Tyrannosauridae

Tyrannosauridae (or tyrannosaurids, meaning "tyrant lizards") is a family of coelurosaurian theropod dinosaurs which comprises two subfamilies containing up to six genera, including the eponymous Tyrannosaurus. The exact number of genera is controversial, with some experts recognizing as few as three. All of these animals lived near the end of the Cretaceous Period and their fossils have been found only in North America and Asia.

Although descended from smaller ancestors, tyrannosaurids were almost always the largest predators in their respective ecosystems, putting them at the apex of the food chain. The largest species was Tyrannosaurus rex, one of the largest known land predators, which measured up to 13 m in length and up to 6.8 t in weight. Tyrannosaurids were bipedal carnivores with massive skulls 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 functional digits.

Unlike most other groups of dinosaurs, very complete remains have been discovered for most known tyrannosaurids. This has allowed a variety of research into their biology. Scientific studies have focused on their ontogeny, biomechanics and ecology, among other subjects. Soft tissue, both fossilized and intact, has been reported from one specimen of Tyrannosaurus rex.

Description
The known tyrannosaurids were all large animals. A single specimen of Alioramus of an individual estimated at between 5 and 6 m long has been discovered, although it is considered by some experts to be a juvenile. Albertosaurus, Gorgosaurus and Daspletosaurus all measured between 8 and 10 m long, while Tarbosaurus reached lengths of 12 m from snout to tail. The massive Tyrannosaurus was the largest, approaching 13 m in the longest specimens.

Tyrannosaurid skull anatomy is well understood as complete skulls are known for all genera but Alioramus, which is known only from partial skull remains. Tyrannosaurus, Tarbosaurus, and Daspletosaurus had skulls which exceeded 1 m in length, The largest discovered Tyrannosaurus skull measures over 1.5 m long. 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 tall premaxillae and fused nasal bones.

Tyrannosaurid skulls had many unique characteristics, including fused parietal bones with a prominent sagittal crest, which ran longitudinally along the sagittal 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 a transverse plane rather than longitudinally. The nuchal crest was especially well-developed in Tyrannosaurus, Tarbosaurus and Alioramus. Albertosaurus, Daspletosaurus and Gorgosaurus had tall crests in front of the eyes on the lacrimal bones, while Tarbosaurus and Tyrannosaurus had extremely thickened postorbital 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 of Daspletosaurus and Tarbosaurus, as well as the more basal tyrannosauroid Appalachiosaurus.

The skull was perched at the end of a thick, S-shaped neck, and a long, heavy tail acted as a counterweight to balance out the head and torso, with the center of mass over the hips. Tyrannosaurids are known for their proportionately very small two-fingered forelimbs, although remnants of a vestigial third digit are sometimes found. Tarbosaurus had the shortest forelimbs compared to its body size, while Daspletosaurus 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 longer tibiae than femora, a characteristic of fast-running dinosaurs like ornithomimids. Larger adults had leg proportions characteristic of slower-moving animals, but not to the extent seen in other large theropods like abelisaurids or carnosaurs. The third metatarsals of tyrannosaurids were pinched between the second and fourth metatarsals, forming a structure known as the arctometatarsus.

It is unclear when the arctometatarsus first evolved; it was not present in the earliest tyrannosauroids like Dilong, but was found in the later Appalachiosaurus. This structure also characterized troodontids, ornithomimids and caenagnathids, but its absence in the earliest tyrannosauroids indicates that it was acquired by convergent evolution.

Teeth
Tyrannosaurids, like their tyrannosauroid ancestors, were heterodont, with premaxillary teeth D-shaped in cross section and smaller than the rest. Unlike earlier tyrannosauroids and most other theropods, the maxillary and mandibular teeth of mature tyrannosaurids are not blade-like but extremely thickened and often circular in cross-section. 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, while Tyrannosaurus had between 54 and 60.

William L. Abler observed in 2001 that Albertosaurus tooth serrations are so thin as to functionally be a crack in the tooth. However, at the base of this crack is round void called an ampulla which would have functioned to distribute force over a larger surface area, hindering the ability of the "crack" formed by the serration to propagate through the tooth. An examination of other ancient predators, a phytosaur and Dimetrodon found similarly crack-like serrations, but no adaptations for preventing crack propagation. Tyrannosaurid teeth were used as holdfasts for pulling meat off a body, rather than knife-like cutting functions. Tooth wear patterns hint that complex head shaking behaviors may have been involved in tyrannosaur feeding. When a tyrannosaur would have pulled back on a piece of meat, the force would tend to push the tip of tooth toward the front of the mouth and the anchored root would experience tension on the posterior side and compression from the front. This would typically incline the tooth to crack formation on the posterior side of the tooth, but the ampullae at the base of the already crack-like serrations would tend to diffuse potential crack-forming forces. This form resembles techniques used by guitar makers to "impart alternating regions of flexibility and rigidity to a stick of wood." The use of a drill to create an "ampulla" of sorts and prevent the propagation of cracks through an important material is also used to protect airplane surfaces. Abler demonstrated that a plexiglass bar with kerfs and drilled holes was more than 25% stronger than one with only regularly placed incisions.

