thehumerus was the only element of the forelimb known.[6] For the initial mounted skeleton as seen by the public in 1915, Osborn substituted longer, three-fingered forelimbs like those ofAllosaurus.[4] A year earlier,Lawrence Lambe described the short, two-fingered forelimbs of the closely relatedGorgosaurus.[145] This strongly suggested thatT. rex had similar forelimbs, but thishypothesis was not confirmed until the first completeT. rex forelimbs were identified in 1989, belonging to MOR 555 (the "Wankel rex").[146][147] The remains of Sue also include complete forelimbs.[51]T. rex arms are very small relative to overall body size, measuring only 1 meter (3.3 ft) long, and some scholars have labelled them asvestigial. However, the bones show large areas formuscle attachment, indicating considerable strength. This was recognized as early as 1906 by Osborn, who speculated that the forelimbs may have been used to grasp a mate duringcopulation.[8] Newman (1970) suggested that the forelimbs were used to assistTyrannosaurus in rising from a prone position.[137] Since then, other functions have been proposed, although some scholars find them implausible.[143]Padian (2022) argued that the reduction of the arms in tyrannosaurids did not serve a particular function but was a secondary adaptation, stating that as tyrannosaurids developed larger and more powerful skulls and jaws, the arms got smaller to avoid being bitten or torn by other individuals, particularly during group feedings.[143]
Diagram illustrating arm anatomy
Another possibility is that the forelimbs held struggling prey while it was killed by the tyrannosaur's enormous jaws. This hypothesis may be supported bybiomechanical analysis.T. rex forelimb bones exhibit extremely thickcortical bone, which has been interpreted as evidence that they were developed to withstand heavy loads. Thebiceps brachii muscle of an adultT. rex was capable of lifting 199 kilograms (439 lb) by itself; other muscles such as thebrachialis would work along with the biceps to make elbow flexion even more powerful. TheM. biceps muscle ofT. rex was 3.5 times as powerful as thehuman equivalent. AT. rex forearm had a limited range of motion, with the shoulder and elbow joints allowing only 40 and 45 degrees of motion, respectively. In contrast, the same two joints inDeinonychus allow up to 88 and 130 degrees of motion, respectively, while a human arm can rotate 360 degrees at the shoulder and move through 165 degrees at the elbow. The heavy build of the arm bones, strength of the muscles, and limited range of motion may indicate a system evolved to hold fast despite the stresses of a struggling prey animal. In the first detailed scientific description ofTyrannosaurus forelimbs, paleontologists Kenneth Carpenter and Matt Smith dismissed notions that the forelimbs were useless or thatTyrannosaurus was an obligate scavenger.[148]
The idea that the arms served as weapons when hunting prey have also been proposed bySteven M. Stanley, who suggested that the arms were used for slashing prey, especially by using the claws to rapidly inflict long, deep gashes to its prey.[149] This was dismissed by Padian, who argued that Stanley based his conclusion on incorrectly estimated forelimb size and range of motion.[143]
Tyrannosaurus, like most dinosaurs, was long thought to have anectothermic ("cold-blooded") reptilianmetabolism. The idea of dinosaur ectothermy was challenged by scientists likeRobert T. Bakker andJohn Ostrom in the early years of the "Dinosaur Renaissance", beginning in the late 1960s.[150][151]T. rex itself was claimed to have beenendothermic ("warm-blooded"), implying a very active lifestyle.[38] Since then, several paleontologists have sought to determine the ability ofTyrannosaurus toregulate its body temperature. Histological evidence of high growth rates in youngT. rex, comparable to those of mammals and birds, may support the hypothesis of a high metabolism. Growth curves indicate that, as in mammals and birds,T. rex growth was limited mostly to immature animals, rather than theindeterminate growth seen in most othervertebrates.[115]
Oxygen isotope ratios in fossilized bone are sometimes used to determine the temperature at which the bone was deposited, as the ratio between certain isotopes correlates with temperature. In one specimen, the isotope ratios in bones from different parts of the body indicated a temperature difference of no more than 4 to 5 °C (7 to 9 °F) between the vertebrae of the torso and thetibia of the lower leg. This small temperature range between the body core and the extremities was claimed by paleontologist Reese Barrick andgeochemist William Showers to indicate thatT. rex maintained a constant internal body temperature (homeothermy) and that it enjoyed a metabolism somewhere between ectothermic reptiles and endothermic mammals.[152] Other scientists have pointed out that the ratio of oxygen isotopes in the fossils today does not necessarily represent the same ratio in the distant past, and may have been altered during or after fossilization (diagenesis).[153] Barrick and Showers have defended their conclusions in subsequent papers, finding similar results in another theropod dinosaur from a different continent and tens of millions of years earlier in time (Giganotosaurus).[154]Ornithischian dinosaurs also showed evidence of homeothermy, whilevaranidlizards from the same formation did not.[155] In 2022, Wiemann and colleagues used a different approach—thespectroscopy of lipoxidation signals, which are byproducts ofoxidative phosphorylation and correlate with metabolic rates—to show that various dinosaur genera includingTyrannosaurus had endothermic metabolisms, on par with that of modern birds and higher than that of mammals. They also suggested that such a metabolism was ancestrally common to all dinosaurs.[156]
