Nagel has, I think, been led astray by a big survivorship bias: the evolutionarylineage that led to us always found a viable mutation, ergo one must, it seems, always be available.
They proposed yanking the prehistoric creature off the "bird" branch of the evolutionary family tree and moving it onto a closely related lineage of birdlike dinosaurs.
That status was called into question two years ago by Chinese scientists, who proposed yanking it off the "bird" branch of the evolutionary family tree and moving it onto a closely related lineage of birdlike dinosaurs.