History of discovery
The first remains of tyrannosaurids were uncovered during expeditions led by the Geological Survey of Canada, which located numerous scattered teeth. These distinctive dinosaur teeth were given the name Deinodon ("terrible tooth") by Joseph Leidy in 1856. The first good specimens of a tyrannosaurid were found in the Horseshoe Canyon Formation of Alberta, and consisted of nearly complete skulls with partial skeletons. These remains were first studied by Edward Drinker Cope in 1876, who considered them a species of the eastern tyrannosauroid Dryptosaurus. In 1905, Henry Fairfield Osborn recognized that the Alberta remains differed considerably from Dryptosaurus, and coined a new name for them: Albertosaurus sarcophagus ("flesh-eating Alberta lizard"). Cope described more tyrannosaur material in 1892, in the form of isolated vertebrae, and gave this animal the name Manospondylus 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. In his 1905 paper naming Albertosaurus, Osborn described two additional tyrannosaur specimens that had been collected from in Montana and Wyoming during a 1902 expedition of the American Museum of Natural History, led by Barnum Brown. Initially, Osborn considered these to be distinct species. The first, he named Dynamosaurus 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 that Dynamosaurus had been found first, the name Tyrannosaurus had appeared one page earlier in his original article describing both specimens. Therefore, according to the International Code of Zoological Nomenclature (ICZN), the name Tyrannosaurus was used.

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 (which Lawrence Lambe named Gorgosaurus libratus, "balanced fierce lizard", in 1914). A second significant find attributed to Gorgosaurus was made in 1942, in the form of a well-preserved, though unusually small, complete skull. The specimen waited until after the end of World War II to be studied by Charles W. Gilmore, who named it Gorgosaurus lancesnis. This skull was re-studied by Robert T. Bakker, Phil Currie, and Michael Williams in 1988, and assigned to the new genus Nanotyrannus. It was also in 1946 that paleontologists from the Soviet Union began expeditions into Mongolia, and uncovered the first tyrannosaur remains from Asia. Evgeny Maleev described new Mongolian species of Tyrannosaurus and Gorgosaurus in 1955, and one new genus: Tarbosaurus ("terrifying lizard"). Subsequent studies, however, showed that all of Maleev's tyrannosaur species were actually one species of Tarbosaurus at different stages of growth. A second species of Mongolian tyrannosaurid was found later, described by Sergei Kurzanov in 1976, and given the name Alioramus remotus ("remote different branch"), though its status as a true tyrannosaurid and not a more primitive tyrannosaur is still controversial.

Distribution
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 misidentified abelisaurid fossils. The exact time and place of origin of the family remain unknown due to the poor fossil record in the middle part of the Cretaceous on both continents, although the earliest confirmed tyrannosaurids lived in the early Campanian stage in western North America.

Tyrannosaurid remains have never been recovered from eastern North America, while more basal tyrannosauroids like Dryptosaurus and Appalachiosaurus persisted there until the end of the Cretaceous, indicating that tyrannosaurids must have evolved in or dispersed into western North America after the continent was divided in half by the Western Interior Seaway in the middle of the Cretaceous. Tyrannosaurid fossils have been found in Alaska, which may have provided a route for dispersal between North America and Asia. Alioramus and Tarbosaurus are found to be related in one cladistic analysis, forming a unique Asian branch of the family. Of the two subfamilies, tyrannosaurines appear to have been more widespread. Albertosaurines are unknown in Asia, which was home to the tyrannosaurines Tarbosaurus and Alioramus. Both subfamilies were present in the Campanian and early Maastrichtian stages of North America, with tyrannosaurines like Daspletosaurus ranging throughout the Western Interior, while the albertosaurines Albertosaurus and Gorgosaurus are currently known only from the northwestern part of the continent.

