Tyrannosaurus rex

Description

Tyrannosaurus rex was the largest representative of a group of theropods belonging to the Tyrannosauridae family, commonly referred to as “tyrannosaurs,” which also included Albertosaurus and Thanatotheristes. It is a member of the tyrannosaurine subfamily, meaning it is more closely related to Thanatotheristes than to Albertosaurus. Members of the Tyrannosaurinae subfamily were more robust and had relatively shorter legs than members of the Albertosaurinae subfamily (Albertosaurus and Gorgosaurus). Because of these differences, some paleontologists have suggested that albertosaurines may have been swifter predators than tyrannosaurines.

Tyrannosaurus rex reached up to 13 m in length — longer than a school bus — and weighed between 5.5 and 10 tonnes, equivalent to from one to two male African elephants. It walked on two powerful hindlimbs at a leisurely pace of 4.6 km/h, and although it couldn’t run, it could walk at a speedy pace of up to 28 km/h. The forelimbs of T. rex were very short but powerful — although it was once claimed that each could lift up to 200 kg, this value was revised to 100 kg. Each hand had two clawed fingers. At the end of their short necks hung a large, robust skull that reached up to 1.4 m in length.

Tyrannosaurus rex had 32–34 teeth in its upper jaw and 24–36 teeth in its lower jaw. Their teeth were serrated, like a steak knife, for slicing through flesh, but very thick to crush through bones. The largest teeth reached 15 cm in length, with a root of similar length. Unlike crocodiles, T. rex had scaly lips covering their teeth entirely, similar to Komodo dragons. Although several theropods were covered with hair-like feathers, there is no evidence that Tyrannosaurus rex, or any other tyrannosaurid, had feathers. Instead, fossilized skin impressions reveal that tyrannosaurids had scales. However, these fossils represent only a small portion of their body, so it is possible that Tyrannosaurus rex had feathers in certain places and used them for courtship or individual identification. Tyrannosaurus rex had bony projections, like small horns, in front of each eye. These horns likely played a role in mating displays or individual identification and, as such, may have been covered by a keratinous sheath or colourful scales in life.

Did you know?
While commonly used, the spellings “T-Rex” and “T. Rex” are technically incorrect. T. rex is the abbreviation of the scientific genus Tyrannosaurus (italicized and spelled with a capital T) and the species rex (italicized but spelled with a lowercase r).

Range and Habitat

Skeletons of Tyrannosaurus rex are found throughout the western interior of North America, from central Alberta and southern Saskatchewan in the north to southernmost New Mexico in the south, with a single jaw bone and isolated teeth identified as belonging to T. rex found as far south as southernmost Texas and northern Mexico. As such, this species is unique among tyrannosaurids in having a transcontinental distribution. With such a vast geographic distribution, Tyrannosaurus rex lived in a variety of habitats: from temperate/subtropical humid forests in the north to tropical arid environments in the south. These habitats bordered the western shore of the Western Interior Seaway, which was retreating due to dropping sea levels.

Although several new Tyrannosaurus species have been proposed in recent years, they all appear to be minor anatomical variations of Tyrannosaurus rex. Such variation is to be expected in a wide-ranging species. As such, most paleontologists believe that only one tyrannosaurid species existed at the end of the Cretaceous in the western interior of North America — T. rex.

Reproduction and Development

Although it is presumed it laid eggs, no eggs or eggshell fragments have been discovered for Tyrannosaurus rex or any tyrannosaurids. This could be because tyrannosaurs nested in environments where it was unlikely their eggs would be buried and eventually fossilized. However, a recent study suggested that it could be because most dinosaurs laid soft-shelled eggs, like many lizards and turtles. Soft-shelled eggs are less likely to fossilize than hard-shelled eggs.

