| Tyrannosaurids | |
|---|---|
 | |
| Montage of six tyrannosaurids, clockwise from top left:Tyrannosaurus,Daspletosaurus,Tarbosaurus,Gorgosaurus,Zhuchengtyrannus, andAlioramus | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Superfamily: | †Tyrannosauroidea |
| Clade: | †Pantyrannosauria |
| Clade: | †Eutyrannosauria |
| Family: | †Tyrannosauridae Osborn, 1906 |
| Type genus | |
| †Tyrannosaurus Osborn, 1905 | |
| Subgroups[1] | |
| Synonyms | |
Tyrannosauridae (ortyrannosaurids, meaning "tyrant lizards") is afamily ofcoelurosauriantheropoddinosaurs that comprises two subfamilies containing up to fifteengenera, including theeponymousTyrannosaurus. The exact number of genera is controversial, with some experts recognizing as few as three. All of these animals lived near the end of theCretaceousPeriod and theirfossils have been found only inNorth America andAsia.
Although descended fromsmaller ancestors, tyrannosaurids were almost always the largestpredators in their respectiveecosystems, putting them at theapex of thefood chain. The largestspecies wasTyrannosaurus rex, the most massive known terrestrial predator, which measured over 13 metres (43 ft) in length[2] and according to most modern estimates up to 8.87 metric tons (9.78 short tons) in weight.[3][4] Tyrannosaurids werebipedal carnivores with massiveskulls filled with large teeth. Despite their large size, their legs were long and proportioned for fast movement. In contrast, their arms were very small, bearing only two functionaldigits.
Unlike most other groups of dinosaurs, very complete remains have been discovered for most known tyrannosaurids. This has allowed a variety of research into theirbiology. Scientific studies have focused on theirontogeny,biomechanics andecology, among other subjects.
The first remains of tyrannosaurids were uncovered during expeditions led by theGeological Survey of Canada, which located numerous scattered teeth. These distinctive dinosaur teeth were given the nameDeinodon ("terrible tooth") byJoseph Leidy in 1856. The first good specimens of a tyrannosaurid were found in theHorseshoe Canyon Formation ofAlberta, and consisted of nearly complete skulls with partial skeletons. These remains were first studied byEdward Drinker Cope in 1876, who considered them a species of the eastern tyrannosauroidDryptosaurus. In 1905,Henry Fairfield Osborn recognized that the Alberta remains differed considerably fromDryptosaurus, and coined a new name for them:Albertosaurus sarcophagus ("flesh-eating Alberta lizard").[5] Cope described more tyrannosaur material in 1892, in the form of isolated vertebrae, and gave this animal the nameManospondylus gigas. This discovery was mostly overlooked for over a century, and caused controversy in the early 2000s when it was discovered that this material actually belonged to, and had name priority over,Tyrannosaurus rex.[6]
In his 1905 paper namingAlbertosaurus, Osborn described two additional tyrannosaur specimens that had been collected inMontana andWyoming during a 1902 expedition of theAmerican Museum of Natural History, led byBarnum Brown. Initially, Osborn considered these to be distinct species. The first, he namedDynamosaurus imperiosus ("emperor power lizard"), and the second,Tyrannosaurus rex ("king tyrant lizard"). A year later, Osborn recognized that these two specimens actually came from the same species. Despite the fact thatDynamosaurus had been found first, the nameTyrannosaurus had appeared one page earlier in his original article describing both specimens. Therefore, according to theInternational Code of Zoological Nomenclature (ICZN), the nameTyrannosaurus was used.[7]
Barnum Brown went on to collect several more tyrannosaurid specimens from Alberta, including the first to preserve the shortened, two-fingered forelimbs characteristic of the group (whichLawrence Lambe namedGorgosaurus libratus, "balanced fierce lizard", in 1914). A second significant find attributed toGorgosaurus was made in 1942, in the form of a well-preserved, though unusually small, complete skull. The specimen waited until after the end ofWorld War II to be studied byCharles W. Gilmore, who named itGorgosaurus lancesnis.[5] This skull was re-studied byRobert T. Bakker,Phil Currie, and Michael Williams in 1988, and assigned to the new genusNanotyrannus.[8] It was also in 1946 that paleontologists from theSoviet Union began expeditions intoMongolia, and uncovered the first tyrannosaur remains from Asia.Evgeny Maleev described new Mongolian species ofTyrannosaurus andGorgosaurus in 1955, and one new genus:Tarbosaurus ("terrifying lizard"). Subsequent studies, however, showed that all of Maleev's tyrannosaur species were actually one species ofTarbosaurus at different stages of growth. A second species of Mongolian tyrannosaurid was found later, described by Sergei Kurzanov in 1976, and given the nameAlioramus remotus ("remote different branch"), though its status as a true tyrannosaurid and not a more primitive tyrannosaur is still controversial.[9][5]
The tyrannosaurids were all large animals, with all species capable of weighing at least 1 metric ton.[10] A single specimen ofAlioramus of an individual estimated at between 5 and 6 metres (16 and 20 ft) long has been discovered,[9] although it is considered by some experts to be a juvenile.[10][11]Albertosaurus,Gorgosaurus andDaspletosaurus all measured between 8 and 9 metres (26 and 30 ft) long,[12] whileTarbosaurus reached lengths of 11 metres (36 ft) from snout to tail.[13] The massiveTyrannosaurus reached 13 metres (43 ft) in the largest specimen,RSM P2523.8.[14]
_of_Daspletosaurus_horneri.jpg)
Tyrannosaurid skull anatomy is well understood, as complete skulls are known for all genera butAlioramus, which is known only from partial skull remains.[15]Tyrannosaurus,Tarbosaurus, andDaspletosaurus had skulls that exceeded 1 m (3.3 ft) in length.[12] Adult tyrannosaurids had tall, massive skulls, with many bones fused and reinforced for strength. At the same time, hollow chambers within many skull bones and large openings (fenestrae) between those bones helped to reduce skull weight. Many features of tyrannosaurid skulls were also found in their immediate ancestors, including tallpremaxillae and fusednasal bones.[10]
