Dinosauria

Dinosauria is a clade of archosaurian reptiles that first appeared early in the Triassic period and went on to become the dominant terrestrial vertebrates for much of the Mesozoic era, through the Jurassic and Cretaceous. The group is extraordinarily diverse, with paleontologists having identified more than 900 genera and over 1,000 species of extinct non-avian dinosaurs alone, ranging from small bipeds to the largest land animals that ever lived, the biggest sauropods reaching nearly 40 meters in length. Dinosaurs share an erect, bird-like leg posture that set them apart from the sprawling stance of most other reptiles, and elaborate display structures such as horns and crests appear across the group, while some lineages evolved bony armor and spines. They were ancestrally bipedal, though many groups became quadrupedal, and the evidence indicates that all laid eggs, with nest-building widespread. Birds are the one dinosaur lineage to have survived the Cretaceous–Paleogene extinction roughly 66 million years ago, so the clade is divided into the avian dinosaurs (birds) and the extinct non-avian dinosaurs.