Even ifT. rex does exhibit evidence of homeothermy, it does not necessarily mean that it was endothermic. Such thermoregulation may also be explained bygigantothermy, as in some livingsea turtles.[157][158][159] Similar to contemporary crocodilians, openings (dorsotemporal fenestrae) in the skull roofs ofTyrannosaurus may have aided thermoregulation.[160]
Soft tissue
T. rex femur (MOR 1125) from which demineralized matrix andpeptides (insets) were obtained
In the March 2005 issue ofScience,Mary Higby Schweitzer ofNorth Carolina State University and colleagues announced the recovery of soft tissue from the marrow cavity of a fossilized leg bone from aT. rex. The bone had been intentionally, though reluctantly, broken for shipping and then not preserved in the normal manner, specifically because Schweitzer was hoping to test it for soft tissue.[161] Designated as the Museum of the Rockies specimen 1125, or MOR 1125, the dinosaur was previously excavated from theHell Creek Formation. Flexible, bifurcatingblood vessels and fibrous but elasticbone matrix tissue were recognized. In addition, microstructures resemblingblood cells were found inside the matrix and vessels. The structures bear resemblance toostrich blood cells and vessels. Whether an unknown process, distinct from normal fossilization, preserved the material, or the material is original, the researchers do not know, and they are careful not to make any claims about preservation.[162] If it is found to be original material, any surviving proteins may be used as a means of indirectly guessing some of the DNA content of the dinosaurs involved, because each protein is typically created by a specific gene. The absence of previous finds may be the result of people assuming preserved tissue was impossible, therefore not looking. Since the first, two more tyrannosaurs and a hadrosaur have also been found to have such tissue-like structures.[161] Research on some of the tissues involved has suggested that birds are closer relatives to tyrannosaurs than other modern animals.[163] The original endogenous chemistry was also found in MOR 1125 based on preservation of elements associated with bone remodeling and redeposition (sulfur, calcium, zinc), which showed that the bone cortices are similar to those of extant birds.[164]
In studies reported inScience in April 2007, Asara and colleagues concluded that seven traces ofcollagen proteins detected in purifiedT. rex bone most closely match those reported inchickens, followed by frogs and newts. The discovery of proteins from a creature tens of millions of years old, along with similar traces the team found in a mastodon bone at least 160,000 years old, upends the conventional view of fossils and may shift paleontologists' focus from bone hunting to biochemistry. Until these finds, most scientists presumed that fossilization replaced all living tissue with inert minerals. Paleontologist Hans Larsson of McGill University in Montreal, who was not part of the studies, called the finds "a milestone", and suggested that dinosaurs could "enter the field of molecular biology and really slingshot paleontology into the modern world".[165]
The presumed soft tissue was called into question by Thomas Kaye of theUniversity of Washington and his co-authors in 2008. They contend that what was really inside the tyrannosaur bone was slimybiofilm created by bacteria that coated the voids once occupied by blood vessels and cells.[166] The researchers found that what previously had been identified as remnants of blood cells, because of the presence of iron, were actuallyframboids, microscopic mineral spheres bearing iron. They found similar spheres in a variety of other fossils from various periods, including anammonite. In the ammonite, they found the spheres in a place where the iron they contain could not have had any relationship to the presence of blood.[167] Schweitzer has strongly criticized Kaye's claims and argues that there is no reported evidence that biofilms can produce branching, hollow tubes like those noted in her study.[168] San Antonio, Schweitzer and colleagues published an analysis in 2011 of what parts of the collagen had been recovered, finding that it was the inner parts of the collagen coil that had been preserved, as would have been expected from a long period of protein degradation.[169] Other research challenges the identification of soft tissue as biofilm and confirms finding "branching, vessel-like structures" from within fossilized bone.[170]
Scientists have produced a wide range of possible maximum running speeds forTyrannosaurus: mostly around 9 meters per second (32 km/h; 20 mph), but as low as 4.5–6.8 meters per second (16–24 km/h; 10–15 mph) and as high as 20 meters per second (72 km/h; 45 mph), though it running this speed is very unlikely.Tyrannosaurus was a bulky and heavy carnivore so it is unlikely to run very fast at all compared to other theropods likeCarnotaurus orGiganotosaurus.[171] Researchers have relied on various estimating techniques because, while there are manytracks of large theropods walking, none showed evidence of running.[172]