Tyrannosaurid skulls had many unique characteristics, including fusedparietal bones with a prominentsagittal crest, which ranlongitudinally along thesagittal suture and separated the two supratemporal fenestrae on the skull roof. Behind these fenestrae, tyrannosaurids had a characteristically tall nuchal crest, which also arose from the parietals but ran along atransverse plane rather than longitudinally. The nuchal crest was especially well-developed inTyrannosaurus,Tarbosaurus andAlioramus.Albertosaurus,Daspletosaurus andGorgosaurus had tall crests in front of the eyes on thelacrimal bones, whileTarbosaurus andTyrannosaurus had extremely thickenedpostorbital bones forming crescent-shaped crests behind the eyes.Alioramus had a row of six bony crests on top of its snout, arising from the nasal bones; lower crests have been reported on some specimens ofDaspletosaurus andTarbosaurus, as well as the morebasal tyrannosauroidAppalachiosaurus.[11][16] The snout and other parts of the skull also sported numerousforamina. According to the 2017 study which describedD. horneri, scaly integument as well as tactile sensitivity was correlated with the multiple rows of neurovascular foramina seen incrocodilians and tyrannosaurids.[17]
Tyrannosaurids, like their tyrannosauroid ancestors, wereheterodonts, with premaxillary teethD-shaped incross section and smaller than the rest. Unlike earlier tyrannosauroids and most other theropods, themaxillary andmandibular teeth of mature tyrannosaurids are not blade-like but extremely thickened and often circular in cross-section, with some species having reduced serrations.[10] Tooth counts tend to be consistent within species, and larger species tend to have lower tooth counts than smaller ones. For example,Alioramus had 76 to 78 teeth in its jaws, whileTyrannosaurus had between 54 and 60.[18]
William Abler observed in 2001 thatAlbertosaurustooth serrations resemble acrack in the tooth ending in a round void called an ampulla.[19]Tyrannosaurid teeth were used as holdfasts for pullingmeat off a body, so when a tyrannosaur would have pulled back on a piece of meat, the tension could cause a purely crack-like serration to spread through the tooth.[19] However, the presence of the ampulla would have distributed these forces over a largersurface area, and lessened the risk of damage to the tooth under strain.[19] The presence of incisions ending in voids has parallels in human engineering.Guitar makers use incisions ending in voids to, as Abler describes, "impart alternating regions of flexibility and rigidity" to the wood they work with.[19] The use of adrill to create an "ampulla" of sorts and prevent the propagation of cracks through material is also used to protectairplane surfaces.[19] Abler demonstrated that a plexiglass bar with incisions called "kerfs" and drilled holes was more than 25% stronger than one with only regularly placed incisions.[19] Unlike tyrannosaurs and other theropods, ancient predators likephytosaurs andDimetrodon had no adaptations to prevent the crack-like serrations of their teeth from spreading when subjected to the forces of feeding.[19]
The skull was perched at the end of a thick,S-shaped neck, and a long, heavy tail acted as acounterweight to balance out the head and torso, with thecenter of mass over the hips. Tyrannosaurids are known for their proportionately very small two-fingered forelimbs, although remnants of avestigial third digit are sometimes found.[10][20]Tarbosaurus had the shortest forelimbs compared to its body size, whileDaspletosaurus had the longest.
Tyrannosaurids walked exclusively on their hindlimbs, so their leg bones were massive. In contrast to the forelimbs, the hindlimbs were longer compared to body size than almost any other theropods. Juveniles and even some smaller adults, like more basal tyrannosauroids, had longertibiae thanfemora, a characteristic ofcursorial (fast-running) dinosaurs likeornithomimids. Larger adults had leg proportions characteristic of slower-moving animals, but not to the extent seen in other large theropods likeabelisaurids orcarnosaurs. The thirdmetatarsals of tyrannosaurids were pinched between the second and fourth metatarsals, forming a structure known as thearctometatarsus.[10] Tyrannosaurids also had large preserved leg muscle attachments and low rotationalinertia relative to their body mass, indicating that they could turn more quickly than other large theropods.[21][22]
It is unclear when the arctometatarsus first evolved; it was not present in the earliest tyrannosauroids likeDilong,[23] but was found in the laterAppalachiosaurus.[16] This structure also characterizedtroodontids,ornithomimids andcaenagnathids,[24] but its absence in the earliest tyrannosauroids indicates that it was acquired byconvergent evolution.[23]
A comparative analysis of the teeth suggests that tyrannosaurids, as well as the other large theropods, had lips that protected their teeth from external damage. This anatomical feature gave these animals more visual resemblance tolepidosaurs than to closely related crocodilians.[25]
The nameDeinodontidae was coined byEdward Drinker Cope in 1866 for this family,[26] and continued to be used in place of the newer name Tyrannosauridae through the 1960s.[27] The type genus of the Deinodontidae isDeinodon, which was named after isolated teeth fromMontana.[28] However, in a 1970 review of North American tyrannosaurs,Dale Russell concluded thatDeinodon was not a valid taxon, and used the name Tyrannosauridae in place of Deinodontidae, stating that this was in accordance withICZN rules.[12] Therefore, Tyrannosauridae is preferred by modern experts.[5]
Tyrannosaurus was named byHenry Fairfield Osborn in 1905, along with the family Tyrannosauridae.[29] The name is derived from theAncient Greek wordsτυραννος (tyrannos) ('tyrant') andσαυρος (sauros) ('lizard'). The very commonsuffix-idae is normally appended tozoological family names and is derived from the Greek suffix-ιδαι-idai, which indicates a plural noun.[30]
Tyrannosauridae is afamily in rank-basedLinnaean taxonomy, within thesuperfamilyTyrannosauroidea and thesuborderTheropoda.
