Prehistoric Groups

Theropods are bipedal saurischian dinosaurs characterized by hollow bones, three-toed feet, and — in most lineages — reduced forelimbs. The group encompasses all meat-eating dinosaurs and is the direct ancestor of modern birds.

Sauropodomorphs are long-necked, herbivorous saurischian dinosaurs. The group includes the prosauropods of the Triassic and the true sauropods — the largest land animals in Earth's history — which dominated from the Jurassic through the end of the Cretaceous.

Ceratopsians are ornithischian dinosaurs defined by a rostral bone at the tip of the upper jaw — a unique structure forming a beak — and, in most species, elaborate horns and a bony neck frill. The group ranges from crow-sized Triassic ancestors to the 9-meter, multi-horned giants of the Late Cretaceous.

Hadrosaurs — the "duck-billed dinosaurs" — are ornithopod dinosaurs characterized by a broad, flat snout and a remarkable dental battery of hundreds of tightly-packed, self-replacing teeth, the most sophisticated chewing apparatus ever evolved by a reptile.

Ankylosaurs are heavily armored ornithischian dinosaurs covered in osteoderms — bony plates, spikes, and knobs fused into the skin. The group is divided into nodosaurids (spiky, no tail club) and ankylosaurids (broader body, bony tail club).

Stegosaurs are thyreophoran ornithischians defined by two rows of bony plates and spikes running along the back and tail. Despite their dramatic appearance, they were low-browsing herbivores with remarkably small skulls and brains relative to their body size.

Pachycephalosaurs are bipedal ornithischians distinguished by a thickened dome of solid bone on top of the skull, sometimes up to 25 cm thick. They are the sister group to ceratopsians within Marginocephalia.

Tyrannosauridae is the terminal family of the tyrannosaur lineage — large-bodied, short-armed apex predators of the Late Cretaceous of Laurasia. Defined by massive skulls with bone-crushing teeth, binocular vision, and highly reduced forelimbs, they are the most biomechanically studied large predators in the fossil record.

Spinosauridae is a family of large tetanuran theropods defined by elongated, crocodile-like skulls with conical, interlocking teeth adapted for catching fish, and in most species tall neural spines forming a prominent dorsal sail or hump. They are the largest theropods ever discovered by length.

Early Dinosauria encompasses the basal lineages that arose in the Middle and Late Triassic — before the great split into Saurischia and Ornithischia was fully established. These include the herrerasaurids, among the earliest large predatory dinosaurs, and a range of small, agile forms that give us the closest look at the dinosaur common ancestor.

Pterosaurs are flying archosaurs — the first vertebrates to achieve powered flight, predating birds by 80 million years. They are not dinosaurs but are the closest relatives of dinosaurs, sharing a common ancestor in the Triassic. Their wings were formed by a membrane of skin and muscle stretched from an enormously elongated fourth finger.

The "marine reptiles" of the Mesozoic are not a single evolutionary group but several independent lineages — ichthyosaurs, plesiosaurs, and mosasaurs — that each independently evolved from land-dwelling ancestors to become fully aquatic ocean predators.