By the late Maastrichtian, albertosaurines appear to have gone extinct, while the tyrannosaurine Tyrannosaurus roamed from Saskatchewan to Texas. This pattern is mirrored in other North American dinosaur taxa. During the Campanian and early Maastrichtian, lambeosaurine hadrosaurs and centrosaurine ceratopsians are common in the northwest, while hadrosaurines and chasmosaurines 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.

Growth
Paleontologist Gregory Erickson and colleagues have studied the growth and life history of tyrannosaurids. Analysis of bone histology 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 tremendous growth spurts for about four years midway through their lives. After the rapid growth phase ended with sexual 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. The smallest known Tyrannosaurus rex individual (LACM 28471, the "Jordan theropod") is estimated to have weighed only 29.9 kg at only 2 years old, while the largest, such as FMNH PR2081 ("Sue") most likely weighed over 5400 kg, estimated to have been 28 years old, an age which may have been close to the maximum for the species. T. rex juveniles remained under 1800 kg 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 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. For example, only 600 kg separated the 28-year-old "Sue" from a 22-year-old Canadian specimen (RTMP 81.12.1). This sudden change in growth rate may indicate physical maturity, a hypothesis which is supported by the discovery of medullary tissue in the femur of a 18-year-old T. rex from Montana (MOR 1125, also known as "B-rex"). Medullary tissue is found only in female birds during ovulation, indicating that "B-rex" was of reproductive age.

Other tyrannosaurids exhibit extremely similar growth curves, although with lower growth rates corresponding to their lower adult sizes. Compared to albertosaurines, Daspletosaurus showed a faster growth rate during the rapid growth period due to its higher adult weight. The maximum growth rate in Daspletosaurus was 180 kg per year, based on a mass estimate of 1800 kg in adults. Other authors have suggested higher adult weights for Daspletosaurus; this would change the magnitude of the growth rate but not the overall pattern. The youngest known Albertosaurus is a two-year-old discovered in the Dry Island bonebed, which would have weighed about 50 kg and measured slightly more than 2 m in length. The 10 m specimen from the same quarry is the oldest and largest known, at 28 years of age. The fastest growth rate is estimated to be around 12–16 years, reaching 122 kg per year, based on an adult 1300 kg which is about five times slower than for T.-rex. For Gorgosaurus the calculated maximum growth rate is about 110 kg during the rapid growth phase, which is comparable to that of Albertosaurus.

Life history
The end of the rapid growth phase suggests the onset of sexual maturity in Albertosaurus, although growth continued at a slower rate throughout the animals' lives. Sexual maturation while still actively growing appears to be a shared trait among small and large dinosaurs as well as in large mammals such as humans and elephants. 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.

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 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. While this could be due to preservation or collection biases, 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 like elephants. 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 enough Daspletosaurus remains for a similar analysis, but Erickson notes that the same general trend seems to apply.

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. 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 modern Komodo dragons, where hatchlings start off as tree-dwelling insectivores and slowly mature into massive apex predators capable of taking down large vertebrates. For example, Albertosaurus have been found in aggregations that some have suggested to represent mixed-age packs.

Locomotion
Locomotion abilities are best studied for Tyrannosaurus 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. The cause of the difficulty is rotational inertia, since much of Tyrannosaurus’ mass was some distance from its center of gravity, like a human carrying a heavy timber.

Scientists have produced a wide range of maximum speed estimates, mostly around 11 m/s, but a few as low as 5 –, and a few as high as 20 m/s. Researchers have to rely on various estimating techniques because, while there are many tracks of very large theropods walking, so far none have been found of very large theropods running — and this absence may indicate that they did not run.

Jack Horner and Don Lessem argued in 1993 that Tyrannosaurus was slow and probably could not run (no airborne phase in mid-stride). However, Holtz (1998) concluded that tyrannosaurids and their close relatives were the fastest large theropods. Christiansen (1998) estimated that the leg bones of Tyrannosaurus 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 m/s, which is about the speed of a human sprinter. Farlow and colleagues (1995) have argued that a 6-8 ton Tyrannosaurus 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 meters/s²) and its tiny arms could not have reduced the impact. 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. Thus it is quite possible that Tyrannosaurus also moved fast when necessary and had to accept such risks; this scenario has been studied for Allosaurus too. Most recent research on Tyrannosaurus locomotion does not narrow down speeds further than a range from 17 to 40 km/h, i.e. from walking or slow running to moderate-speed running. A computer model study in 2007 estimated running speeds, based on data taken directly from fossils, and claimed that T. rex had a top running speed of 8 m/s. (probably a juvenile individual).