Tyrannosauridae is uncontroversially divided into two subfamilies.Albertosaurinae comprises the North American generaAlbertosaurus andGorgosaurus, whileTyrannosaurinae includesDaspletosaurus,Teratophoneus,Bistahieversor,Tarbosaurus,Nanuqsaurus,Zhuchengtyrannus, andTyrannosaurus itself.[31] Some authors include the speciesGorgosaurus libratus in the genusAlbertosaurus andTarbosaurus bataar in the genusTyrannosaurus,[16][5][32] while others prefer to retainGorgosaurus andTarbosaurus as separate genera.[10][11] Albertosaurines are characterized by more slender builds, lower skulls, and proportionately longertibiae than tyrannosaurines.[10] In tyrannosaurines, the sagittal crest on the parietals continues forward onto the frontals.[11] In 2014,Lü Junchanget al. described theAlioramini as atribe within the Tyrannosauridae containing the generaAlioramus andQianzhousaurus. Their phylogenetic analysis indicated that the tribe was located at the base of the Tyrannosaurinae.[33][34] Some authors, such as George Olshevsky and Tracy Ford, have created other subdivisions or tribes for various combinations of tyrannosaurids within the subfamilies.[35][36] However, these have not been phylogenetically defined, and usually consisted of genera that are now considered synonymous with other genera or species.[18]
Additional subfamilies have been named for more fragmentary genera, includingAublysodontinae andDeinodontinae. However, the generaAublysodon andDeinodon are usually considerednomina dubia, so they and their eponymous subfamilies are usually excluded from taxonomies of tyrannosaurids. An additional tyrannosaurid,Raptorex, was initially described as a more primitive tyrannosauroid, but likely represents a juvenile tyrannosaurine similar toTarbosaurus. However, as it is known only from a juvenile specimen, it is also currently considered anomen dubium.[37] However, Thomas Carr maintains its validity and finds it to be similar to tyrannosaurines.[38]
With the advent ofphylogenetic taxonomy in vertebrate paleontology, Tyrannosauridae has been given several explicit definitions. The original was produced byPaul Sereno in 1998, and included all tyrannosauroids closer to Tyrannosaurus than to eitherAlectrosaurus,Aublysodon orNanotyrannus.[39] However,Nanotyrannus is often considered to be a juvenileTyrannosaurus rex, whileAublysodon is usually regarded as anomen dubium unsuitable for use in the definition of aclade.[10] Definitions since then have been based on more well-established genera.
In 2001,Thomas R. Holtz Jr. published acladistic analysis of the Tyrannosauridae.[40] He concluded that there were twosubfamilies: the more primitiveAublysodontinae, characterized byunserratedpremaxillaryteeth; and theTyrannosaurinae.[40] The Aublysodontinae includedAublysodon, the "KirtlandAublysodon", andAlectrosaurus.[40] Holtz also found thatSiamotyrannus exhibited some of thesynapomorphies of the tyrannosauridae, but lay "outside the [family] proper."[40]
Later in the same paper, he proposed that Tyrannosauridae be defined as "all descendants of the most recent common ancestor ofTyrannosaurus andAublysodon".[40] He also criticized definitions previously proposed by other workers, like one proposed byPaul Sereno, that the Tyrannosauridae was "all taxa closer to "Tyrannosaurus" than toAlectrosaurus,Aublysodon, andNanotyrannus".[40] Holtz observed that sinceNanotyrannus was probably a misidentifiedT. rex juvenile, Sereno's proposed definition would have the family Tyrannosauridae as a subtaxon of the genusTyrannosaurus.[40] Further, his proposed definition of the subfamily Tyrannosaurinae would also be limited toTyrannosaurus.[40]
A 2003 attempt byChristopher Brochu includedAlbertosaurus,Alectrosaurus,Alioramus,Daspletosaurus,Gorgosaurus,Tarbosaurus andTyrannosaurus in the definition.[41] Holtz redefined the clade in 2004 to use all of the above as specifiers except forAlioramus andAlectrosaurus, which his analysis could not place with certainty. However, in the same paper, Holtz also provided a completely different definition, including all theropods more closely related toTyrannosaurus than toEotyrannus.[10] The most recent definition is that of Sereno in 2005, which defined Tyrannosauridae as the least inclusive clade containingAlbertosaurus,Gorgosaurus andTyrannosaurus.[42]
Cladistic analyses of tyrannosauridphylogeny often findTarbosaurus andTyrannosaurus to besister taxa, withDaspletosaurus more basal than either. A close relationship betweenTarbosaurus andTyrannosaurus is supported by numerous skull features, including the pattern ofsutures between certain bones, the presence of a crescent-shaped crest on the postorbital bone behind each eye, and a very deep maxilla with a noticeable downward curve on the lower edge, among others.[10][16] An alternativehypothesis was presented in a 2003 study byPhil Currie and colleagues, which found weak support forDaspletosaurus as a basal member of aclade also includingTarbosaurus andAlioramus, both from Asia, based on the absence of a bony prong connecting the nasal and lacrimal bones.[18]Alioramus was found to be the closest relative ofTarbosaurus in this study, based on a similar pattern of stress distribution in the skull.
