Tangential: there were a bunch of big birds around shortly after 65mya (these penguins, terror birds in the americas, hell Australia still has the cassowary) .

Tangential: there were a bunch of big birds around shortly after 65mya (these penguins, terror birds in the americas, hell Australia still has the cassowary) . These are always understood to be modern-bird-like but bigger, not dinosaur-like. Is this really known to be the case? Maybe equivalently - are these big birds known to have evolved from tiny birds (yes birds-are-dinosaurs, look past it) that survived the 65mya cataclysm? Or instead could they just be big dinosaurs that survived? Is the marked distinction between therapods < 65mya and 65mya < birds really backed by evidence, or is at least some of it down to our preconceptions of what animals in these two eras looked like?