Final answer:
The avian pelvis is a highly modified saurischian pelvis, as birds evolved from saurischian "lizard-hipped" dinosaurs, specifically the theropod subgroup, not the ornithischian "bird-hipped" dinosaurs.
Step-by-step explanation:
The avian pelvis is a highly modified saurischian pelvis. Even though there exist two major dinosaur classifications based on pelvic structure, Saurischia ("lizard-hipped") and Ornithischia ("bird-hipped"), it's vital to note that birds evolved from the saurischian lineage, not the ornithischian "bird-hipped" dinosaurs. While the ornithischian pelvis does resemble that of modern birds, birds are direct descendants of theropod dinosaurs, a subgroup of the saurischians.
Notably, some late theropods show features closely resembling modern birds, especially in their hip and wrist bones. Archaeopteryx, an early transitional fossil, shows a mix of dinosaur and bird characteristics, and modern birds are classified as Neoaves, Palaeognathae, or Galloanserae. These classifications and evolutionary relationships highlight the complexity and nuances in the development of the avian skeletal structure.