Tyrannosaurus was a large predatory dinosaur from the Upper Cretaceous, 68 to 66 million years ago.[1]

Tyrannosaurus was a bipedal carnivore with a massive skull balanced by a long, heavy tail. Compared to the large and powerful hind limbs, its forelimbs were small, but powerful for their size. They had two clawed digits.

There is discussion as to whether it was a hunter or a scavenger. Like most dominant meat-eaters of today, such as lions and hyenas, Tyrannosaurus might have been both. It had a very strong jaw, and its bite power could snap the bones of other dinosaurs.

More than 30 specimens of Tyrannosaurus rex have been found. Some of them are nearly complete skeletons, and soft tissue and proteins have been reported in at least one of these specimens. Research is done on its biology, life history and biomechanics. The feeding habits, physiology and potential speed of Tyrannosaurus rex are some topics. Some scientists think  Tarbosaurus bataar from Asia is a second species of Tyrannosaurus, but others think Tarbosaurus is a separate genus.

The estimated size of this dinosaur has changed many times. Packard and colleagues tested dinosaur mass calculations on elephants. They decided that dinosaur estimations are flawed and produce results which were too high. Thus, the weight of Tyrannosaurus could be much less than usually estimated.[2]

Although other theropods rivalled or exceeded Tyrannosaurus rex in size, it was the largest known tyrannosaurid and one of the largest known land predators. It was up to 12.3 m (40 ft) in length,[3] up to 4 metres (13 ft) tall at the hips,[4] and up to 6.8 metric tons (7.5 short tons) in weight.[5] By far the largest carnivore in its environment, Tyrannosaurus rex was a top predator, probably preying on hadrosaurs and ceratopsians, and/or a scavenger. The debate over Tyrannosaurus as apex predator or scavenger is among the longest running in paleontology.

For a long time, Tyrannosaurus was the largest known carnivorous dinosaur. Recently, skeletons of other, slightly larger, carnivores have been found, such as Giganotosaurus, Spinosaurus, and Carcharodontosaurus. Skeletons of Tyrannosaurus were found on the North American continent, but relatives, such as Tarbosaurus, have been found in Asia.

The largest known Tyrannosaurus rex skulls measure up to 5 feet (1.5 m) in length.[6] Large openings ('fenestrae') in the skull reduced weight and gave places for muscle attachment, as in all carnivorous theropods. But in other respects Tyrannosaurus's skull was significantly different from those of large non-tyrannosaurid theropods. It was extremely wide at the rear but had a narrow snout. This permitted good binocular vision.[7][8] The skull bones were massive. Some bones were fused, preventing movement between them; but many were pneumatized, with a "honeycomb" of tiny air spaces. This may have made the bones more flexible as well as lighter. These features are part of the tyrannosaurid trend towards an increasingly powerful bite. Its bite easily surpassed that of all non-tyrannosaurids.[9][10][11]

The tip of the upper jaw was U-shaped (most non-tyrannosauroid carnivores had V-shaped upper jaws), which increased the amount of tissue and bone a tyrannosaur could rip out with one bite, although it also increased the stresses on the front teeth.[12][13]

The earliest Tyrannosaurus skeletons were found in 1902 by Barnum Brown. Henry Fairfield Osborn, president of the American Museum of Natural History, named the species Tyrannosaurus rex (meaning "tyrant lizard king") in 1905. The most complete skeleton was found in 1990 in South Dakota and named "Sue" after its finder, Susan Hendrickson.

Tyrannosaurus has become well known. Many movies and television shows have featured it, such as Jurassic Park. Its skeletons are popular exhibits in many museums.

Tyrannosaurus became extinct in the Cretaceous–Tertiary extinction event, which wiped out half of all species on Earth.

Jane is a fossil specimen of a small tyrannosaurid. It is either Nanotyrannus or a juvenile Tyrannosaurus. The skeleton was found in the Hell Creek Formation in southern Montana in 2001.[14]

After four years of preparation, Jane went on display at Rockford, Illinois in the Burpee Museum of Natural History. Some paleontologists think Jane was a juvenile about 11 years old at its time of death. Its fully restored skeleton measured 6.5 metres (21.5 ft) long, about half as long as the largest known complete T. rex specimen, which measures 13 m (42.6 ft) long. The weight of the specimen in life was probably nearly 680 kg (1,500 lbs). Its large feet and long legs indicate it was built for speed and could possibly run as fast as 20–30 miles per hour. Its lower jaw has 17 curved, serrated teeth.

Despite having a typically female name, Jane's sex is unknown—the specimen was named after Burpee Museum benefactor Jane Solem.[15]

The Jane specimen has been central to the debate regarding the validity of the proposed tyrannosaurid genus Nanotyrannus. Jane's skull is almost identical to the skull of the original Nanotyrannus specimen, confirming that they belong to the same species. A conference was held at the Burpee museum in 2005, during which paleontologists debated whether these "pygmy tyrants" represented adult specimens of a small species, or juvenile specimens of Tyrannosaurus rex. While there were a few dissenters,[16] most paleontologists at the conference decided on the latter, that both Jane and Nanotyrannus were juvenile T. rex.[17][18] However, the Jane material has yet to be properly studied and described by scientists. This research is currently being done, by Robert Bakker and colleagues Peter Larson, and Phil Currie.[19]

In August 1991, Robert Gebhardt was a high school principal. He joined Royal Saskatchewan Museum palaeontologists on a prospecting expedition. They went to the exposed bedrock along the Frenchman River Valley in southwest Saskatchewan, Canada. 
Gebhardt discovered the base of a  worn tooth, and a vertebra from the tail. Both looked like they belonged to a T. rex.

In June 1994, RSM palaeontologists began excavating the T. rex. The 66-million-year-old skeleton was the first T. rex skeleton found in Saskatchewan and one of only 12 known in the world at the time. Scotty is one of the largest and most complete skeletons with almost 70% of the skeleton found. A complete articulated cast of the skeleton was finally completed in 2012 and is now on display at its permanent home at the T.rex Discovery Centre in Eastend, Saskatchewan, Canada.

Scotty's skull has a scar from the eye socket to the nostril. That was probably caused by another T. rex or large carnivore that gripped Scotty's skull in its jaws.[20] The museum staff think this is the biggest T. rex specimen found. It weighed about 8,870 kg (8.87 tonnes).[21]  Scotty was perhaps in its early thirties at the time of  death, and was 13 m (43 ft) long, including tail.[22]