Tyrannosaurus rex

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Tyrannosaurus rex
Conservation status: Fossil

Fossil skeleton at
National Museum of Natural History, Washington, DC
Scientific classification
Species:T. rex
Binomial name
Tyrannosaurus rex
Osborn, 1905

Tyrannosaurus rex ("tyrant lizard king") was a giant carnivorous theropod dinosaur from the Upper Maastrichtian, the last stage of the Cretaceous period, 65 – 66 million years ago. Its fossil remains are rare — as of 2005 only 30 specimens had been found1, including only three complete skulls. The first specimens found played an important role in the Bone Wars. T. rex is probably the best known carnivorous dinosaur, particularly because until the 90s it was considered the largest to have ever existed. New finds have dethroned it as the largest theropod, but T. rex will very likely remain a point of ongoing scientific research and popular culture.



The first specimen (a partial vertebra) was found by Edward Cope in 1892 and was described as Manospondylus gigas. It was assigned to Tyrannosaurus rex in 1912 by Henry Osborn. Barnum Brown, assistant curator of the American Museum of Natural History, found the second T. rex skeleton in Wyoming in 1900. This skeleton resides in London.


Up to 15 metres (45 feet) in length and 5–10 tons in weight, T. rex was one of the largest carnivorous dinosaurs of all time. Compared to other carnivorous dinosaurs, the skull of Tyrannosaurus is heavily modified. Many of the bones are fused together, eliminating movement between them. The bones themselves are much more massive than is typical of a theropod, and the serrated teeth, far from being bladelike, are massive and oval in cross-section. Heavy wear and the bite marks found on bones of other dinosaurs indicate that these teeth could bite into solid bone. The teeth are often worn or broken at the tips from heavy use but, unlike mammals, were continually grown and shed throughout the life of the animal. Compared to other giant carnivorous dinosaurs such as Allosaurus, Tyrannosaurus appears to have had a sizeable brain, but it was probably not particularly intelligent by mammalian standards.

The neck was short and heavily muscled. The arms of T. rex were small, perhaps to make up for the weight of its enormous head, but were very sturdy. Paleontologists continue to argue about what purpose, if any, they served. The legs were relatively long and slender for an animal of this size. Recent research suggests that an adult Tyrannosaurus could not run, but juveniles might have been some of the fastest of all dinosaurs. To compensate for its immense bulk, the interior of many bones has been hollowed out. This considerably reduced the weight of the skeleton while maintaining much of the strength of the bones.


Like with all dinosaurs, much of Tyrannosaurus' biology — its lifespan, breeding strategy, coloration, ecology and physiology — remains unknown. A site in Alberta has at least nine Albertosaurus sarcophagus individuals of different age preserved together, but whether these animals lived together, or simply died together, is open to argument.


There is active debate, with no irrefutable evidence on either side, about whether T. rex was warm or cold blooded. Perhaps the balance falls on the side of the creature being homeothermic (warm-blooded), although probably not as warm blooded as modern mammals. There is some speculation that the creature's homeothermic strategy might have changed at times in its life cycle.

Feathers for T. rex?

See also: Feathered dinosaurs

From the mid-1990s on, feathered tyrannosaurs were a controversial subject; for example, the reaction to a depiction of a downy T. rex chick in the November 1999 issue of National Geographic Magazine. But now, at least some tyrannosaurids appear to have been feathered. Small coelurosaurs from the Yixian Formation in Liaoning, China, have been discovered with either pennaceous feathers or fur-like "protofeathers", which suggested the possibility that tyrannosaurids may also have borne feathers as well. In 2004, the primitive tyrannosaurid Dilong paradoxus was discovered from the same formation with preserved long tail plumes. However, (adult) tyrannosaurs in Alberta and Mongolia have skin impressions which appear to show the pebbly scales typical of other dinosaurs. It is possible that tyrannosaurs lost their feathers as they grew, similar to the hair density of an elephant as it grows, or were only feathered on parts of their bodies. In general, small animals need insulation more than large ones because of their proportionately larger surface areas.

