Dinosauria

Dinosauria is a clade of archosaurs that includes the most recent common ancestor of Triceratops and modern birds and all of its descendants. The group arose in the Late Triassic, roughly 230 million years ago, and subsequently diversified into a wide range of forms that occupied many terrestrial niches for more than 150 million years. In traditional classifications, Dinosauria splits into two principal lineages: Ornithischia, the bird-hipped dinosaurs, and Saurischia, the lizard-hipped dinosaurs. Birds are a surviving lineage that originated within the theropod branch of Saurischia.

Dinosauria encompassed a vast array of body plans, from small, agile predators to enormous long-necked sauropodomorphs and, within ornithischians, armored, horned, and beaked forms. The two main subgroups gave rise to numerous families and species that adapted to diverse environments, from forests and floodplains to arid regions.

Non-avian dinosaurs went extinct at the end of the Cretaceous period, about 66 million years ago, in a mass extinction event widely linked to an asteroid impact and volcanic activity. Birds survived and later diversified into the roughly 10,000 living species today, representing the only major dinosaur lineage to endure.

Fossil evidence from field sites and museums continues to refine understanding of dinosaur evolution, ecology, and biology, including growth patterns, physiology, and the relationships among major dinosaur groups.