Tyrannosaurus, meaning “tyrant lizard,” was a genus of theropod dinosaur from the Late Cretaceous Period (68 to 65 million years ago). It was among the last non-avian dinosaurs to exist prior to the Cretaceous-Tertiary extinction event. Perhaps the most famous Tyrannosaurus species, T. rex, was named in 1905 by Henry Fairfield Osborn, president of the American Museum of Natural History (AMNH).

Teeth belonging to Tyrannosaurus were first discovered in 1874 by A. Lakes near Golden Colorado. Postcranial elements of Tyrannosaurus were collected in the 1890s in Wyoming by J.B. Hatcher. The fossils were first considered to be from a large species of Ornithomimus. More fragments were found by E.D. Cope in South Dakota in 1892 — first classified as Manospondylus.

The first partial skeleton of Tyrannosaurus was unearthed by Barnum Brown, assistant curator of the AMNH, in eastern Wyoming in 1900. Brown found another partial skeleton in the Hell Creek Formation in Montana in 1902. In all, Brown found 5 partial skeletons of Tyrannosaurus. In 1941, his 1902 find was sold to the Carnegie Museum of Natural History in Pittsburgh. His fourth, and largest find, is on display in the AMNH in New York.

Although there are numerous Tyrannosaurus skeletons around the world, only one track has been documented — at Philmont Scout Ranch in northeastern New Mexico, discovered in 1983 and identified in 1994.

An 85-percent complete specimen was discovered by amateur paleontologist Sue Hendrickson in the Hell Creek Formation near Faith, South Dakota on August 12, 1990. This Tyrannosaurus, nicknamed “Sue” in honor of the discoverer, was the object of a legal battle over its ownership. The suit was settled in 1997, favoring Maurice Williams, the original land owner. The Field Museum of Natural History purchased the specimen at auction for $7.6 million, making it the most expensive dinosaur skeleton to date.

Study of “Sue” showed that it reached full size at age 19 and died at age 28, the longest any Tyrannosaur is known to have lived.

Another specimen, nicknamed “Stan,” in honor of amateur paleontologist Stan Sacrison, was found in Hell Creek Formation near Buffalo, SD, in the spring of 1987. After 30,000 hours of excavation, a 65-percent complete specimen was unearthed. “Stan” is on display at the Black Hills Museum of Natural History Exhibit in Hill City, SD.

Five more Tyrannosaurus skeletons were discovered near the Fort Peck Reservoir in Montana in the summer of 2000 by Jack Horner. One of the specimens, dubbed “C. rex,” is reportedly the largest Tyrannosaurus ever found.

A 50-percent complete juvenile specimen was found in 2001 in the Hell Creek Formation in Montana by a crew from the Burpee Museum of Natural History of Rockford, Illinois. It is the most complete and best preserved juvenile example known to date.

Tyrannosaurus was a bipedal carnivore with a massive skull balanced by a long, heavy tail. Relative to the large, powerful hindlimbs, the forelimbs were unusually small, though powerful for their size. Although other theropod exceeded Tyrannosaurus in size, it was the largest known tyrannosaurid, and one of the largest known land predators of its time. An adult measured up to 43 feet long, 13 feet tall at the hips, and weighed up to 7.5 tons. It was by far the largest carnivore in its environment, and may have been a apex predator, preying on hadrosaurs and ceratopsians. However, some experts believe Tyrannosaurus was primarily a scavenger, and the debate over which is more accurate has been ongoing for decades.

The neck of Tyrannosaurus formed a natural S-shaped curve like that of other theropods, but was short and muscular to support the massive head. The forelimbs had only two clawed fingers and a smaller third digit. Its tail was long and heavy, containing more than forty vertebrae. To compensate for the immense bulkiness of the animal, many bones throughout the skeleton were hollow, reducing its weight without significant loss of strength.

The skull was significantly different from those of other large theropod. It was extremely wide at the rear with a narrow snout, allowing unusually good binocular vision. The strong bones of the skull lead to speculation that Tyrannosaurus had an extremely powerful bite, which easily surpassed that of all non-tyrannosaurid dinosaurs. The tip of the upper jaw was U-shaped, which increased the amount of tissue and bone a tyrannosaur could rip out with one bite.

The teeth of this animal displayed marked differences in shape. The teeth at the front of the upper jaw were closely packed, D-shaped in cross-section, had reinforcing ridges on the rear surface, with chisel-like blades and were curved backwards. The D-shaped cross-section, reinforcing ridges and backwards curve reduced the risk that the teeth would snap when Tyrannosaurus bit and pulled. The remaining teeth were robust, like “lethal bananas” rather than daggers.

Tyrannosaurus was historically depicted as a “living tripod,” with the body at 45 degrees or less from vertical and the tail dragging on the ground, similar to a kangaroo. This concept dates back to 1865 when the first reconstruction of a Hadrosaurus was completed, depicting the dinosaur in a bipedal posture. Osborn reinforced this belief by unveiling the first complete Tyrannosaurus skeleton in 1915, also placed in an upright pose. It stood this way for nearly 80 years, until being dismantled in 1992. This pose was realized as incorrect in the 1970s, as scientists argued that no living animal could maintain such a posture, as it would have resulted in the dislocation or weakening of several joints, including the hips.