A 2002 report used a mathematical model (validated by applying it to three living animals:alligators,chickens, andhumans; and eight more species, including emus and ostriches[172]) to gauge the leg muscle mass needed for fast running (over 40 km/h or 25 mph).[171] Scientists who think thatTyrannosaurus was able to run point out that hollow bones and other features that would have lightened its body may have kept adult weight to a mere 4.5 metric tons (5.0 short tons) or so, or that other animals likeostriches andhorses with long, flexible legs are able to achieve high speeds through slower but longer strides.[172] Proposed top speeds exceeded 40 kilometers per hour (25 mph) forTyrannosaurus, but were deemed infeasible because they would require exceptional leg muscles of approximately 40–86% of total body mass. Even moderately fast speeds would have required large leg muscles. If the muscle mass was less, only 18 kilometers per hour (11 mph) for walking or jogging would have been possible.[171] Holtz noted that tyrannosaurids and some closely related groups had significantly longerdistal hindlimb components (shin plus foot plus toes) relative to the femur length than most other theropods, and that tyrannosaurids and their close relatives had a tightly interlockedmetatarsus (foot bones).[173] The third metatarsal was squeezed between the second and fourth metatarsals to form a single unit called anarctometatarsus. This ankle feature may have helped the animal to run more efficiently.[174] Together, these leg features allowedTyrannosaurus to transmit locomotory forces from the foot to the lower leg more effectively than in earlier theropods.[173]
Additionally, a 2020 study indicates thatTyrannosaurus and other tyrannosaurids were exceptionally efficient walkers. Studies by Dececchi et al., compared the leg proportions, body mass, and the gaits of more than 70 species of theropod dinosaurs includingTyrannosaurus and its relatives. The research team then applied a variety of methods to estimate each dinosaur's top speed when running as well as how much energy each dinosaur expended while moving at more relaxed speeds such as when walking. Among smaller to medium-sized species such as dromaeosaurids, longer legs appear to be an adaptation for faster running, in line with previous results by other researchers. But for theropods weighing over 1,000 kg (2,200 lb), top running speed is limited by body size, so longer legs instead were found to have correlated with low-energy walking. The results further indicate that smaller theropods evolved long legs as a means to both aid in hunting and escape from larger predators while larger theropods that evolved long legs did so to reduce the energy costs and increase foraging efficiency, as they were freed from the demands of predation pressure due to their role as apex predators. Compared to more basal groups of theropods in the study, tyrannosaurs likeTyrannosaurus itself showed a marked increase in foraging efficiency due to reduced energy expenditures during hunting or scavenging. This in turn likely resulted in tyrannosaurs having a reduced need for hunting forays and requiring less food to sustain themselves as a result. Additionally, the research, in conjunction with studies that show tyrannosaurs were more agile than other large-bodied theropods, indicates they were quite well-adapted to a long-distance stalking approach followed by a quick burst of speed to go for the kill. Analogies can be noted between tyrannosaurids and modern wolves as a result, supported by evidence that at least some tyrannosaurids were hunting in group settings.[175][176]
A study published in 2021 by Pasha van Bijlert et al., calculated thepreferred walking speed ofTyrannosaurus, reporting a speed of 1.28 meters per second (4.6 km/h; 2.9 mph). While walking, animals reduce theirenergy expenditure by choosing certain step rhythms at which their body partsresonate. The same would have been true fordinosaurs, but previous studies did not fully account for the impact the tail had on their walking speeds. According to the authors, when a dinosaur walked, its tail would slightly sway up and down with each step as a result of theinterspinous ligaments suspending the tail. Like rubber bands, these ligaments stored energy when they are stretched due to the swaying of the tail. Using a 3-D model ofTyrannosaurus specimenTrix, muscles and ligaments were reconstructed to simulate the tail movements. This results in a rhythmic, energy-efficient walking speed forTyrannosaurus similar to that seen in living animals such as humans, ostriches and giraffes.[177]
A 2017 study estimated the top running speed ofTyrannosaurus as 17 mph (27 km/h), speculating thatTyrannosaurus exhausted its energy reserves long before reaching top speed, resulting in a parabola-like relationship between size and speed.[178][179] Another 2017 study hypothesized that an adultTyrannosaurus was incapable of running due to high skeletal loads. Using a calculated weight estimate of 7 tons, the model showed that speeds above 11 mph (18 km/h) would have probably shattered the leg bones ofTyrannosaurus. The finding may mean that running was also not possible for other giant theropod dinosaurs likeGiganotosaurus,Mapusaurus andAcrocanthosaurus.[180]However, studies by Eric Snively and colleagues, published in 2019 indicate thatTyrannosaurus and other tyrannosaurids were more maneuverable than allosauroids and other theropods of comparable size due to low rotational inertia compared to their body mass combined with large leg muscles. As a result, it is hypothesized thatTyrannosaurus was capable of making relatively quick turns and could likely pivot its body more quickly when close to its prey, or that while turning, the theropod could "pirouette" on a single planted foot while the alternating leg was held out in a suspended swing during a pursuit. The results of this study potentially could shed light on how agility could have contributed to the success of tyrannosaurid evolution.[181]
Possible footprints
Depiction ofTyrannosaurus rising from the ground, based on fossil tracks described in 2021.