A related study also noted a locking mechanism in the lower jaw shared between the two genera.[43] In a separate paper, Currie noted the possibility thatAlioramus might represent a juvenileTarbosaurus, but stated that the much higher tooth count and more prominent nasal crests inAlioramus suggest it is a distinct genus. Similarly, Currie uses the high tooth count ofNanotyrannus to suggest that it may be a distinct genus,[11] rather than a juvenileTyrannosaurus as most other experts believe.[10][44] However, the discovery and description ofQianzhousaurus reveals thatAlioramus is not a close relation toTarbosaurus, instead belonging to a newly described tribe of tyrannosaurids; the Alioramini.Qianzhousaurus further reveals that similar long-snouted tyrannosaurids were widely distributed throughout Asia and would have shared the same environment while avoiding competition with larger and more robust tyrannosaurines by hunting different prey.[45]
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PaleontologistGregory Erickson and colleagues have studied the growth and life history of tyrannosaurids. Analysis of bonehistology can determine the age of a specimen when it died. Growth rates can be examined when the age of various individuals are plotted against their size on a graph. Erickson has shown that after a long time as juveniles, tyrannosaurs underwent tremendousgrowth spurts for about four years midway through their lives. After the rapid growth phase ended withsexual maturity, growth slowed down considerably in adult animals. A tyrannosaurid growth curve is S-shaped, with the maximum growth rate of individuals around 14 years of age.[49]
The smallest knownTyrannosaurus rex individual (LACM 28471, the "Jordan theropod") is estimated to have weighed only 29.9 kilograms (66 lb) at only 2 years old, while the largest, such asFMNH PR2081 ("Sue"), most likely weighed about 5,654 kg (12,465 lb), estimated to have been 28 years old, an age which may have been close to the maximum for the species.[49]T. rex juveniles remained 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. This slowed after 16 years, and at 18 years of age, the curve plateaus again, indicating that growth slowed dramatically.[50] For example, only 600 kg (1,300 lb) separated the 28-year-old "Sue" from a 22-year-oldCanadian specimen (RTMP 81.12.1).[49] This sudden change in growth rate may indicate physical maturity, a hypothesis that is supported by the discovery of medullary tissue in thefemur of an 18-year-oldT. rex from Montana (MOR 1125, also known as "B-rex").[51] Medullary tissue is found only in female birds during ovulation, indicating that "B-rex" was of reproductive age.[52]
Other tyrannosaurids exhibit extremely similar growth curves, although with lower growth rates corresponding to their lower adult sizes.[53] Compared to albertosaurines,Daspletosaurus showed a faster growth rate during the rapid growth period due to its higher adult weight. The maximum growth rate inDaspletosaurus was 180 kilograms (400 lb) per year, based on a mass estimate of 1,800 kg (4,000 lb) in adults. Other authors have suggested higher adult weights forDaspletosaurus; this would change the magnitude of the growth rate, but not the overall pattern.[49] The youngest knownAlbertosaurus is a two-year-old discovered in the Dry Island bonebed, which would have weighed about 50 kg (110 lb) and measured slightly more than 2 metres (6.6 ft) in length. The 10-metre (33 ft) specimen from the same quarry is the oldest and largest known, at 28 years of age. The fastest growth rate is estimated to occur around 12–16 years of age, reaching 122 kg (269 lb) per year, based on a 1,300 kg (2,900 lb) adult, which is about a fifth of the rate forT.-rex. ForGorgosaurus, the calculated maximum growth rate is about 110 kilograms (240 lb) during the rapid growth phase, which is comparable to that ofAlbertosaurus.[49]
The discovery of an embryonic tyrannosaur of an as-yet-unknown genus suggests that tyrannosaurids developed their distinctive skeletal features while developing in the egg. Furthermore, the size of the specimen, a 1.1 in (2.8 cm) dentary from the lower jaw found in theTwo Medicine Formation of Montana in 1983 and a foot claw found in theHorseshoe Canyon Formation in 2018 and described in 2020, suggests that neonate tyrannosaurids were born with skulls the size of a mouse or similarly sized rodents and may have been roughly the size of a small dog at birth. The jaw specimen is believed to have come from an animal roughly 2.5 ft (0.76 m) while the claw is believed to belong to a specimen measuring around 3 ft (0.91 m). While eggshells have not been found in association with either specimen, the location where these neonate tyrannosaurids were uncovered suggests these animals were using the same nest sites as other species they lived with and preyed upon.[54] The lack of eggshells associated with these specimens has also opened up speculation to the possibility that tyrannosaurids laid soft-shelled eggs as the generaMussaurus andProtoceratops are believed to have done.[55]
Fossil footprints from the Wapiti Formation suggest that as tyrannosaurids grew, the feet became wider with thicker toes to support their weight. The broader feet suggest that adult tyrannosaurids were slower-moving than their offspring.[56][57]
The end of the rapid growth phase suggests the onset ofsexual maturity inAlbertosaurus, although growth continued at a slower rate throughout the animals' lives.[49][53] Sexual maturation while still actively growing appears to be a shared trait among small[58] and large[59] dinosaurs as well as in large mammals, such as humans andelephants.[59] This pattern of relatively early sexual maturation differs strikingly from the pattern in birds, which delay their sexual maturity until after they have finished growing.[59][60]
By tabulating the number of specimens of each age group, Erickson and his colleagues were able to draw conclusions about life history in tyranosauridae populations. Their analysis showed that while juveniles were rare in the fossil record, subadults in the rapid growth phase and adults were far more common. Over half of the knownT. rex specimens appear to have died within six years of reaching sexual maturity, a pattern that 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. While this could be due to preservation or collectionbiases, Erickson hypothesized that the difference was due to low mortality among juveniles over a certain size, which is also seen in some modern large mammals, likeelephants. This low mortality may have resulted from a lack of predation, since tyrannosaurs surpassed all contemporaneous predators in size by the age of two. Paleontologists have not found enoughDaspletosaurus remains for a similar analysis, but Erickson notes that the same general trend seems to apply.[53]
