Could those massive dinosaurs really have given rise to tiny flightworthy creatures? Some argue that flight couldn’t have developed from the “ground up” — that is, from fast-running animals — but had to start from the “trees down,” that is, from tree-dwelling gliders.
HOWEVER, DOES the “ground up”/”top down” argument really have any bearing on the family tree of birds and dinosaurs? These should legitimately be separate questions: one question concerning the placement of birds within the tree of life, and the second about the lifestyle of the first flying birds and their immediate ancestors.
For comparison, determining the place of whales in the mammal family tree is based on comparisons of the distribution of anatomical and DNA features among many groups of mammals, while understanding the origin of a whale’s swimming ability is based on biomechanical analysis of early whale limbs and vertebrae.
Furthermore, there is a logical problem with this particular argument that dinosaurs could not have been bird ancestors. Specifically, the similarity in the anatomy of birds and coelurosaurian dinosaurs are explained away as convergence: independent evolution of characters due to similar life habits. Yet the same individuals argue that birds and coelurosaurs had very different life habits. If the latter is true, how did these similarities arise?
Note that this is not a problem if the similarities are due to common evolutionary origin, so that these features were inherited and retained by early birds just as they were inherited and retained in raptors, troodonts, and other advanced coelurosaurs.