Feathers
Long filamentous structures have been preserved along with skeletal remains of numerous coelurosaurs from the Early Cretaceous Yixian Formation and other nearby geological formations from Liaoning, China. These filaments have usually been interpreted as "protofeathers," homologous with the branched feathers found in birds and some non-avian theropods, although other hypotheses have been proposed. A skeleton of Dilong was described in 2004 that included the first example of "protofeathers" in a tyrannosauroid. Similarly to down feathers of modern birds, the "protofeathers" found in Dilong were branched but not pennaceous, and may have been used for insulation.

It has also been theoretized that tyrannosaurids had such protofeathers. However, rare skin impressions from adult tyrannosaurids in Canada and Mongolia show pebbly scales typical of other dinosaurs. While it is possible that protofeathers existed on parts of the body which have not been preserved, a lack of insulatory body covering is consistent with modern multi-ton mammals such as elephants, hippopotamus, and most species of rhinoceros. As an object increases in size, its ability to retain heat increases due to its decreasing surface area-to-volume ratio. Therefore, as large animals evolve in or disperse into warm climates, a coat of fur or feathers loses its selective advantage for thermal insulation and can instead become a disadvantage, as the insulation traps excess heat inside the body, possibly overheating the animal. Protofeathers may also have been secondarily lost during the evolution of large tyrannosaurids, especially in warm Cretaceous climates.

Vision
The eye-sockets of Tyrannosaurus are positioned so that the eyes would point forward, giving them binocular vision slightly better than that of modern hawks. Jack Horner also pointed out that the tyrannosaur lineage had a history of steadily improving binocular vision. It is hard to see how natural selection would have favored this long-term trend if tyrannosaurs had been pure scavengers, which would not have needed the advanced depth perception that stereoscopic vision provides. In modern animals, binocular vision is found mainly in predators (the principal exceptions are primates, which need it for leaping from branch to branch). Unlike Tyrannosaurus, Tarbosaurus had a narrower skull more typical of other tyrannosaurids in which the eyes faced primarily sideways. All of this suggests that Tarbosaurus relied more on its senses of smell and hearing than on its eyesight. In Gorgosaurus specimens, the eye socket was circular rather than oval or keyhole-shaped as in other tyrannosaurid genera. In Daspletosaurus this was a tall oval, somewhere in between the circular shape seen in Gorgosaurus and the 'keyhole' shape of Tyrannosaurus.

Bony crests
Bony crests are found on the skulls of many theropods, including many of 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 of Appalachiosaurus, as well as some specimens of Daspletosaurus, Albertosaurus, and Tarbosaurus. In Albertosaurus, Gorgosaurus and Daspletosaurus, there is a prominent horn in front of each eye on the lacrimal bone. The lacrimal horn is absent in Tarbosaurus and Tyrannosaurus, which instead have a crescent-shaped crest behind each eye on the postorbital bone. These head crests may have been used for display, perhaps for species recognition or courtship behavior.

Thermoregulation
Tyrannosaurus, like most dinosaurs, was long thought to have an ectothermic ("cold-blooded") reptilian metabolism but was challenged by scientists like Robert T. Bakker and John Ostrom in the early years of the "Dinosaur Renaissance", beginning in the late 1960s. Tyrannosaurus rex itself was claimed to have been endothermic ("warm-blooded"), implying a very active lifestyle. Since then, several paleontologists have sought to determine the ability of Tyrannosaurus to regulate its body temperature. Histological evidence of high growth rates in young T. 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 the indeterminate growth seen in most other vertebrates. 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 the tibia of the lower leg. This small temperature range between the body core and the extremities was claimed by paleontologist Reese Barrick and geochemist William Showers to indicate that T. rex maintained a constant internal body temperature (homeothermy) and that it enjoyed a metabolism somewhere between ectothermic reptiles and endothermic mammals. Later they found similar results in Giganotosaurus specimens, who lived on a different continent and tens of millions of years earlier in time. Even if Tyrannosaurus 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.

Coexistence of Daspletosaurus and Gorgosaurus
In the Dinosaur Park Formation, Gorgosaurus lived alongside a rarer species of the tyrannosaurine Daspletosaurus. This is one of the few examples of two tyrannosaur genera coexisting. Similarly-sized predators in modern predator guilds are separated into different ecological niches by anatomical, behavioral or geographical differences that limit competition. Niche differentiation between the Dinosaur Park tyrannosaurids is not well-understood.{{Cite journal|last=Farlow |first=James O. |authorlink=James Farlow |coauthors=& Pianka, Eric R. |year=2002 |title=Body size overlap, habitat partitioning and living space requirements of te