The tyrannosaurids spent as much as half its life in the juvenile phase before ballooning up to near-maximum size in only a few years.[49] This, along with the complete lack of predators intermediate in size between huge adult tyrannosaurids and other small theropods, suggests these niches may have been filled by juvenile tyrannosaurids. This is seen in modernKomodo dragons, where hatchlings start off as tree-dwellinginsectivores and slowly mature into massiveapex predators capable of taking down large vertebrates.[10] For example,Albertosaurus have been found in aggregations that some have suggested to represent mixed-agepacks.[61][62]
Locomotion abilities are best studied forTyrannosaurus, and there are two main issues concerning this: how well it could turn; and what its maximum straight-line speed was likely to have been.Tyrannosaurus may have been slow to turn, possibly taking one to two seconds to turn only 45° – an amount that humans, being vertically oriented and tail-less, can spin in a fraction of a second.[63] The cause of the difficulty isrotational inertia, since much ofTyrannosaurus's mass was some distance from its center of gravity, like a human carrying a heavy timber.[64]
Scientists have produced a wide range of maximum speed estimates, mostly around 11 metres per second (25 mph), but a few as low as 5–11 metres per second (11–25 mph), and a few as high as 20 metres per second (45 mph). Researchers have to rely on various estimating techniques because, while there are manytracks of very large theropods walking, so far none have been found of very large theropods running—and this absencemay indicate that they did not run.[65]
Jack Horner and Don Lessem argued in 1993 thatTyrannosaurus was slow and probably could not run (no airborne phase in mid-stride).[66] However, Holtz (1998) concluded that tyrannosaurids and their close relatives were the fastest large theropods.[67] Christiansen (1998) estimated that the leg bones ofTyrannosaurus were not significantly stronger than those of elephants, which are relatively limited in their top speed and never actually run (there is no airborne phase), and hence proposed that the dinosaur's maximum speed would have been about 11 metres per second (25 mph), which is about the speed of a human sprinter.[68] Farlow and colleagues (1995) have argued that a 6- to 8-tonTyrannosaurus would have been critically or even fatally injured if it had fallen while moving quickly, since its torso would have slammed into the ground at a deceleration of 6 g (six times the acceleration due to gravity, or about 60 metres/s2) and its tiny arms could not have reduced the impact.[69][70] However,giraffes have been known to gallop at 50 km/h (31 mph), despite the risk that they might break a leg or worse, which can be fatal even in a "safe" environment such as a zoo.[71][72] Thus it is quite possible thatTyrannosaurus also moved fast when necessary and had to accept such risks; this scenario has been studied forAllosaurus too.[73][74] Most recent research onTyrannosaurus locomotion does not narrow down speeds further than a range from 17 to 40 km/h (11 to 25 mph), i.e. from walking or slow running to moderate-speed running.[65][75][76] A computer model study in 2007 estimated running speeds, based on data taken directly from fossils, and claimed thatT. rex had a top running speed of 8 metres per second (18 mph).[77][78] (Probably a juvenile individual.[79])
Studies by Eric Snivelyet al., published in 2019 indicate that tyrannosaurids such asTarbosaurus andTyrannosaurus itself were more manoeuvrable than allosauroids of comparable size due to low rotational inertia compared to their body mass combined with large leg muscles. As a result, it is hypothesized that tyrannosaurids were capable of making relatively quick turns and could likely pivot their bodies more quickly when close to their prey, or that while turning, they could "pirouette" on a single planted foot while the alternating leg was held out in a suspended swing during pursuit. The results of this study potentially could shed light on how agility could have contributed to the success of tyrannosaurid evolution.[80]
Additionally, a 2020 study indicates that tyrannosaurids were exceptionally efficient walkers. Studies by Dececchiet al., compared the leg proportions, body mass, and the gaits of more than 70 species of theropod dinosaurs including tyrannosaurids. 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 of the study further indicated that smaller theropods evolved long legs for speed as a means to both aid in hunting and escape from larger predators while larger predatory 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, tyrannosaurids showed a marked increase in foraging efficiency due to reduced energy expenditures during hunting and scavenging. This 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 such asAlbertosaurus were hunting in group settings.[81][82]
An ongoing debate in the paleontological community surrounds the extent and nature of tyrannosaurid integumentary covering. Longfilamentous structures have been preserved along with skeletal remains of numerous coelurosaurs from the Early CretaceousYixian Formation and other nearbygeological formations fromLiaoning, China.[83] These filaments have usually been interpreted as "protofeathers,"homologous with the branched feathers found in birds andsome non-avian theropods,[84][85] although other hypotheses have been proposed.[86] A skeleton ofDilong was described in 2004 that included the first example of "protofeathers" in a tyrannosauroid. Similarly todown feathers of modern birds, the "protofeathers" found inDilong were branched but notpennaceous, and may have been used forinsulation.[23] The discovery and description of the 9-metre (30 ft) feathered tyrannosauroidYutyrannus in 2012 indicates the possibility large tyrannosaurids were also feathered as adults.[87]
Based on the principle ofphylogenetic bracketing, it was predicted that tyrannosaurids might also possess such feathering. However, a study in 2017 published by a team of researchers in Biology Letters described tyrannosaurid skin impressions collected in Alberta, Montana, and Mongolia, which came from five genera (Tyrannosaurus,Albertosaurus,Gorgosaurus,Daspletosaurus andTarbosaurus).[88] Although the skin impressions are small, they are widely dispersed across the post-cranium, being collectively located on the abdomen, thoracic region, ilium, pelvis, tail, and neck. They show a tight pattern of fine, non-overlapping pebbly scales (which co-author Scott Persons compared to those seen on the flanks of a crocodile[89]) and preserve no hints of feathering. The basic texture is composed of tiny "basement scales" approximately 1 to 2 mm in diameter, with some impressions showing 7 mm "feature scales" interspersed between them. Additional scales can be seen in tyrannosaurid footprints.[90] Studies find that the facial integument of tyrannosaurids had scales on the dentary and maxilla, cornified epidermis and armor-like skin on the subordinate regions.[91][92]
