Glossary

**Dinosauria** is a clade of archosaurian reptiles that first appeared in the Late Triassic Period (approximately 237 million years ago) and dominated terrestrial ecosystems worldwide for roughly 170 million years until the end of the Cretaceous Period (approximately 66 million years ago). Phylogenetically, the clade is defined as the most recent common ancestor of Triceratops and modern birds (Neornithes) and all of its descendants. Dinosaurs are distinguished from other archosaurs by a suite of skeletal features, including a fully open acetabulum (hip socket), an erect parasagittal limb posture with limbs held directly beneath the body, an elongated deltopectoral crest on the humerus, and three or fewer phalanges on the fourth digit of the hand. Traditionally divided into two major groups based on pelvic morphology—Saurischia ('lizard-hipped') and Ornithischia ('bird-hipped')—dinosaurs achieved extraordinary morphological and ecological diversity, ranging from chicken-sized theropods to sauropods exceeding 70 tonnes. While all non-avian dinosaurs perished at the Cretaceous–Paleogene (K–Pg) boundary, one theropod lineage survived as birds (Aves), of which more than 10,000 species exist today. In strict cladistic terms, dinosaurs are therefore not extinct.