Rare fossil footprints and trackways found in New Mexico and Wyoming that are assigned to the ichnogenusTyrannosauripus have been attributed to being made byTyrannosaurus, based on the stratigraphic age of the rocks they are preserved in. The first specimen, found in 1994 was described by Lockley and Hunt and consists of a single, large footprint. Another pair of ichnofossils, described in 2021, show a large tyrannosaurid rising from a prone position by rising up using its elbows in conjunction with the pads on their feet to stand. These two unique sets of fossils were found in Ludlow, Colorado and Cimarron, New Mexico.[182] Another ichnofossil described in 2018, perhaps belonging to a juvenileTyrannosaurus or the dubious genusNanotyrannus was uncovered in the Lance Formation of Wyoming. The trackway itself offers a rare glimpse into the walking speed of tyrannosaurids, and the trackmaker is estimated to have been moving at a speed of 4.5–8.0 kilometers per hour (2.8–5.0 mph), significantly faster than previously assumed for estimations of walking speed in tyrannosaurids.[183][184]
A study conducted byLawrence Witmer and Ryan Ridgely of Ohio University found thatTyrannosaurus shared the heightened sensory abilities of othercoelurosaurs, highlighting relatively rapid and coordinated eye and head movements; an enhanced ability to sense low frequency sounds, which would allow tyrannosaurs to track prey movements from long distances; and an enhanced sense of smell.[185] A study published by Kent Stevens concluded thatTyrannosaurus had keen vision. By applying modifiedperimetry to facial reconstructions of several dinosaurs includingTyrannosaurus, the study found thatTyrannosaurus had a binocular range of 55 degrees, surpassing that of modern hawks. Stevens estimated thatTyrannosaurus had 13 times the visual acuity of a human and surpassed the visual acuity of an eagle, which is 3.6 times that of a person. Stevens estimated a limiting far point (that is, the distance at which an object can be seen as separate from the horizon) as far as 6 km (3.7 mi) away, which is greater than the 1.6 km (1 mi) that a human can see.[45][46][186]
Thomas Holtz Jr. would note that high depth perception ofTyrannosaurus may have been due to the prey it had to hunt, noting that it had to hunt ceratopsians such asTriceratops, ankylosaurs such asAnkylosaurus, and hadrosaurs. He would suggest that this made precision more crucial forTyrannosaurus enabling it to, "get in, get that blow in and take it down." In contrast,Acrocanthosaurus had limited depth perception because they hunted large sauropods, which were relatively rare during the time ofTyrannosaurus.[122]
Though noTyrannosaurussclerotic ring has been found,Kenneth Carpenter estimated its size based on that ofGorgosaurus. The inferred sclerotic ring for theStan specimen is ~7 cm (2.8 in) in diameter with an internal aperture diameter of ~3.5 cm (1.4 in). Based on eye proportions in living reptiles, this implies a pupil diameter of about 2.5 cm (0.98 in), an iris diameter about that of the sclerotic ring, and an eyeball diameter of 11–12 cm (4.3–4.7 in). Carpenter also estimated an eyeball depth of ~7.7–9.6 cm (3.0–3.8 in). Based on these calculations, thef-number for Stan's eye is 3–3.8; sincediurnal animals have f-numbers of 2.1 or higher, this would indicate thatTyrannosaurus had poor low-light vision and hunted during the day.[187]
Tyrannosaurus had very largeolfactory bulbs andolfactory nerves relative to their brain size, the organs responsible for a heightened sense of smell. This suggests that the sense of smell was highly developed, and implies that tyrannosaurs could detect carcasses by scent alone across great distances. The sense of smell in tyrannosaurs may have been comparable to modernvultures, which use scent to track carcasses for scavenging. Research on the olfactory bulbs has shown thatT. rex had the most highly developed sense of smell of 21 sampled non-avian dinosaur species.[188]
Somewhat unusually among theropods,T. rex had a very longcochlea. The length of the cochlea is often related to hearing acuity, or at least the importance of hearing in behavior, implying that hearing was a particularly important sense to tyrannosaurs. Specifically, data suggests thatT. rex heard best in the low-frequency range, and that low-frequency sounds were an important part of tyrannosaur behavior.[185] A 2017 study by Thomas Carr and colleagues found that the snout of tyrannosaurids was highly sensitive, based on a high number of small openings in the facial bones of the relatedDaspletosaurus that containedsensory neurons. The study speculated that tyrannosaurs might have used their sensitive snouts to measure the temperature of their nests and to gently pick up eggs and hatchlings, as seen in modern crocodylians.