Bellet al. performed an ancestral character reconstruction based on what is known about integument distribution in tyrannosauroids. Despite an 89% probability that tyrannosauroids started out with feathers, they determined that scaly tyrannosaurids have a 97% probability of being true. The data "provides compelling evidence of an entirely squamous covering in Tyrannosaurus," the team wrote, although they conceded that plumage may have still been present on the dorsal region where skin impressions haven't been found yet. Bellet al. hypothesizes that the scale impressions of tyrannosaurids are possibly reticula which are secondarily derived from feathers though evidence is needed to support this.[88] However, others argue that this is because of taphonomic bias in tyrannosaurids.[93]
It has yet to be determined why such an integumentary change might have occurred. A precedent for feather loss can be seen in other dinosaur groups such asornithischians, in which filamentous structures were lost, and scales reappeared.[94] Although gigantism has been suggested as a mechanism, Phil R. Bell, who co-authored the study, noted that the featheredYutyrannus overlapped in size withGorgosaurus andAlbertosaurus. "The problem here is that we have big tyrannosaurs, some with feathers, some without that live in pretty similar climates. So what's the reason for this difference? We really don't know."[95]
The eye-sockets ofTyrannosaurus are positioned so that the eyes would point forward, giving thembinocular vision slightly better than that of modernhawks. While predatory theropods in general had binocular vision directly in front of their skull, tyrannosaurs had a significantly larger area of overlap.Jack Horner also pointed out that the tyrannosaur lineage had a history of steadily improving binocular vision. It is hard to see hownatural 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.[96][97] In modern animals, binocular vision is found mainly in predators (the principal exceptions areprimates, which need it for leaping from branch to branch). UnlikeTyrannosaurus,Tarbosaurus had a narrower skull more typical of other tyrannosaurids in which the eyes faced primarily sideways. All of this suggests thatTarbosaurus relied more on its senses of smell and hearing than on its eyesight.[98] InGorgosaurus specimens, theeye socket was circular rather than oval or keyhole-shaped as in other tyrannosaurid genera.[11] InDaspletosaurus, this was a tall oval, somewhere in between the circular shape seen inGorgosaurus and the 'keyhole' shape ofTyrannosaurus.[10][11][44]
Based on comparisons ofbone texture ofDaspletosaurus with extantcrocodilians, a detailed study in 2017 byThomas D. Carret al. found that tyrannosaurs had large, flatscales on theirsnouts.[99][100]At the center of thesescales were smallkeratinised patches. Incrocodilians, such patches cover bundles ofsensory neurons that can detect mechanical, thermal and chemicalstimuli.[101][102] They proposed thattyrannosaurs probably also had bundles ofsensory neurons under their facialscales and may have used them to identify objects, measure thetemperature of theirnests and gently pick-upeggs andhatchlings.[99]
Bony crests are found on the skulls of many theropods, including many tyrannosaurids.Alioramus, a possible tyrannosaurid from Mongolia, bears a single row of five prominent bony bumps on the nasal bones; a similar row of much lower bumps is present on the skull ofAppalachiosaurus, as well as some specimens ofDaspletosaurus,Albertosaurus, andTarbosaurus.[16] InAlbertosaurus,Gorgosaurus andDaspletosaurus, there is a prominent horn in front of each eye on the lacrimal bone. The lacrimal horn is absent inTarbosaurus andTyrannosaurus, which instead have a crescent-shaped crest behind each eye on thepostorbital bone. These head crests may have been used fordisplay, perhaps for species recognition orcourtship behavior.[10]
Tyrannosaurus, like most dinosaurs, was long thought to have anectothermic ("cold-blooded") reptilianmetabolism but was challenged by scientists likeRobert T. Bakker andJohn Ostrom in the early years of the "Dinosaur Renaissance", beginning in the late 1960s.[103][104]Tyrannosaurus rex itself was claimed to have beenendothermic ("warm-blooded"), implying a very active lifestyle.[105] Since then, several paleontologists have sought to determine the ability ofTyrannosaurus toregulate its bodytemperature. 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.[50] It has been indicated that the temperature difference may have been 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.[106] Later they found similar results inGiganotosaurus specimens, who lived on a different continent and tens of millions of years earlier in time.[107] Even ifTyrannosaurus 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.[108][109][110]
In the Dinosaur Park Formation,Gorgosaurus lived alongside a rarer species of the tyrannosaurineDaspletosaurus. This is one of the few examples of two tyrannosaur genera coexisting. Similarly sized predators in modern predatorguilds are separated into differentecological niches by anatomical, behavioral or geographical differences that limit competition.Niche differentiation between the Dinosaur Park tyrannosaurids is not well understood.[111] In 1970, Dale Russellhypothesized that the more commonGorgosaurus actively hunted fleet-footedhadrosaurs, while the rarer and more troublesomeceratopsians andankylosaurians (horned and heavilyarmoured dinosaurs) were left to the more heavily builtDaspletosaurus.[12] However, a specimen ofDaspletosaurus (OTM 200) from the contemporaneousTwo Medicine Formation of Montana preserves the digested remains of a juvenile hadrosaur in its gut region.[112]Unlike some other groups of dinosaurs, neither genus was more common at higher or lower elevations than the other.[111] However,Gorgosaurus appears more common in northern formations like the Dinosaur Park, with species ofDaspletosaurus more abundant to the south. The same pattern is seen in other groups of dinosaurs. Chasmosaurine ceratopsians and hadrosaurine hadrosaurs are also more common in the Two Medicine Formation of Montana and in southwestern North America during the Campanian, while centrosaurines and lambeosaurines dominate in northern latitudes. Holtz has suggested that this pattern indicates shared ecological preferences between tyrannosaurines, chasmosaurines and hadrosaurines. At the end of the later Maastrichtian stage, tyrannosaurines likeTyrannosaurus rex, hadrosaurines likeEdmontosaurus and chasmosaurines likeTriceratops were widespread throughout western North America, while albertosaurines and centrosaurines became extinct, and lambeosaurines were rare.[10]
There is limited evidence of social behavior among the tyrannosaurids. Researchers reported that a subadult and a juvenile skeleton were found in the same quarry as the "Sue" specimen, which has been used to support the hypothesis that tyrannosaurs may have lived in social groups of some kind.[113] While there is no evidence of gregarious behavior inGorgosaurus,[61][62] there is evidence of some pack behavior forAlbertosaurus andDaspletosaurus.
