Just say the word "tyrannosaur," and most people immediately picture the king of all carnivores, T. 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 the biggest). From the perspective of a small, quivering herbivore, Daspletosaurus, Alioramus, and a dozen or so other genuses were every bit as dangerous, and their teeth were just as sharp. (See a gallery of tyrannosaur pictures.)
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As with other broad classifications of dinosaurs, the definition of a tyrannosaur (which means "tyrant lizard") amounts to a mix of arcane anatomical features and broad swathes of physiology. For most purposes, tyrannosaurs are best described as large, bipedal meat eaters with 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 the members of other dinosaur families (such as ceratopsians), but there are some exceptions, as noted below.
The First Tyrannosaurs
As you might already have guessed, tyrannosaurs are closely related to dromaeosaurs--the two-legged, vicious predators better known as raptors. In this light, it's not surprising that the oldest tyrannosaur 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.
There’s one other fact about Dilong that may permanently change your conception of tyrannosaurs. Based on analysis of its fossil remains, this small, Asian carnivore of the early Cretaceous (about 130 million years ago) appears to have sported a coat of primitive, hair-like feathers. This discovery has led some experts to speculate that all juvenile tyrannosaurs, even the mighty T. Rex, had coats of feathers, which they shed on reaching adulthood.
Initial similarities aside, tyrannosaurs and dromaeosaurs quickly diverged along separate evolutionary paths. Most notably, the advanced tyrannosaurs of the late Cretaceous reached enormous sizes: a full-grown T. Rex was about 40 feet long and weighed 7 tons, while the biggest dromaeosaur, 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 of tyrannosaurs were their numerous, sharp teeth and crushing jaws.
The Tyrannosaur Lifestyle
Tyrannosaurs truly came into their own during the late Cretaceous period (90 to 65 million years ago), when they prowled what is now modern-day North America and Asia. Based on numerous (and often 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 T. Rex (to take just one example) actively hunted for its food, scavenged already-dead remains, or both, or whether the average tyrannosaur could run faster than a relatively poky 10 miles per hour.
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 the tyrannosaur 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!
How Many Tyrannosaurs?
Because later tyrannosaurs like T. 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 believe this was really an individual species of Albertosaurus. And in a similar vein, some paleontologists believe the dinosaur known as Nanotyrannus ("tiny tyrant") may actually have been a juvenile T. Rex, the offspring of a closely related genus, or perhaps a new kind of dromaeosaur and not a tyrannosaur at all.
Genus confusion aside, here’s a list of the most notable tyrannosaurs; you can 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.
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.
Nanotyrannus Was it a new kind of tyrannosaur, or a juvenile T. Rex?
Tarbosaurus The second-biggest tyrannosaur after T. Rex.
T. Rex The once--and always--king of the dinosaurs.
