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 the Cretaceous period (although it was certainly one of the biggest). From the perspective of a small, quivering herbivorous dinosaur, Daspletosaurus, Alioramus, and a dozen or so other tyrannosaur genera were every bit as dangerous, and their teeth were just as sharp. (See a gallery of tyrannosaur pictures.)
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, though, 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 did 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 Cretaceous period; 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 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), were also fairly petite, if no less vicious. (The middle to late Jurassic period witnessed other small, tyrannosaur-like dinosaurs, including Kileskus and Aviatyrannis.)
There’s one other fact about Dilong that may permanently change your image of mighty tyrannosaurs. Based on analysis of its fossil remains, paleontologists believe 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 some experts to speculate that all juvenile tyrannosaurs, even the mighty Tyrannosaurus Rex, may have had early feather coats, which they shed on reaching adulthood. (Recently, the discovery in China's Liaoning fossil beds of the large, feathered Yutyrannus has lent 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 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 main weapons used by tyrannosaurs were their numerous, sharp teeth and crushing jaws.
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 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 plus-sized tyrannosaur could run faster than a relatively poky 10 miles per hour, about the speed of a kid 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 dinosaurs). 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.
For example, the genus known as Gorgosaurus isn’t accepted by everyone in the dinosaur community, some experts believing this was really an individual species of Albertosaurus. And in a similar vein, some paleontologists think the dinosaur known as Nanotyrannus ("tiny tyrant") may actually have been a juvenile Tyrannosaurus Rex, the offspring of a closely related genus, or perhaps a new kind of raptor and not a tyrannosaur at all!
The following is a list of the most notable tyrannosaurs; just click on the links for more information.
Albertosaurus This carnivorous dinosaur was a close relative of T. Rex.
Alectrosaurus Few specimens of this "unmarried lizard" have been found.
Alioramus All we know about this tyrannosaur is based on a single skull.
Appalachiosaurus One of the few dinosaurs ever to be found in Alabama.
Aublysodon This tyrannosaur was named after a single tooth.
Aviatyrannis This "grandmother tyrant" was one of the first tyrannosaurs.
Bagaraatan No one is quite sure how to classify this theropod.
Bistahieversor A new tyrannosaur from North America.
Daspletosaurus This "frightful lizard" was a close cousin of T. Rex.
Dilong This "emperor dragon" may have been an ancestor of T. Rex.
Eotyrannus An early tyrannosaur that looked more like a raptor.
Gorgosaurus Might this tyrannosaur have been a species of Albertosaurus?
Guanlong Probably the first tyrannosaur ever to walk the earth.
Juratyrant This early tyrannosaur was discovered in England.
Kileskus Yet another "basal" tyrannosaur from central Asia.
Labocania It may or may not have been a true tyrannosaur.
Nanotyrannus Was it a new kind of tyrannosaur, or a juvenile T. Rex?
Tarbosaurus The second-biggest tyrannosaur after T. Rex.
Teratophoneus This "monstrous murderer" wasn't all that big.
Tyrannosaurus Rex The once--and always--king of the dinosaurs.
Yutyrannus The largest feathered tyrannosaur yet identified.
Zhuchengtyrannus This Asian tyrannosaur was the size of T. Rex.