dinosaurs

Dinosaurs are a group of archosaurs that first appeared in the Triassic period, about 230 million years ago. Over millions of years they diversified into a wide range of forms and became the dominant terrestrial vertebrates until their broad extinction at the end of the Cretaceous.

They are traditionally divided into two major clades: Ornithischia (bird-hipped dinosaurs) and Saurischia (lizard-hipped dinosaurs). Saurischia includes theropods, the mostly carnivorous and often bipedal lineage that gave rise to birds, and sauropodomorphs, the long-necked herbivores. Birds are now considered living dinosaurs, descended from theropod ancestors.

Dinosaurs varied greatly in size and posture, from tiny feathered compsognathids to massive sauropods such as Argentinosaurus. They occupied diverse ecosystems and included both terrestrial and, in some lineages, possibly aquatic members. Many theropods and some ornithischians show evidence of feathers or filamentous coverings, while others had scaly skin.

Fossils have been found on every continent, and the groups persisted through the Triassic, Jurassic, and Cretaceous periods. The non-avian dinosaurs became extinct about 66 million years ago in a mass extinction event, likely caused by a combination of volcanic activity and an asteroid impact. Birds survived and are considered the only surviving dinosaurs.

Studying dinosaurs helps scientists understand evolution, growth, biomechanics, and the rise of birds, and they remain a central focus of paleontology and popular interest.