Tyrannosaurus

Tyrannosaurus Temporal range: Ma PreꞒ Ꞓ O S D C P T J K Pg N
Reconstruction of the T. rex type specimen at the Carnegie Museum of Natural History
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Dinosauria
Clade:	Saurischia
Clade:	Theropoda
Family:	†Tyrannosauridae
Subfamily:	†Tyrannosaurinae
Clade:	† Tyrannosaurini
Genus:	†Tyrannosaurus Osborn, 1905
Type species
†Tyrannosaurus rex Osborn, 1905
Other species
T. mcraeensis Dalman et al., 2024 See text
Synonyms
Genus synonymy Dinotyrannus Olshevsky, 1995 Dynamosaurus Osborn, 1905 Manospondylus Cope, 1892 Nanotyrannus Currie , 1988 Stygivenator Olshevsky, 1995 Tarbosaurus? Maleev, 1955b Species synonymy Aublysodon amplus? Marsh , 1892 Deinodon amplus ? (Marsh, 1892) Hay, 1902 Manospondylus amplus ? (Marsh, 1892) Olshevsky, 1978 Stygivenator amplus ? (Marsh, 1892) Olshevsky, 1995 Tyrannosaurus amplus ? (Marsh, 1892) Hay, 1930 Aublysodon cristatus ? Marsh, 1892 Deinodon cristatus ? (Marsh, 1892) Hay, 1902 Stygivenator cristatus ? (Marsh, 1892) Olshevsky, 1995 Manospondylus gigas Cope, 1892 Dynamosaurus imperiosus Osborn, 1905 Tyrannosaurus imperiosus (Osborn, 1905) Swinton, 1970 Gorgosaurus lancensis Gilmore, 1946 Albertosaurus lancensis (Gilmore, 1946) Russell, 1970 Deinodon lancensis (Gilmore, 1946) Kuhn, 1965 Aublysodon lancensis (Gilmore, 1946) Charig in Appleby, Charig, Cox, Kermack & Tarlo, 1967 Nanotyrannus lancensis (Gilmore, 1946) Bakker, Williams & Currie, 1988 Albertosaurus "megagracilis" Paul, 1988a (nomen nudum) Dinotyrannus megagracilis Olshevsky, 1995 Aublysodon molnaris Paul, 1988a Aublysodon molnari Paul, 1988a emend Paul, 1990 Stygivenator molnari (Paul, 1988a emend Paul, 1990) Olshevsky, 1995

Tyrannosaurus (

Earliest finds

A tooth from what is now documented as a Tyrannosaurus rex was found in July 1874 upon South Table Mountain (Colorado) by Jarvis Hall (Colorado) student Peter T. Dotson under the auspices of Prof. Arthur Lakes near Golden, Colorado.^[1] In the early 1890s, John Bell Hatcher collected postcranial elements in eastern Wyoming. The fossils were believed to be from the large species Ornithomimus grandis (now Deinodon) but are now considered T. rex remains.^[2]

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 them Manospondylus gigas, meaning "giant porous vertebra", in reference to the numerous openings for blood vessels he found in the bone.^[2] The M. gigas remains were, in 1907, identified by Hatcher as those of a theropod rather than a ceratopsid.^[3]

Osborn named the other specimen Dynamosaurus imperiosus in a paper in 1905.

From the 1910s through the end of the 1950s, Barnum's discoveries remained the only specimens of Tyrannosaurus, as the Great Depression and wars kept many paleontologists out of the field.^[5]

Resurgent interest

Beginning in the 1960s, there was renewed interest in Tyrannosaurus, 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 the Museum 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 and Stan.^[5]

Sue Hendrickson, an amateur paleontologist, discovered the most complete (approximately 85%) and largest Tyrannosaurus skeleton in the Hell 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 the Field 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 to New 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]

Another Tyrannosaurus, 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 a Tyrannosaurus tooth.^[15]

In 1998, Bucky Derflinger noticed a T. rex toe exposed above ground, making Derflinger, who was 20 years old at the time, the youngest person to discover a Tyrannosaurus. The specimen, dubbed Bucky in honor of its discoverer, was a young adult, 3.0 metres (10 ft) tall and 11 metres (35 ft) long. Bucky is the first Tyrannosaurus to be found that preserved a furcula (wishbone). Bucky is permanently displayed at The Children's Museum of Indianapolis.^[16]

