The 12 Main Dinosaur Groups

Ankylosaurs were among the last dinosaurs standing 65 million years ago, after the K/T Extinction, and with good reason: these otherwise gentle herbivores were the Cretaceous equivalent of Sherman tanks, complete with armor plates, sharp spikes and heavy clubs. Ankylosaurs (which were closely related to stegosaurs) seem to have evolved their armament mainly to ward off predators, though it's possible that males fought each other for dominance in the herd. Read more about ankylosaurs

Among the oddest-looking dinosaurs that ever lived, ceratopsians--"horned faces"--included such familiar herbivores as Triceratops and Pentaceratops, and were characterized by their huge, frilled, horned skulls. Most ceratopsians were comparable in size to modern cattle or elephants, but one of the most common genuses of the Cretaceous period, Protoceratops, only weighed a few hundred pounds. Read more about ceratopsians

During the Mesozoic Era, there wasn't just one "missing link" that connected dinosaurs and birds, but dozens of them: small, feathered theropods that showed a tantalizing mixture of dinosaur and bird features. Exquisitely preserved dino-birds like Sinornithosaurus and Sinosauropteryx have recently been unearthed in China, prompting paleontologists to revise their opinions about bird (and dinosaur) evolution. Read more about dino-birds

Among the last--and most numerous--dinosaurs to roam the earth, hadrosaurs (commonly known as duck-billed dinosaurs) were large, oddly shaped, low-slung plant eaters with tough beaks on their snouts for shredding vegetation. Most hadrosaurs are believed to have lived in herds, and some genuses (like the North American Maiasaura) seem to have paid especially close attention to their children. Read more about hadrosaurs

Ornithomimids ("bird mimics") didn't resemble flying birds, but landbound varieties like modern ostriches and emus. These two-legged dinosaurs were the speed demons of the Cretaceous period; some genuses (like Dromiceiomimus) may have been capable of hitting top speeds of 50 miles per hour. Oddly, ornithomimids were among the few dinosaurs to enjoy omnivorous diets, feasting on meat and vegetation with equal gusto. Read more about ornithomimids

Ornithopods--small- to medium-sized, mostly bipedal herbivores--were among the most common dinosaurs of the Mesozoic Era, roaming the plains and woodlands in vast herds. By an accident of history, ornithopods like Iguanodon and Mantellisaurus were among the first dinosaurs ever to be excavated, reconstructed and named, putting this reptile family at the center of innumerable disputes in paleontology. Read more about ornithopods

Twenty million years before the dinosaurs went extinct, a strange new breed evolved: small- to medium-sized, two-legged herbivores with unusually thick skulls. It's believed that pachycephalosaurs like Colepiocephale (Greek for "knucklehead") used their thick noggins to battle each other for dominance in the herd, although it's possible they also came in handy for butting away curious predators. Read more about pachycephalosaurs

Among the most feared predators of prehistoric times, raptors (which go by the technical name dromaeosaurs) were closely related to modern birds. Raptors were distinguished by their bipedal posture, grasping, three-fingered hands, larger-than-average brains, and the signature, single curved toes on each of their feet; they may also have been covered with feathers. The most famous raptors were Deinonychus, Velociraptor and the giant Utahraptor. Read more about raptors

Sauropods were the true giants of the dinosaur family, some of them attaining lengths of over 100 feet and weights of over 100 tons. Most sauropods were characterized by their extremely long necks and tails and thick, squat bodies; they were the dominant herbivores of the Jurassic period, though an armored branch (known as the titanosaurs) flourished during the Cretaceous. Among the most well-known sauropods were Brachiosaurus, Apatosaurus and Argentinosaurus. Read more about sauropods

Stegosaurus is far and away the most famous example, but at least a dozen genuses of stegosaurs (spiked and plated herbivorous dinosaurs closely related to the armored ankylosaurs) lived during the Jurassic and early Cretaceous periods. The function of the stegosaurs' famous plates is still a matter of dispute; they may have been used for mating displays, or as a way to dissipate excess heat. Read more about stegosaurs

Tyrannosaurs and raptors made up only a small percentage of the bipedal, carnivorous dinosaurs known as theropods, which included such exotic families as ceratosaurs, abelisaurs, megalosaurs and allosaurs. The exact evolutionary relationships among these big theropods is still a matter of debate, but there's no doubt they were equally deadly to any herbivorous dinosaurs that wandered into their path. Read more about theropods

Tyrannosaurs were the killing machines of the Cretaceous period: these huge, powerful carnivores were all legs, trunk and teeth, and they preyed relentlessly on smaller, herbivorous dinosaurs. The most famous tyrannosaur was T. Rex, though less well-known genuses (such as Albertosaurus and Daspletosaurus) were equally deadly. Technically, tyrannosaurs were theropods, placing them in the same larger group as dino-birds and raptors. Read more about tyrannosaurs