What Dinosaurs Are

Summary

Dinosaurs are a diverse clade of archosaurian reptiles that first appeared during the Late Triassic, about 233 million years ago. They became the dominant terrestrial vertebrates after the Triassic–Jurassic extinction event and remained dominant throughout the Jurassic and Cretaceous periods. Dinosaurs included herbivores and carnivores, bipedal and quadrupedal species, and animals ranging from small, bird-sized forms to the enormous long-necked sauropods. Their characteristic features included erect hind limbs, egg-laying reproduction, and varied adaptations such as horns, crests, armor, and spines. Modern birds are the only surviving dinosaur lineage and are classified as avian dinosaurs, while all other dinosaurs are considered non-avian and became extinct during the Cretaceous–Paleogene extinction event about 66 million years ago. Dinosaurs are defined through evolutionary relationships and include groups such as theropods, sauropodomorphs, ornithopods, ceratopsians, stegosaurs, and ankylosaurs. The term “dinosaur,” meaning “terrible” or “fearfully great lizard,” was coined by Sir Richard Owen in 1842.

Key Points

• 1

  Dinosaurs are warm-blooded archosaurian reptiles belonging to the clade Dinosauria.

• 2

  They lived from the Late Triassic to the present, because modern birds are surviving avian dinosaurs.

• 3

  Dinosaurs had erect hind limbs, laid eggs, and included both herbivorous and carnivorous species.

• 4

  Birds evolved from theropod dinosaurs and are the only dinosaur lineage to survive the Cretaceous–Paleogene extinction event.

• 5

  Dinosaurs displayed enormous diversity in size, body shape, locomotion, habitats, and features such as horns, crests, armor, and feathers.