How are birds and dinosaurs related?

Jingmai O’Connor: There’s this small dinosaur named Anchiornis that was originally thought to be a Troodontid dinosaur; birds are most closely related to Troodontid dinosaurs. And then as more of these Anchiornis-like dinosaurs were found, they were then resolved as forming their own little group. And that group is sister taxon to Archaeopteryx and all other birds. So you could then say, well, birds are Archaeopteryx plus Anchiornis, or you could just remove them and just consider Archaeopteryx to be birds. And I strongly think that the latter is more likely because what really makes Archaeopteryx a bird dinosaur is the appearance of flight in this particular lineage of dinosaur. And if you look at the wings of Anchiornis, the “proto wings,” they are definitely not capable of flight. It’s not a flying animal, without a doubt. And Archaeopteryx shows all the signs that it was able to fly. So I would say that nobody has made a convincing argument for these slightly older, probably Troodontid dinosaurs actually being avian Troodontid dinosaurs.