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Just say the word "tyrannosaur," and most people immediately picture the king of all dinosaurs, Tyrannosaurus Rex. However, as any paleontologist worth his pickaxe will tell you, T. Rex was far from the only tyrannosaur roaming the forests, plains, and swamplands of Cretaceous North America and Eurasia (although it was certainly one of the biggest). From the perspective of the average small, quivering plant-eating dinosaur, Daspletosaurus, Alioramus, and a dozen or so other tyrannosaur genera were every bit as dangerous as T. Rex, and their teeth were just as sharp.
What Defines a Tyrannosaur?
As with other broad classifications of dinosaurs, the definition of a tyrannosaur (Greek for "tyrant lizard") involves a combination of arcane anatomical features and broad swathes of physiology. Generally speaking, tyrannosaurs are best described as large, bipedal, meat-eating theropod dinosaurs possessing powerful legs and torsos; large, heavy heads studded with numerous sharp teeth; and tiny, almost vestigial-looking arms. As a general rule, tyrannosaurs tended to resemble one another more closely than do the members of other dinosaur families (such as ceratopsians), but there are some exceptions, as noted below. (By the way, tyrannosaurs weren't the only theropod dinosaurs of the Mesozoic Era; other members of this populous breed included raptors, ornithomimids and feathered "dino-birds.")
The First Tyrannosaurs
As you might already have guessed, tyrannosaurs were closely related to dromaeosaurs-the relatively small, two-legged, vicious dinosaurs better known as raptors. In this light, it's not surprising that one of the oldest tyrannosaurs yet discovered-Guanlong, which lived in Asia about 160 million years ago--was only about the size of your average raptor, about 10 feet long from head to tail. Other early tyrannosaurs, like Eotyrannus and Dilong (which both lived in the early Cretaceous period), were also fairly petite, if no less vicious.
There's one other fact about Dilong that may permanently change your image of supposedly mighty tyrannosaurs. Based on analysis of its fossil remains, paleontologists believe that this small, Asian dinosaur of the early Cretaceous period (about 130 million years ago) sported a coat of primitive, hair-like feathers. This discovery has led to speculation that all juvenile tyrannosaurs, even the mighty Tyrannosaurus Rex, may have possessed feather coats, which they shed, or perhaps kept, on reaching adulthood. (Recently, the discovery in China's Liaoning fossil beds of the large, feathered Yutyrannus has lent added weight to the feathered tyrannosaur hypothesis.)
Their initial similarities notwithstanding, tyrannosaurs and raptors quickly diverged along separate evolutionary paths. Most notably, the tyrannosaurs of the late Cretaceous period attained enormous sizes: a full-grown Tyrannosaurus Rex measured about 40 feet long and weighed 7 or 8 tons, while the biggest-ever raptor, the middle Cretaceous Utahraptor, punched in at 2,000 pounds, max. Raptors were also far more agile, slashing at prey with their arms and legs, while the primary weapons used by tyrannosaurs were their numerous, sharp teeth and crushing jaws.
Tyrannosaur Lifestyles and Behavior
Tyrannosaurs truly came into their own during the late Cretaceous period (90 to 65 million years ago), when they prowled modern-day North America and Eurasia. Thanks to numerous (and often surprisingly complete) fossil remains, we know a lot about how these tyrannosaurs looked, but not as much about their day-to-day behavior. For example, there's still intense debate about whether Tyrannosaurus Rex actively hunted for its food, scavenged already-dead remains, or both, or whether the average five-ton tyrannosaur could run faster than a relatively poky 10 miles per hour, about the speed of a grade-schooler on a bicycle.
From our modern perspective, perhaps the most puzzling feature of tyrannosaurs is their tiny arms (especially compared to the long arms and flexible hands of their raptor cousins). Today, most paleontologists think the function of these stunted limbs was to lever their owner to an upright position when it was lying on the ground, but it's also possible that tyrannosaurs used their short arms to clutch prey tightly to their chests, or even to get a good grip on females during mating! (By the way, tyrannosaurs weren't the only dinosaurs possessing comically short arms; the arms of Carnotaurus, a non-tyrannosaur theropod, were even shorter.)
How Many Tyrannosaurs?
Because later tyrannosaurs like Tyrannosaurus Rex, Albertosaurus and Gorgosaurus closely resembled one another, there's some disagreement among paleontologists about whether certain tyrannosaurs really merit their own genus (a "genus" is the next step up above an individual species; for example, the genus known as Stegosaurus comprises a handful of closely related species). This situation isn't improved by the occasional discovery of (very) incomplete tyrannosaur remains, which can make assigning a likely genus an impossible bit of detective work.
To take one notable case, the genus known as Gorgosaurus isn't accepted by everyone in the dinosaur community, some paleontologists believing this was really an individual species of Albertosaurus (probably the best-attested tyrannosaur in the fossil record). And in a similar vein, some experts think the dinosaur known as Nanotyrannus ("tiny tyrant") may actually have been a juvenile Tyrannosaurus Rex, the offspring of a closely related tyrannosaur genus, or perhaps a new kind of raptor and not a tyrannosaur at all!