I am curious how one decides, based on an individual skeleton, where a given bird skeleton should be classified in a lineage as regards flight. For instance, if a given skeleton looks like an ostrich, how does one decide that this bird had ancestors that could fly, versus "this is a bird whose ancestors that could not fly but this species is developing characteristics that could lead to flight." I ask because ostrich skeletons seem so therapod like that I wonder why it is believed that they had ancestors that could fly, rather than postulating that they are a transitional species between non-flying therapods and flight capable birds--or to put it a different way, whether they are "proto-flight" birds rather than "post-flight" birds. If there are specific osseos markers looked for by paleontologists I would be most interesting in knowing of them. Thanks to anyone willing to help.