Family Tyrannosauridae

The largest terrestrial carnivores that ever lived belonged to this family of "tyrant lizards." It was a small, specialized group, with less than a dozen types (genera) identified to date. Yet its members have inspired the popular notion of what flesh-eating dinosaurs looked like and how they behaved.

The remains of tyrannosaurs have been found in Asia and western North America. As a group they were shortlived, appearing in Late Cretaceous times and disappearing at the end of that period in the mysterious mass-extinction of all the dinosaurs. Their existence, spanning less than 15 million years, was only a "moment" in terms of the evolution of life.

name: Albertosaurus time: Late Cretaceous locality: North America (Alberta) size: 26 ft/8 m long

This "small" tyrannosaur shows all the features common to the family. It was massively built, with a big head and short body, balanced at the hips by a long, strong tail. Pillarlike legs, with 3-toed, spreading feet, supported the great body weight.

Albertosaurus and other members of its family were specialized theropods. Their arms were puny in comparison to the body size. They were so short that they could not have reached up to the mouth. There were only 2 fingers on each hand, which would not have been very effective for grasping prey. And although the jaws could be opened wide, the bones of the skull were rigidly fixed, without the same degree of flexibility as between those in the skulls of the allosaurs (see p. 117).

Tyrannosaurs had a well-developed second set of ribs on the underside of the body. A possible explanation for these extra ribs and the short arms could be that when a tyrannosaur rested, it lay on its belly. The innards would have been supported by the extra ribs, and not crushed by the great weight of the body. When the animal got up, its tiny arms stopped the bulky frame from sliding forward, and steadied it as it rose to its feet.

There is also a suggestion that the tiny arms were used by the males to hold onto the females while mating.

Prehistoric Animals Found Alive

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Rex Alioramus

name: Alioramus time: Late Cretaceous locality: Asia (Mongolia) size: 20 ft/6 m long

A group of Asiatic tyrannosaurs, represented by Alioramus, differed from "typical" tyrannosaurs in the shape of their skulls. Whereas most members of the family had deep skulls with short snouts, Alioramus and its relatives had shallow skulls with long snouts. There were also some bony knobs or spikes on the face between the eyes and tip of the snout. These may have been display features to distinguish the sexes, perhaps with the males having larger structures than the females.

Alioramus and its tyrannosaur relatives lived in Asia and western North America during the Late Cretaceous period. At that time, the modern continents of Asia and North America were joined into one landmass; the Bering Strait that separates them today was then dry land (see pp. 10-11). But the North American part was divided in two by a shallow sea that ran north-south, and halved the land into "Asiamerica" to the west, and "Euramerica" to the east. Animals, including the great tyrannosaurs, could have migrated freely between Asia and western North America, but relatively few seem to have managed to cross the great divide over to the eastern half of the landmass. However, the remains of one tyrannosaur, similar to Alberto-saurus (above), have been found in the eastern USA.

name: Daspletosaurus time: Late Cretaceous locality: North America (Alberta) size: 28 ft/8.5 m long

The short, deep jaws of this massive meat-eater held even larger teeth than other tyrannosaurs, though they were fewer in number. Each tooth was

;r-sharp, curved and saw-edged. Formidable jaws, clawed feet and sheer bulk (with a body weight of up to 4 US tons/3.6 tonnes — these were the weapons used by Daspletosaurus. It was capable of killing the large, horned dinosaurs (ceratopians) that browsed in the forests of northern North America at the time.

name: Tarbosaurus time: Late Cretaceous locality: Asia (Mongolia) size: up to 46 ft/14 m long

This giant carnosaur lumbered around the lands of central Asia, eating anything it came across, whether dead or alive. Its skills as a hunter are suspect, because of its sheer bulk. It could have preyed on the herbivorous duckbilled and armored dinosaurs that lived in its environment. It could also have supplemented its diet with the kills of other carnosaurs. It was big enough to scare off even the most tenacious predator, with the exception of its larger relative Tyrannosaurus (below), which lived in central Asia at the same time.

Many skeletons of Tarbosaurus have been unearthed in Mongolia, some of them complete. In structure, it was almost identical to Tyrannosaurus, but it was more lightly built and had a longer skull. Because the specimens are so well preserved in the rock, it is possible to analyze the upright posture of this typical tyrannosaur. The back would have been held almost horizontal, with the body balanced at the hips; the long, flexible neck curved abruptly upward from the body, with the heavy head held almost at right angles to it — a birdlike pose.

In the fossil finds, Tarbosaurus' head was pulled right back, toward its shoulders. This often happens when an animal dies, because the ligaments of the neck dry out and shrink, pulling the head backward into an unnatural position.

name: Tyrannosaurus time: Late Cretaceous locality: North America (Alberta,

Montana, Saskatchewan, Texas and

Wyoming) and Asia (Mongolia) size: up to 49 ft/15 m long "The most terrifying engine of destruction ever to walk the earth" sums up the popular notion of this awesome thero-pod. It was the largest of the carnosaur dinosaurs, and certainly the largest terrestrial carnivore yet known.

On average, Tyrannosaurus was 39 ft/ 12 m long, up to 20 ft/6 m tall, and weighed about 8 US tons/7 tonnes (heavier than a modern, adult African bull elephant). Its head alone was over

4 ft/1.25 m long, and was armed with numerous fangs, each measuring some 6 in/15 cm in length.

No complete skeleton of Tyrannosaurus has yet been found, although innumerable bones and teeth have surfaced since it was first discovered in the western USA in 1902. Early reconstructions were often inaccurate; mounted skeletons show the animal propped up on a whiplike tail, its body sloping backward at an angle of 45°. Since the discoveries of complete tyrannosaur skeletons, such as those of Tarbosaurus in Mongolia (above), paleontologists now have a more accurate idea of the stance of these dinosaurs.

The popular notion, that Tyrannosaurus was the most fearsome predator of the Cretaceous, was investigated during the 1960s, with particular attention paid to the structure of the animal's hips and legs. The indications were that Tyrannosaurus may have been no more than a slow-moving scavenger, only able to take small, mincing steps and commandeering the carcasses killed by other predators.

However, this view has since been revised by some paleontologists, who believe that the purpose of the unusually wide area of the skull behind the eyes was to anchor extremely powerful jaw muscles. Taken together with other features — such as the robust, saw-edged teeth; the strong, flexible neck; the large areas of the brain that were associated with the senses of sight and smell; and the possibility of binocular vision — the findings argue in favor of an active, predatory lifestyle.

It is surmised that Tyrannosaurus' diet could have consisted primarily of the duckbilled dinosaurs, or hadrosaurs, that browsed in the hardwood forests of North America (see pp. 146-153). These animals lived in herds, and were always on the alert, sprinting away on 2 legs when danger threatened. So it is likely that Tyrannosaurus hid among the trees, to ambush its prey. It would have leapt out on a passing victim in a short burst of speed. Charging with mouth open wide, the force of the impact would have been absorbed by its strong teeth, sturdy skull and powerful neck.

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  • Maria
    How many members are there in the tyrannosauridae?
    2 years ago

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