In the summer of 2000, crews organized by

In 2006, Montana State University revealed that it possessed the largest Tyrannosaurus 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]

Footprints

Two isolated fossilized

A second footprint that may have been made by a Tyrannosaurus was first reported in 2007 by British paleontologist Phil Manning, from the Hell 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 by Tyrannosaurus is unclear, though Tyrannosaurus 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 the Maastrichtian stage of the Late Cretaceous and hailing from the Lance Formation were described by Scott Persons, Phil Currie and colleagues in 2016, and are believed to belong to either a juvenile T. rex or the dubious tyrannosaurid Nanotyrannus 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 m (5.1 ft) to 2.06 m (6.8 ft).^[25][26][27] A follow-up paper appeared in 2017, increasing the speed estimations by 50–80%.^[28]

Description

Size

T. rex was one of the largest land carnivores of all time. One of the largest and the most complete specimens, nicknamed Sue (FMNH PR2081), is located at the Field Museum of Natural History in Chicago. Sue measured 12.3–12.4 m (40.4–40.7 ft) long,^[29][30] was 3.66–3.96 meters (12–13 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 metric tons (9.26–9.33 short tons).^[34][35] A specimen nicknamed Scotty (RSM P2523.8), located at the Royal Saskatchewan Museum, is reported to measure 13 m (43 ft) in length. Using a mass estimation technique that extrapolates from the circumference of the femur, Scotty was estimated as the largest known specimen at 8.87 metric tons (9.78 short tons) in body mass.^[34][36]

Not every adult Tyrannosaurus specimen recovered is as big. Historically average adult mass estimates have varied widely over the years, from as low as 4.5 metric tons (5.0 short tons),^[37][38] to more than 7.2 metric tons (7.9 short tons),^[39] with most modern estimates ranging between 5.4 metric tons (6.0 short tons) and 8.0 metric tons (8.8 short tons).^{[29][40][41][42][43]}

Skull

The largest known T. rex skulls measure up to 1.54 meters (5 ft) in length.

The teeth of T. rex displayed marked

Life restoration showing scaly skin with sparse feathering, and lipped jaws

Skeletal reconstruction of specimen "Sue"

The vertebral column of Tyrannosaurus 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.^[50] The neck of T. 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 by air sacs, on their sides.^[50] 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 massive locomotor 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.^[50]

The shoulder 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 the coracoid, which was rounded. Both shoulder blades were connected by a small furcula. The paired breast bones possibly were made of cartilage only.^[50]

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, the

The

Tyrannosaurus is the type genus of the superfamily Tyrannosauroidea, the family Tyrannosauridae, 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 American Daspletosaurus and the Asian Tarbosaurus,^[18][60] both of which have occasionally been synonymized with Tyrannosaurus.^[61]

Tyrannosaurids were once commonly thought to be descendants of earlier large theropods such as

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.[62]

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.^[63]

Many

Tyrannosauridae	Albertosaurinae Gorgosaurus libratus Albertosaurus sarcophagus Tyrannosaurinae Dinosaur Park tyrannosaurid Daspletosaurus torosus Two Medicine tyrannosaurid Teratophoneus curriei Bistahieversor sealeyi Lythronax argestes Tyrannosaurus rex Tarbosaurus bataar Zhuchengtyrannus magnus

In their 2024 description of Tyrannosaurus mcraeensis, Dalman et al. recovered similar results to previous analyses, with Tyrannosaurus as the sister taxon to the clade formed by Tarbosaurus and Zhuchengtyrannus, called the Tyrannosaurini. They also found support for a

	Albertosaurus sarcophagus Gorgosaurus libratus Daspletosaurus horneri Thanatotheristes Daspletosaurus torosus Daspletosaurus wilsoni Teratophoneus Nanuqsaurus Bistahieversor Lythronax Tyrannosaurini Tyrannosaurus mcraeensis Tyrannosaurus rex Zhuchengtyrannus Tarbosaurus

Additional species

In 1955, Soviet

Paul rejected the objections raised by critics, insisting that they are unwilling to consider that Tyrannosaurus might represent more than one species.[83] In a subsequent paper awaiting publication, Paul maintained the conclusion that Tyrannosaurus consists of three species. He pointed out that the criticism of the study naming T. imperator and T. regina only focused on two of the features used to distinguish the two new species (the number of small incisiform teeth and femur robustness), while the original study also compared the robustness of other bones as well (the maxilla, dentary, humerus, ilium and metatarsals). Furthermore, Paul argued that Tyrannosaurus can be separated into three different species based on the shape of knob-like bumps ('postorbital bosses') behind the eyes. Paul also argued that past research concluding that Tyrannosaurus only consists of one species (T. rex) has simply assumed that all Tyrannosaurus skeletons are a single species, and that many new dinosaur species have been named on the basis of fewer differences than he and his colleagues used when proposing T. imperator and T. regina.^[84]

