Dinosaurs

Dinosaurs (Dinosauria) are a large group of vertebrates that appeared during the Triassic period and spread across land during the Jurassic and Cretaceous periods. The group is identified by distinctive skeletal features, particularly the structure of the pelvis and the way the legs stand vertically beneath the body — unlike other vertebrates whose legs extend to the sides.

Scientists classify dinosaurs into two major branches: Saurischia (pelvis resembling that of lizards) and Ornithischia (pelvis resembling that of birds). All living bird species today are classified within the Saurischia branch — for this reason, modern science does not regard dinosaurs as a completely extinct group.

Structure and diversity

Size variation was extreme. Some small theropods resembled ordinary birds, while long-necked sauropods were among the heaviest land animals ever to exist. Diet varied: some were herbivorous, some carnivorous, and some omnivorous.

Relationship to birds

Fossil evidence from China and elsewhere has shown that many theropods had feathers. This supported the theory that birds evolved from small theropods. For this reason, the term "non-avian dinosaurs" is used to refer to extinct groups.

End of the Cretaceous period

At the end of the Cretaceous period, a major environmental change eliminated many species, including all non-avian dinosaurs. The most widely cited explanation points to the impact of an asteroid (related to the Chicxulub crater in Mexico), likely acting in combination with volcanic activity and sea-level changes.