Upon hatching out of the egg, young tyrannosaurs measured only 1 m in length. They had tiny heads, long, skinny legs, and long tails. As they grew, their heads slowly became larger, and their legs gradually became proportionally shorter. They began their teenage years at age 10 and reached the age of reproduction around 15 when they attained their maximum growth rate of 767 kg per year (approximately 2 kg per day!). Subsequently, T. rex became dramatically more robust with large heads by age 18, reached adulthood around 20, and could live up to 30. Because young individuals looked so different from adults, some paleontologists believe that they represent instead a dwarf species called Nanotyrannus. However, because nearly all those differences change during growth, most paleontologists believe that Nanotyrannus is not a valid species.

Diet

Tyrannosaurus rex was an obligate carnivore. Tooth marks left on bones by the predator and even healed bone injuries with broken T. rex teeth trapped inside indicate that mature individuals commonly fed on dinosaurian megaherbivores, such as the hadrosaurid Edmontosaurus and the ceratopsid Triceratops. The bite of Tyrannosaurus rex was 15 times more powerful than that of an alligator and 5 times more powerful than that of other giant theropods, like Giganotosaurus. This allowed it to pulverize and ingest bones while feeding. Indeed, bone fragments have been found in coprolites (i.e., fossilized feces) of Tyrannosaurus rex and other tyrannosaurids. Being more lightly built and having weaker bites, young tyrannosaurs fed on small and young dinosaurs. Tooth marks left on the bones of conspecifics reveal that T. rex was occasionally a cannibal.

Behaviour

Tyrannosaurus rex’s strong jaws allowed it to capture and hold onto struggling prey. By comparison, most other theropods only delivered slashing bites, much like Komodo dragons today. The small forelimbs of tyrannosaurids were too short to capture prey. They were likely vestigial features, as the jaws rather than the claws were the primary method of catching prey.

Although some documentaries have suggested that tyrannosaurs hunted in packs or took care of their young, very little is known about the social behaviour of Tyrannosaurus rex. Rare bonebeds and track sites suggest that tyrannosaurs were at least occasionally gregarious and congregated in an area, but we don’t know if this was related to a special environmental event (e.g., drought, flood), an occasional behaviour (e.g., reproductive season, seasonal behaviour/migration), or habitual behaviour (e.g., group living). Evidence from various sources and from many different sites are needed to confirm social behaviour. Based on jaw adaptations, tooth wear, and prey preferences, young and old tyrannosaurs had different diets, suggesting they were probably solitary animals.

Discovery

Although isolated bones and teeth had been discovered earlier, Barnum Brown discovered the first skull of Tyrannosaurus rex in Montana in 1902. The fossil was described and given its name in 1905 by Henry Fairfield Osborn in the same publication that named Albertosaurus. Although most T. rex skeletons have been discovered in rocks of the Hell Creek Formation exposed in Montana, Wyoming, North Dakota and South Dakota, at least three skeletons have been discovered in Canada: two in Alberta and one in Saskatchewan. The first T. rex, nicknamed “Huxley,” was first discovered in 1946 near the hamlet of Huxley, in south-central Alberta, by paleontologist Charles M. Sternberg but was not excavated because it was incomplete (16 per cent of the skeleton) and situated on a high cliff. The specimen was refound in 1979 by geophysicist J.F. Lerbekmo and determined to be situated 10.5 m below the Cretaceous-Paleogene boundary. The skeleton was later excavated in 1981 by the Royal Tyrrell Museum of Palaeontology. The second T. rex, nicknamed “Black Beauty,” was discovered in 1981 near the village of Cowley, in southwestern Alberta, by three teenagers while fishing after school and was excavated by the Royal Tyrrell Museum (28 per cent of the skeleton). The third T. rex, nicknamed “Scotty,” was discovered in 1991 near the town of Eastend, in southwestern Saskatchewan, by high school teacher Robert Gebhardt and was excavated by the Royal Saskatchewan Museum. This last specimen is the largest T. rex individual discovered so far, slightly larger although less complete (65 per cent of the skeleton) than the famous Sue specimen at the Field Museum of Chicago, Illinois, the most complete large T. rex known (73 per cent of the skeleton).

Tyrannosaurus rex became Saskatchewan’s Provincial Fossil in 2017.