Tyrannosaurus mcraeensis

In 2024, Dalman and colleagues

Lehman and Carpenter (1990) suggested that NMMNH P-3698 belonged to a new tyrannosaurid genus,[87] while Carr and Williamson (2000) disagreed with their claim.^[88] Sullivan and Lucas (2015) argued that there is little evidence to support NMMNH P-3698 as a specimen of Tyrannosaurus rex, so they tentatively classified it as cf. Tyrannosaurus sp.; they also considered that the McRae tyrannosaur lived before Lancian (before 67 million years ago) based on its coexistence with Alamosaurus.^[89]

Dalman et al. (2024) proposed the new name Tyrannosaurus mcraeensis for the holotype (NMMNH P-3698), referencing the McRae Group, the rock layers to which the Hall Lake Formation belongs. These rock layers were estimated to date to between 72.7 and 70.9 Ma, correlating to the latest Campanian or earliest Maastrichtian. This is approximately 5–7 million years before T. rex, which existed at the end of the Maastrichtian. T. mcraeensis was estimated at 12 metres (39 ft) long, which is similar to the size of an adult T. rex. The two are distinguished by characters of the skull. Amongst these, the dentary of T. mcraeensis is proportionately longer and possesses a less prominent chin, and the lower jaw shallower than that of T. 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 with Tarbosaurus and Zhuchengtyrannus.^[67][90]

Nanotyrannus

Other tyrannosaurid fossils found in the same formations as T. rex were originally classified as separate taxa, including Aublysodon and Albertosaurus megagracilis,^[61] the latter being named Dinotyrannus megagracilis in 1995.^[91] These fossils are now universally considered to belong to juvenile T. rex.^[92] 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 of Gorgosaurus (G. lancensis) by Charles W. Gilmore in 1946.^[93] In 1988, the specimen was re-described by Robert 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 named Nanotyrannus (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.^[94] However, In 1999, a detailed analysis by Thomas Carr revealed the specimen to be a juvenile, leading Carr and many other paleontologists to consider it a juvenile T. rex individual.^[95][96]

In 2001, a more complete juvenile tyrannosaur (nicknamed "

In 2016, analysis of limb proportions by Persons and Currie suggested Nanotyrannus specimens to have differing cursoriality levels, potentially separating it from T. rex.^[103] However, paleontologist Manabu Sakomoto has commented that this conclusion may be impacted by low sample size, and the discrepancy does not necessarily reflect taxonomic distinction.^[104] In 2016, Joshua Schmerge argued for Nanotyrannus' validity based on skull features, including a dentary groove in BMRP 2002.4.1's skull. According to Schmerge, as that feature is absent in T. rex and found only in Dryptosaurus and albertosaurines, this suggests Nanotyrannus is a distinct taxon within the Albertosaurinae.^[105] The same year, Carr and colleagues noted that this was not sufficient enough to clarify Nanotyrannus' validity or classification, being a common and ontogenetically variable feature among tyrannosauroids.^[106]

A 2020 study by Holly Woodward and colleagues showed the specimens referred to Nanotyrannus were all ontogenetically immature and found it probable that these specimens belonged to T. rex.^[107] The same year, Carr published a paper on T. 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.^[108] Discussing the paper's results, Carr described how all Nanotyrannus 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 of Tyrannosaurus. Carr concluded that "the 'nanomorphs' are not all that similar to each other and instead form an important bridge in the growth series of T. 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."^[109]

However, a 2024 paper published by Nick Longrich and Evan Thomas Saitta reexamined the holotype and referred specimens of Nanotyrannus. Based on several factors, including differences in morphology, ontogeny, and phylogeny, Longrich and Saitta suggest that Nanotyrannus is a distinct taxon which may fall outside of Tyrannosauridae, based on some of their phylogenetic analyses.^[110]

Paleobiology

Life history

The identification of several specimens as juvenile T. rex has allowed scientists to document

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. A T. 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 young T. 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.^[111]

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".