Predator, Scavenger or Both?

The discussion about the feeding patterns of T. rex and other large carnivorous dinosaurs remains active. Most paleontologists have portrayed them as highly active predators, while others see them as obligate scavengers (Lambe 1917; Colinvaux 1978; Halstead and Halsted 1981; Barsbold 1983; Horner and Lessem 1993; Horner 1994; Horner and Dobb 1997). The scavenger hypothesis has been re-proposed by Jack Horner in the 1990s and appeared in Horner's 1993 book "The Complete T. rex".

Clues proposed by Horner speaking in favour of the scavenger hypothesis are their large (relative to their brain-size) olfactory bulbs and olfactory nerves, as wide as the spinal cord. These suggest a highly developed sense of smell, allegedly used to sniff out carcasses over great distances, like modern vultures. Their teeth could crack bone; a skill perhaps needed most when you are last to a kill and in need of extracting as much food (marrow) as possible from a carcass's least nutritious parts. Horner also pointed out that modern hunters use their forelimbs to capture prey, while T. rex could hardly manipulate carcasses with its short and useless forelimbs.

Adapted for running?

Horner's main argument, which is generally agreed upon, is that T. rex was a slow walker and not a runner. Therefore, Horner said, it is more likely to be a scavenger. This isn't necessarily true. On the other hand, presupposing that a predator must be fast is scientifically injust.

Speed can be measured in some ways, using an analogy with living animals and sports (the femur/tibia ratio), using biomechanics, or using footprints (trace fossils). For instance, bicyclists with longer thighs are said to have better endurance.

The femur (thigh bone) to tibia (shin bone) ratio (>1, like in almost all large theropods) suggests a specialized walker, rather than a runner — hence a slow scavenger rather than a fast running predator.

Another argument from Jack Horner regarding T. rex's slow speed are its useless forelimbs mentioned above. It could not catch itself, should it fall over in a high speed hunt, (and perhaps sustain severe injuries due to its heavy skull size) and would therefore have to play it safe by walking rather than running. This claim has been substantiated by Farlow et al. (1995): they used a mathematical model using impact forces and decelerations for an animal weighing 6000 kg to gauge that a fall at very high speed (72 km/h, the top speed used in most models) would kill it. They speculate a top speed of adult individuals of about 36 km/h.

The fact that T. rex's legs were not suitably adapted for high speed is an important point independent of the predator/scavenger debate. A paper in Nature (Hutchinson amd Garcia 2002) — Tyrannosaurus was not a fast runner — used a mathematical model (based on chickens and alligators) to gauge the leg muscle mass needed for some top speeds. They found that some proposed top speeds (40 km/h or 25 mph, or even 72 km/h or 45 mph) are quite unfeasible, because they require very large leg muscles (needing ~86% of total body mass as leg muscles[1] (http://www.priweb.org/ed/ICTHOL/ICTHOL04papers/86.htm)). They specify a very rough upper estimate of 18 km/h or 11 mph.

While T. rex or other large theropods were probably slow (Farlow et al. 1995; Hutchinson and Garcia 2002), it does not necessarily mean they were incapable of hunting prey.

Discussion of Horner's claims

Powerful forelimbs are not necessary for all living predators, crocodiles and the Secretary Bird being prime examples. Obviously, these animals use their heads as a main weapon, and T. rex's enormously powerful jaws were its main weapon.

The available evidence of bite marks in other animals and even other T. rex, combined with the enormous serrated teeth and large jaw, seem to speak in favour of a role as predator. When examining Sue, paleontologist Peter Larson found a broken and healed fibula (calf bone) and tail vertebrae, scarred facial bones and a tooth from another T. rex embedded in a neck vertebra. This is strong evidence for aggressive behaviour between tyrannosaurs, but whether it's competition for food/mates or active hunting (cannibalism) is unclear. In the Sue excavation site, an Edmontosaurus annectens skeleton was also found with healed scars. The fact that the scars seem healed suggests active predation instead of scavenging a previous kill.

