Saurischia is a clade of dinosaurs that includes two main groups: the theropods and the sauropodomorphs. Theropods are primarily carnivorous and include well-known species like Tyrannosaurus rex and Velociraptor, while sauropodomorphs are mostly herbivorous and include the long-necked giants like Brachiosaurus. Saurischians are characterized by their hip structure, which resembles that of modern reptiles. This group is significant in the study of dinosaur evolution, as it includes the ancestors of modern birds, making it an important link in understanding the transition from dinosaurs to avian species.