Hell Creek discoveries

Fossils of T. rex are found only in the Hell Creek Formation of Garfield county, Montana, and adjacent areas of the United States, in deposits dating from the Maastrichtian Age, the last time unit of the Cretaceous Period—although slightly earlier relatives such as Tarbosaurus and Raptorex are known from Asia. Found in the same deposits as T. rex are fossils of the ceratopsians (giant horned dinosaurs) on which they likely preyed. There is some question about whether tyrannosaurs killed their food or simply scavenged it. However, neither predatory nor scavenging behaviour need be excluded, since T. rex, like many large carnivores today, probably fed opportunistically, scavenging when it could and hunting when it had to. One argument for predation emphasizes T. rex’s vision. The eye sockets tend to be keyhole-shaped and directed forward, which has been taken as evidence for accurate depth perception, because the fields of view of the eyes would overlap. Other evidence supporting predation is the well-protected skull and formidable jaws. Wounds in the bones of its prey indicate that T. rex ate by using a “puncture and tear” stroke, planting its feet and using the powerful muscles of the neck and legs to anchor itself and pull flesh off bones.

Before 1980 all knowledge of T. rex was based on only four skeletons, none very complete. The Latin name was given to the first specimen by American paleontologist Henry Fairfield Osborn in 1905 and was based on partial specimens collected from the Hell Creek Formation by renowned fossil hunter Barnum Brown. Remains found by Brown are on display at the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania, the American Museum of Natural History in New York City, and the Natural History Museum in London. Since 1980 more than two dozen other specimens of T. rex have been discovered in western North America, some very complete; however, some are in private collections and are therefore lost to science and education. Two of the best specimens, consisting of almost complete adult skeletons, were unearthed in 1990. One, the 85-percent-complete “Wankel” T. rex, is on display at the Museum of the Rockies in Bozeman, Montana, and the other, the 90-percent-complete “Sue,” is displayed at the Field Museum in Chicago. Other T. rex specimens are mounted at other natural history museums in North America, such as the Denver Natural History Museum, the University of California Museum of Paleontology in Berkeley, the Natural History Museum of Los Angeles County, and the Royal Tyrrell Museum in Drumheller, Alberta, near Dinosaur Provincial Park.

In 2000 five T. rex specimens were discovered in the Hell Creek Formation. Several are now on display at the Museum of the Rockies; one of them, the “B-rex,” preserves soft tissues and also medullary bone that indicates the specimen was female. The soft tissues preserve transparent, flexible, hollow blood vessels that contain small round microstructures highly reminiscent in structure of red blood cells. The preservation of these structures is one of the most amazing features of the entire known fossil record.

Classification

Tyrannosaurs are generally divided into the large but more lightly built and slightly earlier albertosaurines and the still larger, more robust, and later tyrannosaurines. Most tyrannosaurs are known from the end of the Cretaceous, but some basal forms are now known from the Early Cretaceous and even the Late Jurassic, though these earlier forms share few features with their later, well-known relatives. Guanlong, an animal about three metres long from the Late Jurassic of Xinjiang province, western China, is the earliest well-known member of the group; it has some primitive and unique features—the most notable being a complex skull crest consisting of a hollow bone running atop the midline of its skull. Dilong, an early tyrannosaur 1.5 metres (5 feet) long from the spectacular Liaoning deposits of northeastern China, is preserved with a covering of simple filamentous “protofeathers” like those seen on many other Early Cretaceous theropod dinosaurs. Eotyrannus, from Early Cretaceous deposits on Britain’s Isle of Wight, is also lightly built and relatively small (some 4.5 metres, or 15 feet, long). These three tyrannosaurs are so primitive that they retain three fingers on their hands.

Several small tyrannosaur fossils from the end of the Cretaceous of western North America were once assigned to separate taxa, but most scholars now consider them to be merely young tyrannosaurs. For example, specimens once given the names Nanotyrannus and Stygivenator are now considered to be juvenile tyrannosaurs, and the former Dinotyrannus is now seen as a subadult T. rex. T. rex is the only tyrannosaur known from the late Maastrichtian Age (i.e., the end of the Cretaceous Period) in North America. As mentioned above, Tarbosaurus is a slightly earlier and very similar form dated to the end of the Cretaceous of Mongolia.

Since 1902 paleontologists have recognized the T. rex as the only formal member of the genus Tyrannosaurus. This classification remained largely unchallenged until 2022, when some paleontologists suggested on the basis of variation in femurs and teeth among T. rex that T. rex should be separated into three species (T. rex, T. imperator, and T. regina). However, this division remains controversial. Another specimen, which was discovered in New Mexico’s Hall Lake Formation in 1983 and initially classified as a T. rex, was reexamined by paleontologists in 2024. They observed that the fossils, made up of a partial skull and lower jaw, predated those of T. rex by 6–7 million years and whose shape were different enough from those of T. rex to justify their placement in a new and separate species, which they called T. mcraeensis.

Formally described in 2009, a new genus of earlier and smaller tyrannosaurs, Raptorex kriegsteini, is based on a single specimen from the Early Cretaceous of the Inner Mongolia Autonomous Region. It stood roughly 3 metres (10 feet) tall, and its weight was roughly 40 to 70 kg (90 to 150 pounds), about one-hundredth the weight of Tyrannosaurus. Although R. kriegsteini is small for a tyrannosaur, as most of the basal ones are, its body plan—with smaller, D-shaped tooth cross sections and short arms with reduced fingers—closely resembles a reduced version of T. rex.

Tyrannosaurs were long thought to be one of the carnosaurs (“flesh-eating lizards”), related to other large theropods such as the allosaurs (see also Allosaurus). These resemblances have proved to be superficial, related to large size alone. Today tyrannosaurs are considered to be gigantic members of the coelurosaurs (“hollow-tailed lizards”), a group largely composed of smaller, more-slender forms. Frequently they have been related to the largely toothless ostrichlike ornithomimids, mainly because tyrannosaurs and ornithomimids share a peculiar foot with “pinched” foot bones; however, some paleontologists suggest that tyrannosaurs could be related to the dromaeosaurs, the “raptors” of Jurassic Park.