Early sample: Aurornis

Although the exact early avialan boundary is famously messy, we do know birds evolved from theropos. In the “Great Divide” of dinosaur classification, birds are members of the Saurischia (“lizard-hipped”) branch, specifically the Theropods.

These ornithischians, or “bird-hipped” dinosaurs, are dinosaurs like Triceratops and Stegosaurus. Evolution produced a similar hip structure here through convergence, but they are not the ancestors of birds. The lineage of theropods that led to birds were mostly carnivorous dinosaurs like T. rex and Velociraptor. Birds emerged from a specific subgroup of small, feathered maniraptoran theropods.