A young specimen of the Dinosaur ParkDaspletosaurus species (TMP 94.143.1) shows bite marks on the face that were inflicted by another tyrannosaur. The bite marks are healed over, indicating that the animal survived the bite. A full-grown Dinosaur ParkDaspletosaurus (TMP 85.62.1) also exhibits tyrannosaur bite marks, showing that attacks to the face were not limited to younger animals. While it is possible that the bites were attributable to other species, intraspecific aggression, including facial biting, is very common among predators. Facial bites are seen in other tyrannosaurs likeGorgosaurus andTyrannosaurus, as well as in other theropod genera likeSinraptor andSaurornitholestes.Darren Tanke and Phil Currie hypothesize that the bites are due tointraspecific competition for territory or resources, or for dominance within a social group.[61]
Evidence thatDaspletosaurus lived in social groups comes from a bonebed found in the Two Medicine Formation of Montana. The bonebed includes the remains of threeDaspletosaurus, including a large adult, a small juvenile, and another individual of intermediate size. At least five hadrosaurs are preserved at the same location. Geologic evidence indicates that the remains were not brought together byriver currents but that all of the animals were buried simultaneously at the same location. The hadrosaur remains are scattered and bear many marks from tyrannosaur teeth, indicating that theDaspletosaurus were feeding on the hadrosaurs at the time of death. The cause of death is unknown. Currie speculates that the daspletosaurs formed apack, although this cannot be stated with certainty.[62] Other scientists are skeptical of the evidence for social groups inDaspletosaurus and other large theropods;[114] Brian Roach and Daniel Brinkman have suggested thatDaspletosaurus social interaction would have more closely resembled the modernKomodo dragon, where non-cooperative individuals mob carcasses, frequently attacking and evencannibalizing each other in the process.[115]
The Dry Island bonebed discovered by Barnum Brown and his crew contains the remains of 22Albertosaurus, the most individuals found in one locality of any Cretaceous theropod, and the second-most of any large theropod dinosaur behind theAllosaurus assemblage at theCleveland-Lloyd Dinosaur Quarry inUtah. The group seems to be composed of one very old adult; eight adults between 17 and 23 years old; seven sub-adults undergoing their rapid growth phases at between 12 and 16 years old; and six juveniles between the ages of 2 and 11 years, who had not yet reached the growth phase.[53] The near-absence ofherbivore remains and the similar state of preservation between the many individuals at theAlbertosaurus bonebed quarry led Phil Currie to conclude that the locality was not a predator trap like theLa Brea Tar Pits inCalifornia, and that all of the preserved animals died at the same time. Currie claims this as evidence of pack behavior.[116] Other scientists are skeptical, observing that the animals may have been driven together by drought, flood or for other reasons.[53][114][117]
While it generally remains controversial, evidence does exist that supports the theory that at least some tyrannosaurids were social. InBritish Columbia'sWapiti Formation, atrackway composed of the footprints of three individual tyrannosaurids (named as the ichnogenusBellatoripes fredlundi) was discovered by a local outfitter named Aaron Fredlund and described in the journalPLOS One by Richard McCrea et al. An examination of the trackway found no evidence of one trackway being left long after another had been made, further supporting the hypothesis that three individual tyrannosaurs were traveling together as a group. Further research revealed the animals were traveling at a speed of between 3.9 and 5.2 mph (6.3 and 8.4 km/h) and likely had a hip height of around 7 to 9 feet. As three different genera of tyrannosaurids (Gorgosaurus,Daspletosaurus, andAlbertosaurus, respectively) are known from the formation, it is unknown which genus was the maker of the trackway.[118][119][120] Additional evidence in the form of a bone-bed from the Rainbows and Unicorns Quarry in Southern Utah's Kaiparowits Formation described in 2021 attributed toTeratophoneus suggests other tyrannosaurids were also social animals. The fossils, consisting of four or possibly five different animals ranging from 4–22 years of age, suggest a mass mortality event, possibly caused by flooding, or less likely bycyanobacterial toxicosis, fire, or drought. The fact that all of the animals preserved seemed to have perished within a short timespan further strengthens the argument for gregarious behavior in tyrannosaurids, with the bone beds of such genera asTeratophoneus,Albertosaurus,Tyrannosaurus andDaspletosaurus showcasing suggested social behavior may have been widespread amongst tyrannosauridae in general.[121][122][123]
Tyrannosaur tooth marks are the most commonly preserved feeding traces of carnivorous dinosaurs.[124] They have been reported fromceratopsians,hadrosaurs and other tyrannosaurs.[124]Tyrannosaurid bones with tooth marks represent about 2% of known fossils with preserved tooth marks.[124] Tyrannosaurid teeth were used as holdfasts for pullingmeat off a body, rather thanknife-like cutting functions.[125] Tooth wear patterns hint that complex head shaking behaviors may have been involved in tyrannosaur feeding.[125]
Speculation on the pack-hunting habits ofAlbertosaurus were made by a few researchers who suggest that the younger members of the pack may have been responsible for driving their prey towards the adults, who were larger and more powerful, but also slower.[116] Juveniles may also have had different lifestyles than adults, filling predatorniches between those of the enormous adults and the smaller contemporaneous theropods, the largest of which were twoorders of magnitude smaller than an adultAlbertosaurus in mass.[10] However, as the preservation of behavior in the fossil record is exceedingly rare, these ideas cannot readily be tested.Phil Currie speculates that theDaspletosaurus formedpacks to hunt, although this cannot be stated with certainty.[62] There is no evidence of such gregarious behavior inGorgosaurus.[61][62]
The debate about whetherTyrannosaurus was apredator or a purescavenger is as old as the debate about its locomotion. Lambe (1917) described a good skeleton ofTyrannosaurus's close relativeGorgosaurus and concluded that it and therefore alsoTyrannosaurus was a pure scavenger, because theGorgosaurus's teeth showed hardly any wear.[126] This argument is no longer taken seriously, because theropods replaced their teeth quite rapidly. Ever since the first discovery ofTyrannosaurus most scientists have agreed that it was a predator, although like modern large predators it would have been happy to scavenge or steal another predator's kill if it had the opportunity.[127][128]
