Neornithes, or crown birds, are the branch that includes all living birds today. Once the “Crown Bird” body plan was perfected, it became an incredibly stable design.
From here, evolution did its thing. That means the ostrich, the chicken, and the house sparrow all belong here, along with every duck, eagle, hummingbird, penguin, and parrot alive now. In broad terms, living birds fall into three great groups within Neornithes: Palaeognathae (ostriches, emus, kiwis, and tinamous), Galloanserae (landfowl and waterfowl), and Neoaves (nearly all other living birds).
The deeper story comes from the genetic clock. DNA-based studies often suggest that crown birds began diverging well before the asteroid impact, with some major analyses placing the common ancestor of modern birds around 95 million years ago. That is why a timeline label such as ~90 mya ±10 million years is a fair, DNA-leaning estimate: it reflects the idea that the modern bird line may have already existed for tens of millions of years before the end-Cretaceous extinction, even if those early crown birds did not yet look like today’s familiar forms.
The fossil record, however, is what keeps the debate alive. The K–Pg extinction event happened about 66 million years ago, and the clearest Late Cretaceous fossils close to or within crown birds, such as Asteriornis from 66.8–66.7 mya and Vegavis from roughly 69.2–68.4 mya, sit right near that boundary. That leaves paleontologists still debating whether these dates reflect the true emergence of crown birds, their survival through the extinction, or simply the earliest fossils we have so far. In other words, the argument continues because the rocks point to a late appearance, while the clocks often hint at a deeper origin.