An additional study published in 2020 by Woodward and colleagues, for the journal Science 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 for Tyrannosaurus 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 that Tyrannosaurus and the dubious genus Nanotyrannus are synonymous, due to analysis of the growth rings in the bones of the two specimens studied.^[116][117]

Over half of the known T. 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 juvenile T. 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 the

The discovery of

A conference abstract published in 2016 posited that theropods such as Tyrannosaurus had their upper teeth covered in lips, instead of bare teeth as seen in crocodilians. This was based on the presence of enamel, which according to the study needs to remain hydrated, an issue not faced by aquatic animals like crocodilians.^[56] 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 just like modern crocodiles.^[54][124] 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.^[125][126]

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 modern

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, or morphs, 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 '

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 between T. rex sexes.[129] 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. As T. rex specimens have been found from Saskatchewan to New 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.^[50]

Only a single Tyrannosaurus specimen has been conclusively shown to belong to a specific sex. Examination of B-rex demonstrated the preservation of soft 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 of calcium for the production of eggshell during ovulation. 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 reproductive hormones like estrogen. This strongly suggests that B-rex was female and that she died during ovulation.^[112] 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 close evolutionary relationship between the two.^[130]

Posture

Like many

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.[137]^[133] 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^[138][139] and as evidence that they were indeed used and acquired wounds,^[140] like the rest of the body.

Arms

When T. rex was first discovered, the

Another possibility is that the forelimbs held struggling prey while it was killed by the tyrannosaur's enormous jaws. This hypothesis may be supported by

The idea that the arms served as weapons when hunting prey have also been proposed by Steven 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.^[145] This was dismissed by Padian, who argued that Stanley based his conclusion on incorrectly estimated forelimb size and range of motion.^[139]

Thermoregulation

Tyrannosaurus, like most dinosaurs, was long thought to have an

Even if T. rex does exhibit evidence of homeothermy, it does not necessarily mean that it was endothermic. Such thermoregulation may also be explained by gigantothermy, as in some living sea turtles.^{[153][154][155]} Similar to contemporary crocodilians, openings (dorsotemporal fenestrae) in the skull roofs of Tyrannosaurus may have aided thermoregulation.^[156]

Soft tissue

In the March 2005 issue of Science, Mary Higby Schweitzer of North Carolina State University and colleagues announced the recovery of soft tissue from the marrow cavity of a fossilized leg bone from a T. 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.^[157] Designated as the Museum of the Rockies specimen 1125, or MOR 1125, the dinosaur was previously excavated from the Hell Creek Formation. Flexible, bifurcating blood vessels and fibrous but elastic bone matrix tissue were recognized. In addition, microstructures resembling blood cells were found inside the matrix and vessels. The structures bear resemblance to ostrich 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.^[158] 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.^[157] Research on some of the tissues involved has suggested that birds are closer relatives to tyrannosaurs than other modern animals.^[159]

In studies reported in Science in April 2007, Asara and colleagues concluded that seven traces of collagen proteins detected in purified T. rex bone most closely match those reported in chickens, 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".^[160]

The presumed soft tissue was called into question by Thomas Kaye of the

Femur (thigh bone)

Tibia (shin bone)

Metatarsals

(foot bones)

Dewclaw

Phalanges

(toe bones)

Skeletal anatomy of a T. rex right leg

Scientists have produced a wide range of possible maximum running speeds for Tyrannosaurus: 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 like

Additionally, a 2020 study indicates that Tyrannosaurus 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 including Tyrannosaurus 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 like Tyrannosaurus 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.[170]^[171]

A study published in 2021 by Pasha van Bijlert et al., calculated the

A 2017 study estimated the top running speed of Tyrannosaurus as 17 mph (27 km/h), speculating that Tyrannosaurus exhausted its energy reserves long before reaching top speed, resulting in a parabola-like relationship between size and speed.[173]^[174] Another 2017 study hypothesized that an adult Tyrannosaurus 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 of Tyrannosaurus. The finding may mean that running was also not possible for other giant theropod dinosaurs like Giganotosaurus, Mapusaurus and Acrocanthosaurus.^[175] However, studies by Eric Snively and colleagues, published in 2019 indicate that Tyrannosaurus 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 that Tyrannosaurus 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.^[176]