Although not much is known about the vision of T. rex, the skulls clearly show that the eye sockets are positioned in such a way that they had binocular vision. Binocular (or stereoscopic) vision is typically seen in active predators such as hawks, owls, cats, and humans. But these animals have the benefit of being fast and relatively small, so they can sneak up on their prey first. An animal the size of T. rex could unlikely hunt in this manner, although T. rex is sometimes depicted as a "forest stalker". The option of being an active predator still remains open; however, because prey animals like Triceratops and duck-billed dinosaurs were possibly also comparably slow. Additionally, most predators prefer weakened opponents. Sharks and Komodo dragons all hunt on the weaker members of the prey species. They rely on one surprise bite followed by a steady pursuit, watching the injured, fear-stricken prey until they can attack again. Tyrannosaurus may have used a similar strategy.

Being a scavenger has important ecological implications. If Tyrannosaurus was a scavenger, another dinosaur had to be the top predator in the Amerasian Upper Cretaceous. Top prey were the larger marginocephalians and ornithopods. The other tyrannosaurids share so many characteristics, only small dromaeosaurs remain a choice as top predators. In this light, scavenger hypothesis adherents have hypothesised that T. rex bully size and power allowed them to steal kills from smaller predators. While this is certainly a possibility, because (obviously) few animals will pass up a chance for a free meal, so T. rex probably did scavenge — the question remains whether it hunted at all. But a T. rex, depending solely on small agile killers and prey dying of age, disease and accident, would die soon.

A consideration that should be made is that living carnivores are seldom strict predators or scavengers. Lions, for example, sometimes scavenge prey that hyenas killed (and vice versa). Scavenging behaviour depends on prey availability, among other causes.

[2] (http://dml.cmnh.org/2004Feb/msg00278.html) [3] (http://www.enchantedlearning.com/dinotalk/old/Sept01b.shtml)

The World of Tyrannosaurus rex

North America in the times of T. rex had both familiar and strange elements. The soft-shelled turtles, crocodiles, pike (Esocidae), and gar (Lepisosteidae) alive at the time are quite similar to those living today. Frogs and monitor lizards were other familiar animals. Ferns, palms, and shrubs were some of the dominant plants, but grasses had not yet evolved. Other inhabitants of the landscape are more unfamiliar. Giant pterosaurs soared overhead, some, like Quetzalcoatlus, had wingspans up to 35 feet. Herds of Triceratops and duck-billed dinosaurs (hadrosaurs) roamed the land, while toothed birds flew in the forests. Mammals (predominantly multituberculates and marsupials) were still small, shrew- to rat-sized nocturnal animals and a relatively inconspicuous part of the fauna.

Individual specimens

In total Barnum Brown found five T. rex partial skeletons. Brown collected his second T. rex in 1902 and 1905 in Hell Creek, Montana. This is the holotype used to describe Tyrannosaurus rex Osborn, 1905. In 1941 it was sold to the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania. Brown's fourth and largest find, also from Hell Creek, is on display in the American Museum of Natural History in New York.

Susan Hendrickson, amateur paleontologist, discovered the most complete (more than 90%) and largest T. rex fossil skeleton currently known, in the Hell Creek Formation near Faith, South Dakota, on August 12, 1990 . The T. rex, now named Sue, in her honor, became embroiled in a legal battle over its ownership. In 1997 this was settled in favor of Maurice Williams, the original land owner, and the fossil collection was sold at auction for $7.6 million. It has now been reassembled and is currently exhibited at the Field Museum of Natural History. Based on Sue's fossilized bones, she died at age 28 years, having reached her full size at age 19 years1. Researchers report that a sub-adult and a juvenile skeleton were found in the same quarry as Sue; this lends evidence to the possibility that T. rex ran in packs or other groups.