[55] Another study published in 2021 further suggests thatTyrannosaurus had an acute sense of touch, based on neurovascular canals in the front of its jaws, which it could utilize to better detect and consume prey. The study, published by Kawabe and Hittori et al., suggests thatTyrannosaurus could also accurately sense slight differences in material and movement, allowing it to utilize different feeding strategies on different parts of its prey's carcasses depending on the situation. The sensitive neurovascular canals ofTyrannosaurus also likely were adapted to performing fine movements and behaviors such as nest building, parental care, and other social behavior such as intraspecific communication. The results of this study also align with results made in studying the related tyrannosauridDaspletosaurus horneri and theallosauroidNeovenator, which have similar neurovascular adaptations, suggesting that the faces of theropods were highly sensitive to pressure and touch.[189][190] However, a more recent study reviewing the evolution of the trigeminal canals among sauropsids notes that a much denser network of neurovascular canals in the snout and lower jaw is more commonly encountered in aquatic or semiaquatic taxa (e.g.,Spinosaurus,Halszkaraptor,Plesiosaurus), and taxa that developed a rhamphotheca (e.g.,Caenagnathasia), while the network of canals inTyrannosaurus appears simpler, though still more derived than in most ornithischians, and overall terrestrial taxa such as tyrannosaurids andNeovenator may have had average facial sensitivity for non-edentulous terrestrial theropods, although further research is needed. The neurovascular canals inTyrannosaurus may instead have supported soft tissue structures for thermoregulation or social signaling, the latter of which could be confirmed by the fact that the neurovascular network of canals may have changed during ontogeny.[191]
A study by Grant R. Hurlburt, Ryan C. Ridgely and Lawrence Witmer obtained estimates forEncephalization Quotients (EQs), based on reptiles and birds, as well as estimates for the ratio of cerebrum to brain mass. The study concluded thatTyrannosaurus had the relatively largest brain of all adult non-avian dinosaurs with the exception of certain small maniraptoriforms (Bambiraptor,Troodon andOrnithomimus). The study found thatTyrannosaurus's relative brain size was still within the range of modern reptiles, being at most 2standard deviations above the mean of non-avian reptile EQs. The estimates for the ratio of cerebrum mass to brain mass would range from 47.5 to 49.53 percent. According to the study, this is more than the lowest estimates for extant birds (44.6 percent), but still close to the typical ratios of the smallest sexually mature alligators which range from 45.9–47.9 percent.[192] Other studies, such as those by Steve Brusatte, indicate the encephalization quotient ofTyrannosaurus was similar in range (2.0–2.4) to achimpanzee (2.2–2.5), though this may be debatable as reptilian and mammalian encephalization quotients are not equivalent.[193]
Currie's pack-huntingT. rex hypothesis has been criticized for not having beenpeer-reviewed, but rather was discussed in a television interview and book calledDino Gangs.[197] The Currie theory for pack hunting byT. rex is based mainly by analogy to a different species,Tarbosaurus bataar. Evidence of gregariousness inT. bataar itself has not been peer-reviewed, and to Currie's own admission, can only be interpreted with reference to evidence in other closely related species. According to Currie gregariousness inAlbertosaurus sarcophagus is supported by the discovery of 26 individuals with varied ages in the Dry Island bonebed. He ruled out the possibility of a predator trap due to the similar preservation state of individuals and the near absence of herbivores.[197][198]
Additional support of tyrannosaurid gregariousness can be found in fossilizedtrackways from the Upper CretaceousWapiti Formation of northeasternBritish Columbia, Canada, left by three tyrannosaurids traveling in the same direction.[199][200] According to scientists assessing the Dino Gangs program, the evidence for pack hunting inTarbosaurus andAlbertosaurus is weak and based on group skeletal remains for which alternate explanations may apply (such as drought or a flood forcing dinosaurs to die together in one place).[197] Others researchers have speculated that instead of large theropod social groups, some of these finds represent behavior more akin toKomodo dragon-like mobbing of carcasses, even going as far as to say true pack-hunting behavior may not exist in any non-avian dinosaurs due to its rarity in modern predators.[201]