Possible footprints

Rare fossil footprints and trackways found in New Mexico and Wyoming that are assigned to the ichnogenus Tyrannosauripus have been attributed to being made by Tyrannosaurus, 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.^[177] Another ichnofossil described in 2018, perhaps belonging to a juvenile Tyrannosaurus or the dubious genus Nanotyrannus 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.^[178][179]

Brain and senses

A study conducted by

Thomas Holtz Jr. would note that high depth perception of Tyrannosaurus may have been due to the prey it had to hunt, noting that it had to hunt ceratopsians such as Triceratops, ankylosaurs such as Ankylosaurus, and hadrosaurs. He would suggest that this made precision more crucial for Tyrannosaurus 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 of Tyrannosaurus.^[118]

Though no Tyrannosaurus sclerotic ring has been found, Kenneth Carpenter estimated its size based on that of Gorgosaurus. The inferred sclerotic ring for the Stan 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, the f-number for Stan's eye is 3–3.8; since diurnal animals have f-numbers of 2.1 or higher, this would indicate that Tyrannosaurus had poor low-light vision and hunted during the day.^[182]

Tyrannosaurus had very large olfactory bulbs and olfactory 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 modern vultures, which use scent to track carcasses for scavenging. Research on the olfactory bulbs has shown that T. rex had the most highly developed sense of smell of 21 sampled non-avian dinosaur species.^[183]

Somewhat unusually among theropods, T. rex had a very long cochlea. 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 that T. rex heard best in the low-frequency range, and that low-frequency sounds were an important part of tyrannosaur behavior.^[180] 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 related Daspletosaurus that contained sensory 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.^[54] Another study published in 2021 further suggests that Tyrannosaurus 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 that Tyrannosaurus 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 of Tyrannosaurus 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 tyrannosaurid

), while the network of canals in Tyrannosaurus appears simpler, though still more derived than in most ornithischians, and overall terrestrial taxa such as tyrannosaurids and Neovenator may have had average facial sensitivity for non-edentulous terrestrial theropods, although further research is needed. The neurovascular canals in Tyrannosaurus 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.[186]

A study by Grant R. Hurlburt, Ryan C. Ridgely and Lawrence Witmer obtained estimates for

standard 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.^[187] Other studies, such as those by Steve Brusatte, indicate the encephalization quotient of Tyrannosaurus was similar in range (2.0–2.4) to a chimpanzee (2.2–2.5), though this may be debatable as reptilian and mammalian encephalization quotients are not equivalent.^[188]

Social behavior

Most paleontologists accept that Tyrannosaurus was both an active

A debate exists, however, about whether Tyrannosaurus was primarily a predator or a pure scavenger. The debate originated in a 1917 study by Lambe which argued that large theropods were pure scavengers because Gorgosaurus teeth showed hardly any wear.^[206] This argument disregarded the fact that theropods replaced their teeth quite rapidly. Ever since the first discovery of Tyrannosaurus 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.^[207]

Paleontologist Jack Horner has been a major proponent of the view that Tyrannosaurus was not a predator at all but instead was exclusively a scavenger.^{[142][208][209]} 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.^[210] Other paleontologists such as Thomas Holtz Jr. argued that there are plenty of modern-day predators that do not use their forelimbs to hunt such as wolves, hyenas, and secretary birds as well as other extinct animals thought to be predators that would not have used their forelimbs such as phorusrhacids.^[211][212]
• Tyrannosaurs had large olfactory bulbs and olfactory nerves (relative to their brain size). These suggest a highly developed sense of smell which could sniff out carcasses over great distances, as modern vultures do. Research on the olfactory bulbs of dinosaurs has shown that Tyrannosaurus had the most highly developed sense of smell of 21 sampled dinosaurs.^[183]
• 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 in coprolites (fossilized feces) that they attribute to tyrannosaurs, but point out that a tyrannosaur's teeth were not well adapted to systematically chewing bone like hyenas do to extract marrow.^[213]
• Since at least some of Tyrannosaurus's potential prey could move quickly, evidence that it walked instead of ran could indicate that it was a scavenger.
  ceratopsians.^[166][24]

A skeleton of the hadrosaurid

Tyrannosaurus may have had infectious saliva used to kill its prey, as proposed by William Abler in 1992. Abler observed that the serrations (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, giving Tyrannosaurus a deadly, infectious bite much like the Komodo dragon was thought to have.^[221][222] Jack Horner and Don Lessem, in a 1993 popular book, questioned Abler's hypothesis, arguing that Tyrannosaurus's tooth serrations as more like cubes in shape than the serrations on a Komodo monitor's teeth, which are rounded.^{[142]: 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 of allosauroids, due to flat joints of the neck vertebrae.^[223]