A more accurate depiction was introduced in the 1990s, showing Tyrannosaurus with a body approximately parallel to the ground and tail extended behind the body to balance the head.

In 2004, the journal Nature published a report describing an early tyrannosaurid, Dilong paradoxus, from the Yixian Formation in China. The fossil was well preserved with a coat of filamentous structures which are commonly recognized as the precursors of feathers. It has been proposed that Tyrannosaurus and other closely related tyrannosaurids had such protofeathers as well. However, skin impressions from large tyrannosaurid specimens show mosaic scales. While it is possible that protofeathers existed on parts of the body of Tyrannosaurus, a lack of insulatory body covering is consistent with modern multi-ton animals such as elephants, hippos, and rhinos.

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.

Tyrannosaurus, like most dinosaurs, was long believed to have an ectothermic (cold-blooded) reptilian metabolism. This was challenged by scientists in the early years of the “Dinosaur Renaissance,” beginning in the late 1960s. Tyrannosaurus was claimed to have been endothermic (warm-blooded), implying a very-active lifestyle. Scientists have sought to determine if this ability to regulate body temperature was accurate. Evidence of high growth rates in young Tyrannosaurus’, compared to birds and mammals, may support the theory of a high metabolism.

By studying oxygen isotope ratios in fossilized bone to determine the temperature at which the bone was deposited, scientists hoped to get a clear picture of thermal regulation in Tyrannosaurus. The isotope ratios in bones from different parts of the body indicated a temperature difference of no more than 7 to 9 degrees Fahrenheit between the vertebrae of the torso and the tibia of the lower leg. Scientists claimed, based on the findings, that Tyrannosaurus maintained a constant internal body temperature and that it enjoyed a metabolism somewhere between ectothermic reptiles and endothermic mammals.

However, some scientists do not believe the oxygen isotope ratios seen today accurately represent the same ratio in the distant past, and may have been altered during or after fossilization.

In relevance to the debate of the hunting or scavenger behavior of Tyrannosaurus, comes locomotion. Tyrannosaurs may have been slow at turning, taking about one to two seconds to turn only 45 degrees — an amount that humans 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.

Scientists have produced a wide range of maximum speed estimates, most of which are around 25 mph, but a few as low as 11 mph, and a few as high as 45 mph. Researchers so far have to rely on various estimating techniques because there have been no tracks found of a Tyrannosaurus running. Some suggest that absence of running tracks may indicate it never ran. But scientists supporting the running theory say its bones were hollow, lightening the body enough to provide relatively quick locomotion.

Some estimates describe the Tyrannosaurus leg bone as not significantly stronger than those of elephants, which are limited in their top speed and never actually run — there is no airborne phase in elephant locomotion. With this information, the proposed maximum speed of Tyrannosaurus is limited to 25 mph, which is about the speed of a human sprinter.

Some further estimations conclude that Tyrannosaurus, based on its hefty weight of 5 to 7 tons, would have been critically or perhaps fatally injured if it fell during fast movement, since its torso would have slammed into the ground at six times the acceleration due to gravity, and its tiny arms could not have reduced the impact. However, giraffes have been known to gallop at 31 mph, despite the risk that they might break a leg or worse, which can be fatal. Thus it is quite possible that Tyrannosaurus also moved fast when necessary and had to accept such risks.

Still, most recent research on Tyrannosaurus locomotion does not support speeds faster than 25 mph. A 2007 study using computer models to estimate running speeds, and based on data taken from fossils, claimed that Tyrannosaurus had a top speed of 18 mph.

Those who still argue that Tyrannosaurus was incapable of such speeds, estimate the actual number is closer to 11 mph. This is still faster than its most likely prey species, hadrosaurids and ceratopsians. In addition, some advocates of the idea that Tyrannosaurus was a predator claim that tyrannosaur running speed is not important, since it may have been slow but still faster than its probable prey.

The debate over locomotion is almost as old as the debate over whether Tyrannosaurus was a predator or a pure scavenger. Some studies show Tyrannosaurus teeth with hardly any wear, indicating it was a scavenger. But this argument is no longer taken seriously, because theropod continually replaced teeth throughout life, and quite rapidly. Most scientists consider Tyrannosaurus a predator, although like modern predators, it would have been happy to scavenger or steal another predator’s kill if it had the chance.

Other evidence suggest hunting behavior as well. The eye sockets of Tyrannosaurus are positioned so that the eyes point forward, giving binocular vision slightly better than modern day hawks. Scientists argue that binocular vision is a pronounced feature of top predators. In the modern world, binocular vision is found mainly in predators.

Since it was first described in 1905, Tyrannosaurus 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, and the scientific abbreviation T. rex.

Image Caption: Tyrannosaurus rex holotype specimen at the Carnegie Museum of Natural History, Pittsburgh. Credit: ScottRobertAnselmo/Wikipedia (CC BY-SA 3.0)