Evidence of intraspecific attack was found by Joseph Peterson and his colleagues in the juvenileTyrannosaurus nicknamedJane. Peterson and his team found that Jane's skull showed healed puncture wounds on the upper jaw and snout which they believe came from another juvenileTyrannosaurus. Subsequent CT scans of Jane's skull would further confirm the team's hypothesis, showing that the puncture wounds came from a traumatic injury and that there was subsequent healing.[202] The team would also state that Jane's injuries were structurally different from the parasite-induced lesions found in Sue and that Jane's injuries were on its face whereas the parasite that infected Sue caused lesions to the lower jaw.[203] Pathologies of otherTyrannosaurus specimens have been suggested as evidence of conspecific attack, including "Wyrex" with a hole penetrating its jugual and severe trauma on its tail that shows signs ofbone remodeling (not regrowth).[204][205]
Most paleontologists accept thatTyrannosaurus was both an activepredator and ascavenger like most largecarnivores.[206] By far the largest carnivore in its environment,T. rex was most likely anapex predator, preying uponhadrosaurs, armored herbivores likeceratopsians andankylosaurs, and possiblysauropods.[207] Enamel δ44/42Ca values also suggest the possibility thatT. rex occasionally fed on carcasses of marine reptiles and fish washed up on the shores of the Western Interior Seaway.[208] A study in 2012 by Karl Bates and Peter Falkingham found thatTyrannosaurus had the most powerful bite of any terrestrial animal that has ever lived, finding an adultTyrannosaurus could have exerted 35,000 to 57,000N (7,868 to 12,814lbf) of force in the back teeth.[209][210][211] Even higher estimates were made by Mason B. Meers in 2003.[48] This allowed it to crush bones during repetitive biting and fully consume the carcasses of large dinosaurs.[21] Stephan Lautenschlager and colleagues calculated thatTyrannosaurus was capable of a maximum jaw gape of around 80 degrees, a necessary adaptation for a wide range of jaw angles to power the creature's strong bite.[212][213]
A debate exists, however, about whetherTyrannosaurus was primarily apredator or a purescavenger. The debate originated in a 1917 study by Lambe which argued that large theropods were pure scavengers becauseGorgosaurus teeth showed hardly any wear.[214] This argument disregarded the fact that theropods replaced their teeth quite rapidly. Ever since the first discovery ofTyrannosaurus most scientists have speculated that it was a predator; like modern large predators it would readily scavenge or steal another predator's kill if it had the opportunity.[215]
PaleontologistJack Horner has been a major proponent of the view thatTyrannosaurus was not a predator at all but instead was exclusively a scavenger.[146][216][217] He has put forward arguments in the popular literature to support the pure scavenger hypothesis:
Tyrannosaur arms are short when compared to other known predators. Horner argues that the arms were too short to make the necessary gripping force to hold on to prey.[218] Other paleontologists such asThomas Holtz Jr. argued that there are plenty of modern-day predators that do not use their forelimbs to hunt such aswolves,hyenas, andsecretary birds as well as other extinct animals thought to be predators that would not have used their forelimbs such asphorusrhacids.[219][220]
Tyrannosaurs had largeolfactory bulbs andolfactory nerves (relative to their brain size). These suggest a highly developed sense of smell which could sniff out carcasses over great distances, as modernvultures do. Research on the olfactory bulbs of dinosaurs has shown thatTyrannosaurus had the most highly developed sense of smell of 21 sampled dinosaurs.[188]
Tyrannosaur teeth could crush bone, and therefore could extract as much food (bone marrow) as possible from carcass remnants, usually the least nutritious parts. Karen Chin and colleagues have found bone fragments incoprolites (fossilized feces) that they attribute to tyrannosaurs, but point out that a tyrannosaur's teeth were not well adapted to systematically chewing bone likehyenas do to extract marrow.[221]
Since at least some ofTyrannosaurus's potential prey could move quickly, evidence that it walked instead of ran could indicate that it was a scavenger.[216] On the other hand, recent analyses suggest thatTyrannosaurus, while slower than large modern terrestrial predators, may well have been fast enough to prey on largehadrosaurs andceratopsians.[171][24]
Other evidence suggests hunting behavior inTyrannosaurus. The eye sockets of tyrannosaurs are positioned so that the eyes would point forward, giving thembinocular vision slightly better than that of modernhawks. It is not obvious whynatural selection would have favored this long-term trend if tyrannosaurs had been pure scavengers, which would not have needed the advanceddepth perception thatstereoscopic vision provides.[45][46] In modern animals, binocular vision is found mainly in predators.