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.

metatarsals, 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 the Tyrannosaurus was feeding on a cadaver in which the more fleshy parts already had been consumed. They were also open to the possibility that other tyrannosaurids practiced cannibalism.^[224]

Parenting

While there is no direct evidence of Tyrannosaurus 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.[226] Direct evidence of parental behavior exists in other dinosaurs such as Maiasaura peeblesorum, the first dinosaur to have been discovered to raise its young, as well as more closely related Oviraptorids, the latter suggesting parental behavior in theropods.^{[227][228][229][230][231]}

Pathology

MOR 980

) with parasite infections

In 2001, Bruce Rothschild and others published a study examining evidence for

teres major muscles. The presence of avulsion injuries being limited to the forelimb and shoulder in both Tyrannosaurus and Allosaurus 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.^[232]

A 2009 study showed that smooth-edged holes in the skulls of several specimens might have been caused by Trichomonas-like parasites that commonly infect birds. 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 infection Actinomycosis or by intraspecific attacks.^[233] 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 of Trichomonas-infected birds are dissimilar to the reactive bone seen in the affected T. 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 a Trichomonas-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 a T. rex.^[234] 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 reptile Stagonolepis, in the absence of infection. The possible cause may instead have been intraspecific combat.^[235]

One study of Tyrannosaurus specimens with tooth marks in the bones attributable to the same genus was presented as evidence of

tyrannosaurids may also have practiced cannibalism.^[224]

Paleoecology

Tyrannosaurus lived during what is referred to as the

titanosaurian sauropod Alamosaurus

"dominated" its southern range. Tyrannosaurus remains have been discovered in different ecosystems, including inland and coastal subtropical, and semi-arid plains.

Several notable Tyrannosaurus remains have been found in the

pachycephalosaurs Pachycephalosaurus and Sphaerotholus, and the theropods Ornithomimus, Struthiomimus, Acheroraptor, Dakotaraptor, Pectinodon and Anzu.^[237]

Another formation with Tyrannosaurus remains is the Lance Formation of Wyoming. This has been interpreted as a bayou environment similar to today's Gulf Coast. The fauna was very similar to Hell Creek, but with Struthiomimus replacing its relative Ornithomimus. The small ceratopsian Leptoceratops also lived in the area.^[238]

In its southern range Tyrannosaurus lived alongside the titanosaur Alamosaurus, the ceratopsians Torosaurus, Bravoceratops and Ojoceratops, hadrosaurs which consisted of a species of Edmontosaurus, Kritosaurus and a possible species of Gryposaurus, the nodosaur Glyptodontopelta, the oviraptorid Ojoraptosaurus, possible species of the theropods Troodon and Richardoestesia, and the pterosaur Quetzalcoatlus.^[239] The region is thought to have been dominated by semi-arid inland plains, following the probable retreat of the Western Interior Seaway as global sea levels fell.^[240]

Tyrannosaurus may have also inhabited Mexico's Lomas 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 of Tyrannosaurus. If true, the evidence indicates the range of Tyrannosaurus was possibly more extensive than previously believed.^[241] It is possible that tyrannosaurs were originally Asian species, migrating to North America before the end of the Cretaceous period.^[242]

Population estimates

According to studies published in 2021 by Charles Marshall et al., the total population of adult Tyrannosaurus 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.^[243][244]

In the same paper, it is suggested that in a population of Tyrannosaurus 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 adult Tyrannosaurus. 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 million Tyrannosaurus 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.^[243][244]

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.^[245] 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 the uncertainty in the maximum body mass.^[246]

Cultural significance

Main article: Tyrannosaurus in popular culture

Since it was first described in 1905, T. rex has become the most widely recognized dinosaur species in popular culture. It is the only dinosaur that is commonly known to the general public by its full scientific name (binomial name) and the scientific abbreviation T. rex has also come into wide usage.^[50] Robert T. Bakker notes this in The Dinosaur Heresies and explains that, "a name like 'T. rex' is just irresistible to the tongue."^[38]

See also

• History of paleontology
• Sue (dinosaur) (FMNH-PR-2081)
• Tyrannosauridae

Notes

1. Ancient Greek τύραννος (túrannos) 'tyrant', and σαῦρος
   (saûros) 'lizard'

References