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. Nor was it part of the pterosaur line. Pterosaurs were a separate and much earlier flying branch, close to dinosaurs within Ornithodira but not dinosaurs themselves. So this animal belongs specifically on the theropod-to-bird path, not on the ornithischian path and not on the pterosaur path either.
An avialan from about 155 million years ago would likely have looked broadly Archaeopteryx-like, but not in the polished way popular art often suggests. It would still have been very much a small feathered theropod: long-tailed, toothed, clawed, and only partly birdlike in overall impression. In other words, if it truly belonged on the avialan branch, it was probably already close to that classic transitional look, because Archaeopteryx itself was not yet a fully modern bird but a remarkably dinosaurian one, with feathers layered onto a body plan that still retained many ancestral theropod traits. Archaeopteryx fossils date to roughly 150 million years ago, and even it still carried teeth, a long bony tail, and unfused dinosaurian bones.
About 6 million years after this, the world of Archaeopteryx appears more clearly in the fossil record. By then, the outline of the bird story is easier to see: feathered forelimbs are more visibly functioning as wings, the body is more balanced for aerial maneuvering, and the ancient dinosaur frame is beginning to read, at a glance, like the first true bird experiment. Yet even there, the future is only beginning. The beak is not yet modern, the tail is still long, the claws are still prominent, and the age of birds has barely lifted off.