Notedhadrosaur expertJack Horner is currently the major advocate of the idea thatTyrannosaurus was exclusively a scavenger and did not engage in active hunting at all.[66][129][130] Horner has presented several arguments to support the pure scavenger hypothesis. The presence of largeolfactory bulbs andolfactory nerves suggests a highly developed sense of smell for sniffing out carcasses over great distances. The teeth could crush bone, and therefore could extract as much food (bone marrow) as possible from carcass remnants, usually the least nutritious parts. At least some of its potential prey could move quickly, while evidence suggests that Tyrannosaurus walked instead of ran.[129][131]
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. Tyrannosaur-inflicted damage has been found on skeletons of hadrosaurs andTriceratops that seemed to have survived initial attacks.[132][133][134] Some researchers argue that ifTyrannosaurus were a scavenger, another dinosaur had to be the top predator in the Amerasian Upper Cretaceous. The top prey were the largermarginocephalians andornithopods. The other tyrannosaurids share so many characteristics withTyrannosaurus that only smalldromaeosaurs remain as feasible top predators. In this light, scavenger hypothesis adherents have suggested that the size and power of tyrannosaurs allowed them tosteal kills from smaller predators.[131]
In 2023, a juvenileGorgosaurus with itsin situ stomach contents containing twoCitipes juveniles about a year old intact has been reported from theDinosaur Park Formation. This juvenile would have been 5-7 years old at the time of death, measuring about 4 metres (13 ft) long and weighing around 335 kilograms (739 lb). It is much larger than the twoCitipes juveniles that weigh about 9–12 kilograms (20–26 lb), contrary to the assumption that tyrannosaurids fed on prey of their size once they reached 16–32 kilograms (35–71 lb), indicating that juvenile tyrannosaurids still consumed much smaller prey after exceeding a certain size threshold. The discovery of this specimen indicated that tyrannosaurids probably did not hunt in multigenerational packs, since its prey size is too small to share with the conspecifics. It is also a direct dietary evidence that reinforces the theory of 'ontogenetic dietary shift' for tyrannosaurids, as previously inferred by ecological modeling and anatomical features among different age groups. Only the remains of the hindlimbs and caudal vertebrae of juvenileCitipes were present in the tyrannosaurid's stomach cavity, suggesting that a juvenileGorgosaurus may have had preferential consumption of the muscular hindlimbs.[135]
Evidence also strongly suggests that tyrannosaurids 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.[136] Fossils from theFruitland Formation,Kirtland Formation (both Campanian in age), and Maastichtian-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.[137]
The earliest known tyranosaur remains occurred between 129.4 and 125 million years ago in Ishikawa, Japan near Lat 36.166668lng 136.633331. Found byMarsh in 1881 and identified by H.F. Osborne in 1906, the age of the remains was determined by H. Matsuoka et al. in 2002.
While earlier tyrannosauroids are found on all three northern continents, tyrannosaurid fossils are known only from North America and Asia. Sometimes fragmentary remains uncovered in the Southern Hemisphere have been reported as "Southern Hemisphere tyrannosaurids," although these seem to have been misidentifiedabelisaurid fossils.[138]
Tyrannosaurid remains have never been recovered from eastern North America, while more basal tyrannosauroids, likeDryptosaurus andAppalachiosaurus, persisted there until the end of the Cretaceous, indicating that tyrannosaurids must have evolved in ordispersed into western North America after the continent was divided in half by theWestern Interior Seaway in the middle of the Cretaceous.[16] Tyrannosaurid fossils have been found inAlaska, which may have provided a route for dispersal between North America and Asia.[139]Alioramus andTarbosaurus are found to be related in one cladistic analysis, forming a unique Asian branch of the family.[18] This was later disproven with the discovery ofQianzhousaurus and the description of the tyrannosaur family Alioramini. Tyrannosaurid teeth from a large species of unknown variety were discovered in the Nagasaki Peninsula by researchers from the Fukui Prefectural Dinosaur Museum, further expanding the range of the group. The teeth were estimated to be 81 million years old (Campanian Age).[140]
Of the two subfamilies, tyrannosaurines appear to have been more widespread. Albertosaurines are unknown in Asia, which was home to the tyrannosaurines, such asTarbosaurus andZhuchengtyrannus, andQianzhousaurus andAlioramus of theAlioramini. Both the Tyrannosaurinae and Albertosaurinae subfamilies were present in the Campanian and earlyMaastrichtian stages of North America, with tyrannosaurines likeDaspletosaurus ranging throughout the Western Interior, while the albertosaurinesAlbertosaurus andGorgosaurus are currently known only from the northwestern part of the continent.[141]
By the late Maastrichtian, albertosaurines appear to have gone extinct, while the tyrannosaurineTyrannosaurus roamed fromSaskatchewan toTexas. This pattern is mirrored in other North American dinosaur taxa. During the Campanian and early Maastrichtian,lambeosaurinehadrosaurs andcentrosaurineceratopsians are common in the northwest, whilehadrosaurines andchasmosaurines were more common to the south. By the end of the Cretaceous, centrosaurines are unknown and lambeosaurines are rare, while hadrosaurines and chasmosaurines were common throughout the Western Interior.[10] A study published in the journalScientific Reports on February 2, 2016, by Steve Brusatte, Thomas Carret al. indicates that during the later Maastrichtian,Tyrannosaurus itself might have been partially responsible for the extinction of the other tyrannosaurids in most of western North America. The study indicates thatTyrannosaurus might have been an immigrant from Asia as opposed to having evolved in North America (possibly a descendant of the closely relatedTarbosaurus) that supplanted and outcompeted other tyrannosaurids. This theory is further supported by the fact that few to no other types of tyrannosaurid are found withinTyrannosaurus' known range.[142]
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