The damage to the tail vertebrae of thisEdmontosaurus annectens skeleton (on display at the Denver Museum of Nature and Science) indicates that it may have been bitten by aTyrannosaurus
A skeleton of the hadrosauridEdmontosaurus annectens has been described from Montana with healed tyrannosaur-inflicted damage on its tailvertebrae. The fact that the damage seems to have healed suggests that theEdmontosaurus survived a tyrannosaur's attack on a living target, i.e. the tyrannosaur had attempted active predation.[222] Despite the consensus that the tail bites were caused byTyrannosaurus, there has been some evidence to show that they might have been created by other factors. For example, a 2014 study suggested that the tail injuries might have been due toEdmontosaurus individuals stepping on each other,[223] while another study in 2020 backs up the hypothesis that biomechanical stress is the cause for the tail injuries.[224] There is also evidence for an aggressive interaction between aTriceratops and aTyrannosaurus in the form of partially healed tyrannosaur tooth marks on aTriceratops brow horn andsquamosal (a bone of theneck frill); the bitten horn is also broken, with new bone growth after the break. It is not known what the exact nature of the interaction was, though: either animal could have been the aggressor.[225] Since theTriceratops wounds healed, it is most likely that theTriceratops survived the encounter and managed to overcome theTyrannosaurus. In a battle against a bullTriceratops, theTriceratops would likely defend itself by inflicting fatal wounds to theTyrannosaurus using its sharp horns.[226] Studies ofSue found a broken and healedfibula and tail vertebrae, scarred facial bones and a tooth from anotherTyrannosaurus embedded in a neck vertebra, providing evidence for aggressive behavior.[227] Studies on hadrosaur vertebrae from the Hell Creek Formation that were punctured by the teeth of what appears to be a late-stage juvenileTyrannosaurus indicate that despite lacking the bone-crushing adaptations of the adults, young individuals were still capable of using the same bone-puncturing feeding technique as their adult counterparts.[228]
Tyrannosaurus may have had infectioussaliva used to kill its prey, as proposed byWilliam Abler in 1992. Abler observed that theserrations (tiny protuberances) on the cutting edges of the teeth are closely spaced, enclosing little chambers. These chambers might have trapped pieces of carcass with bacteria, givingTyrannosaurus a deadly, infectious bite much like theKomodo dragon was thought to have.[229][230] Jack Horner and Don Lessem, in a 1993 popular book, questioned Abler's hypothesis, arguing thatTyrannosaurus's tooth serrations as more like cubes in shape than the serrations on a Komodo monitor's teeth, which are rounded.[146]: 214–215
Tyrannosaurus, and most other theropods, probably primarily processed carcasses with lateral shakes of the head, like crocodilians. The head was not as maneuverable as the skulls ofallosauroids, due to flat joints of the neck vertebrae.[231]
Cannibalism
Evidence also strongly suggests that tyrannosaurs were at least occasionally cannibalistic.Tyrannosaurus itself has strong evidence pointing towards it having been cannibalistic in at least a scavenging capacity based on tooth marks on the foot bones, humerus, and metatarsals of one specimen.[232] Fossils from theFruitland Formation,Kirtland Formation (both Campanian in age) and the Maastrichtian agedOjo Alamo Formation suggest that cannibalism was present in various tyrannosaurid genera of the San Juan Basin. The evidence gathered from the specimens suggests opportunistic feeding behavior in tyrannosaurids that cannibalized members of their own species.[233] A study from Currie, Horner, Erickson and Longrich in 2010 has been put forward as evidence of cannibalism in the genusTyrannosaurus.[232] They studied someTyrannosaurus specimens with tooth marks in the bones, attributable to the same genus. The tooth marks were identified in thehumerus, foot bones andmetatarsals, and this was seen as evidence for opportunistic scavenging, rather than wounds caused by intraspecific combat. In a fight, they proposed it would be difficult to reach down to bite in the feet of a rival, making it more likely that the bitemarks were made in a carcass. As the bitemarks were made in body parts with relatively scantly amounts of flesh, it is suggested that theTyrannosaurus was feeding on a cadaver in which the more fleshy parts already had been consumed. They were also open to the possibility that othertyrannosaurids practiced cannibalism.[232]
Parenting
While there is no direct evidence ofTyrannosaurus raising their young (the rarity of juvenile and nest Tyrannosaur fossils has left researchers guessing), it has been suggested by some that like its closest living relatives, modern archosaurs (birds and crocodiles)Tyrannosaurus may have protected and fed its young. Crocodilians and birds are often suggested by some paleontologists to be modern analogues for dinosaur parenting.[234] Direct evidence of parental behavior exists in other dinosaurs such asMaiasaura peeblesorum, the first dinosaur to have been discovered to raise its young, as well as more closely relatedOviraptorids, the latter suggesting parental behavior in theropods.[235][236][237][238][239]
Pathology
Restoration of an individual (based onMOR 980) with parasite infections
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. Of the 81Tyrannosaurus foot bones examined in the study, one was found to have a stress fracture, while none of the 10 hand bones were found to have stress fractures. The researchers found tendon avulsions only amongTyrannosaurus andAllosaurus. An avulsion injury left a divot on the humerus of Sue theT. rex, apparently located at the origin of thedeltoid orteres major muscles. The presence of avulsion injuries being limited to the forelimb and shoulder in bothTyrannosaurus andAllosaurus suggests that theropods may have had a musculature more complex than and functionally different from those of birds. The researchers concluded that Sue's tendon avulsion was probably obtained from struggling prey. The presence of stress fractures and tendon avulsions, in general, provides evidence for a "very active" predation-based diet rather than obligate scavenging.[240]
A 2009 study showed that smooth-edged holes in the skulls of several specimens might have been caused byTrichomonas-like parasites that commonly infectbirds. According to the study, seriously infected individuals, including "Sue" and MOR 980 ("Peck's Rex"), might therefore have died from starvation after feeding became increasingly difficult. Previously, these holes had been explained by the bacterious bone infectionActinomycosis or by intraspecific attacks.[241] A subsequent study found that while trichomoniasis has many attributes of the model proposed (osteolytic, intra oral) several features make the assumption that it was the cause of death less supportable by evidence. For example, the observed sharp margins with little reactive bone shown by the radiographs ofTrichomonas-infected birds are dissimilar to the reactive bone seen in the affectedT. rex specimens. Also, trichomoniasis can be very rapidly fatal in birds (14 days or less) albeit in its milder form, and this suggests that if aTrichomonas-like protozoan is the culprit, trichomoniasis was less acute in its non-avian dinosaur form during the Late Cretaceous. Finally, the relative size of this type of lesions is much larger in small bird throats, and may not have been enough to choke aT. rex.[242] A more recent study examining the pathologies concluded that the osseous alteration observed most closely resembles those around healing human cranial trepanations and healing fractures in the Triassic reptileStagonolepis, in the absence of infection. The possible cause may instead have been intraspecific combat.[243]
One study ofTyrannosaurus specimens with tooth marks in the bones attributable to the same genus was presented as evidence ofcannibalism.[232] Tooth marks in thehumerus, foot bones andmetatarsals, may indicate opportunistic scavenging, rather than wounds caused by combat with anotherT. rex.[232][244] Othertyrannosaurids may also have practiced cannibalism.[232]
Paleoecology
Fauna of Hell Creek (Tyrannosaurus in dark red, left).
