Tyrannosaurus rex ("tyrant lizard king") was a giant carnivorous theropod dinosaur from the Upper Maastrichtian, the last stage of the Cretaceous period, 66-65 million years ago. Its remains are rare - as of 2005 only 30 specimens had been found, including only three complete skulls. 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 meters (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. Relative to other 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?
From the mid-1990s on, feathered tyrannosaurs were a controversial subject. This was shown for instance with the reaction on the November 1999 National Geographic Magazine, were a downy T. rex chick was depicted. 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.
See also: Feathered dinosaurs
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. The scavenger hypothesis has been re-proposed by Jack Horner in the 1990's and appeared in Horner's 1993 book, "The Complete T. rex".
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, also a Edmontosaurus annectens skeleton was found with healed scars. The fact that the scars seem healed is suggesting active predation instead of scavenging a previous kill.
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 nutricious parts. 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. Lastly, Horner pointed out that modern hunters use their forelimbs to capture prey, while T. rex could hardly manipulate carcasses with its short and useless forelimbs.
Another argument from Jack Horner regarding T. rex's slow speed is their 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 cranium size) and would therefore have to play it safe by walking rather than running. It has also been calculated by Farlow et al. (1995) that falling at high speeds could be fatal.
Powerful forelimbs are not necessary for all living predators, crocodiles and the Secretary Bird being prime examples. Obviously, these animals use their head as main weapon. Characteristic of T. rex is its giant head.
The lack of speed (adaptations) is an important point. 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 a better endurance. A paper in Nature (Hutchinson & 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, 25 miles/h or even 72 km/h or 45 miles/h) are quite unfeasible, because they require very large leg muscles. They specify a very rough upper estimate of 18 km/h or 11 miles/h. While T. rex or other large theropods were probably (conform these latest findings) slow, it does not necessarily mean they were incapable of hunting prey.
If Tyrannosaurus was a scavenger, which dinosaur was 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 is 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.
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 and thus stereoscopic vision is typically seen in active predators such as hawks, owls, cats, and humans. Sharks and Komodo dragons all hunt on the weaker members of the prey species. They rely on one bite followed by a steady pursuit, watching the injured, fear-stricken prey until they can attack again. Tyrannosaurus may have used a similar strategy.
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.
The World of T. rex
North America in the times of Tyrannosaurus 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 with wingspans up to 35 feet across. 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.
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 find, his largest, 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 a 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 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. 
T. rex was not the only tyrannosaurid. The following species have been identified:
(measurements given are based on found fossils and estimates)
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.
These genera are members of the Tyrannosauridae. 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. (Scientific American, 290, no. 2, February 2004 pp. 23-24)
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.
Paleontologist James Farlow calculated the number of lawyers a grown Tyrannosaurus had to eat (based on a scene from the movie Jurassic Park, in which a lawyer became T. rex fodder) to stay alive. Taken an average weight of 68 kilograms, 292 lawyers would be needed to keep one T. rex happy for a year!
NotesIn the Field
, January-February 2005
issue, Field Museum of Natural History
- Holtz, T. R. (1994). The phylogenetic position of the Tyrannosauridae: implications for theropod systematics. Journal of Paleontology 68(5): 1100-1117.
- Horner, John R. and Lessem, Don. The Complete T. Rex. How Stunning New Discoveries Are Changing Our Understanding of the World's Most Famous Dinosaur. Simon and Schuster, 1993. ISBN 0-671-74185-3
- Osborn, H. F. (1905). Tyrannosaurus and other Cretaceous carnivorous dinosaurs. Bulletin of the American Museum of Natural History 35:733-771.
- Osborn, H. F. (1917). Skeletal adaptations of Ornitholestes, Struthiomimus, Tyrannosaurus. Bulletin of the American Museum of Natural History 35: 733-71.
- Hutchinson, J. R., Garcia, M. (2002). Tyrannosaurus was not a fast runner. Nature 415:1018 - 1021. 
- Schweitzer, M. H., Wittmeyer, J. L., Horner, J. R., Toporski, J. K. (2005). Soft-Tissue Vessels and Cellular Preservation in Tyrannosaurus rex. Science, 5717, 1952-1955. 
Last updated: 08-04-2005 19:02:16