Another T. rex, nicknamed Stan in honor of amateur paleontologist Stan Sacrison, was found in the Hell Creek Formation near Buffalo, South Dakota, in the spring of 1987. After 30,000 hours of digging and preparing, a 65% complete skeleton emerged. Stan currently is on display in the Black Hills Museum of Natural History Exhibit in Hill City, South Dakota, after an extensive world tour. This tyrannosaur too was found to have many bone pathologies, including broken and healed ribs, a broken (and healed) neck and a spectacular hole in the back of its head, about the size of a T. rex tooth. Both Stan and Sue were examined by Peter Larson.

In the March 2005 Science magazine, Mary Higby Schweitzer of North Carolina State University and colleagues announced the recovery of soft tissue from the marrow cavity of a fossilised leg bone from a 68-million-year-old 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. 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 recognised. In addition, microstructures resembling blood cells were found inside the matrix and vessels. The structures bear resemblance to ostrich blood cells and vessels. However, since an unknown process distinct from normal fossilisation seems to have preserved the material, the researchers are being careful not to claim that it is original material from the dinosaur. [4] (http://news.bbc.co.uk/1/hi/sci/tech/4379577.stm) The presence of medullary bones in this specimen is also of interest. [5] (http://www.washingtonpost.com/wp-dyn/content/article/2005/06/03/AR2005060300141.html)

If it turns out 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 merely be the result of people assuming preserved tissue was impossible, and simply not looking; since the first, two more tyrannosaurs and a hadrosaur have also been found to have such tissue-like structures.

Other Tyrannosauridae

Tyrannosaurus rex was not the only member of the Tyrannosauridae. The following species have been identified:

(measurements given are based on found fossils and estimates)

Skull length Total length Hip height Weight Location Time
T. torosus
(Russell, 1970)
1.1 m 9 m 2.5 m 2.3 tonnes Alberta, Montana Upper Campanian
T. bataar
(Maleev, 1955)
1.35 m 10 m 2.9 m 5 tonnes China, Mongolia Lower Maastrichtian
T. rex
(Osborn, 1905)
1.75 m 13.6 m 4.4 m 12 tonnes Alberta, Saskatchewan, Colorado,
Montana, New Mexico, N. Dakota,
S. Dakota, Wyoming, Texas?
Upper Maastrichtian

' ' T. Imperator height unknown 55 feet weight unknown Montana upper Maastriichtian

The classification of these varies a little (for instance, T. bataar is sometimes called Tarbosaurus, and T. torosus is nearly always classified as a distinct genus Daspletosaurus. Other species include Dilong paradoxus, Eotyrannus lengi, Gorgosaurus libratus, Albertosaurus sarcophagus, and Alectrosaurus olseni.

They are believed to have required extensive geographic feeding ranges—almost as large as a continent, and that theropods the size of T. rex arose in response to the retreat of the Western Interior Seaway of North America, 69 million years ago, which would have increased the size of the feeding range 2.

The T. rex in Jurassic Park

  • In the film Jurassic Park the T. rex seems to be running nearly as fast as the jeep, at around 40 mph (64 km/h). However, the time between strides is too small to reach that speed, unless it is taking impossibly large steps. The real speed of the animal (10–15 mph, 16–24 km/h) seems much higher by some kind of optical illusion. This has been asserted by the moviemakers and Hutchinson and Garcia [6] (http://tam.cornell.edu/students/garcia/.trex_www/JPconnection.html).
  • Paleontologist James Farlow calculated the number of lawyers a grown T. rex would need to eat (based on the scene where the lawyer becomes T. rex fodder) to stay alive. Taking an average weight of 68 kilograms (150 pounds, a bit light for American lawyers), 292 lawyers would be needed to keep one T. rex alive for a year!

Other giant theropods

A number of other giant carnivorous dinosaurs have been discovered, including Carcharodontosaurus, Giganotosaurus, Acrocanthosaurus, and a giant species of Allosaurus. Giganotosaurus appears to have been larger than Tyrannosaurus. There is still no clear scientific explanation for exactly why these animals grew so much larger than the predators that came before and after them.


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