Tyrannosaurus lived during what is referred to as theLancian faunal stage (Maastrichtian age) at the end of the Late Cretaceous.Tyrannosaurus ranged fromCanada in the north to at least New Mexico in the south ofLaramidia.[5] During this timeTriceratops was the major herbivore in the northern portion of its range, while thetitanosauriansauropodAlamosaurus "dominated" its southern range.Tyrannosaurus remains have been discovered in different ecosystems, including inland and coastal subtropical, and semi-arid plains.
Tyrannosaurus and other animals of the Hell Creek Formation
Another formation withTyrannosaurus remains is theLance Formation of Wyoming. This has been interpreted as abayou environment similar to today's Gulf Coast. The fauna was very similar to Hell Creek, but withStruthiomimus replacing its relativeOrnithomimus. The small ceratopsianLeptoceratops also lived in the area.[255]
Tyrannosaurus may have also inhabited Mexico'sLomas Coloradas Formation in Sonora. Though skeletal evidence is lacking, six shed and broken teeth from the fossil bed have been thoroughly compared with other theropod genera and appear to be identical to those ofTyrannosaurus. If true, the evidence indicates the range ofTyrannosaurus was possibly more extensive than previously believed.[262] It is possible that tyrannosaurs were originally Asian species, migrating to North America before the end of the Cretaceous period.[263]
Population estimates
Chart of the time-averaged census for large-bodied dinosaurs from the entire Hell Creek Formation in the study area
According to studies published in 2021 by Charles Marshall et al., the total population of adultTyrannosaurus at any given time was perhaps 20,000 individuals, with computer estimations also suggesting a total population no lower than 1,300 and no higher than 328,000. The authors themselves suggest that the estimate of 20,000 individuals is probably lower than what should be expected, especially when factoring in that disease pandemics could easily wipe out such a small population. Over the span of the genus' existence, it is estimated that there were about 127,000 generations and that this added up to a total of roughly 2.5 billion animals until their extinction.[264][265]
In the same paper, it is suggested that in a population ofTyrannosaurus adults numbering 20,000, the number of individuals living in an area the size of California could be as high as 3,800 animals, while an area the size of Washington D.C. could support a population of only two adultTyrannosaurus. The study does not take into account the number of juvenile animals in the genus present in this population estimate due to their occupation of a different niche than the adults, and thus it is likely the total population was much higher when accounting for this factor. Simultaneously, studies of living carnivores suggest that some predator populations are higher in density than others of similar weight (such as jaguars and hyenas, which are similar in weight but have vastly differing population densities). Lastly, the study suggests that in most cases, only one in 80 millionTyrannosaurus would become fossilized, while the chances were likely as high as one in every 16,000 of an individual becoming fossilized in areas that had more dense populations.[264][265]
Meiri (2022) questioned the reliability of the estimates, citing uncertainty in metabolic rate, body size, sex and age-specific survival rates, habitat requirements and range size variability as shortcomings Marshall et al. did not take into account.[266] The authors of the original publication replied that while they agree that their reported uncertainties were probably too small, their framework is flexible enough to accommodate uncerainty in physiology, and that their calculations do not depend on short-term changes in population density and geographic range, but rather on their long-term averages. Finally, they remark that they did estimate the range of reasonable survivorship curves and that they did include uncertainty in the time of onset of sexual maturity and in the growth curve by incorporating theuncertainty in the maximum body mass.[267]
Since it was first described in 1905,T. rex has become the most widely recognized dinosaur species inpopular culture. It is the only dinosaur that is commonly known to the general public by its full scientific name (binomial name) and the scientific abbreviationT. rex has also come into wide usage.[51]Robert T. Bakker notes this inThe Dinosaur Heresies and explains that, "a name like'T. rex' is just irresistible to the tongue."[38]
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