Final answer:
Dinosaurs are classified into two main groups, Saurischia ('lizard-hipped') and Ornithischia ('bird-hipped'), with the former including the ancestors of modern birds, despite their names.
Step-by-step explanation:
Dinosaurs are usually divided into two major groups, the Saurischia and Ornithischia. These two clades represent the most significant subdivision within the Dinosauria, differing primarily in pelvic structure and further evolutionary pathways. The Saurischia, or 'lizard-hipped' dinosaurs, retained the ancestral three-pronged pelvic structure and include both the long-necked herbivores, such as Apatosaurus, and the theropod dinosaurs which eventually gave rise to birds.
On the other hand, Ornithischia, or 'bird-hipped' dinosaurs, developed a more complex pelvis which, despite its name, is not linked to the evolution of modern birds. Instead, birds descended from a branch of saurischian theropods known as maniraptoran theropods, exemplified by fossils like that of Archaeopteryx. Both Saurischia and Ornithischia evolved into a multitude of diverse forms, occupying a variety of ecological niches during the Mesozoic Era.