Quote Originally Posted by Xei View Post
I'm pretty sure that two different species cannot mate to produce fertile offspring. That would serve as one defintion.
No, that is not always the case. Sometimes the offspring are infertile; sometimes they are fertile, such as coyotes and wolves, as one example. In the wild, occasionally individuals of closely related species hybridize (mule deer and white tails; closely related bird species), but the offspring are usually less fit and are not as successful as the pure-breds, which tends to keep the species seperate, but not always.

Quote Originally Posted by tkdyo View Post
moonbeam- This may be true, but in so aspects we need to show species producing new offspring to show evolution as an explination for the diversity...so I think being able to show these skeletons as seperate species should be essential.
I don't get what you are saying. It's easy to show skeletons from species that aren't closely related to be different species. But just like with living animals, it may get harder if the skeletons are more similar, especially over time, as they evolve. I don't see the problem, just because the definition of "species" is a human description, and somewhat arbitrary.