There are some great posts about issues of creature animation vs paleoanimation to be found at the moment… in part due to an upcoming book about Ray Harryhausen. How much liberty does a concept artist / animator have in designing creatures that once existed?
Over at Pterosaur.net, Mark Witton quotes Harryhausen:

…we tried to find a compromise between strict scientific accuracy and the need to achieve certain cinematic effects, and I believe we did the right thing. It gave them a fantasy element and after all we weren’t making pictures for palaeontologists, although today filmmakers make all their creatures so real – too real for my tastes.

Fred Tepper paraphrases from another Harryhausen encounter:

His effects were the type of thing where you were aware that it wasn’t real but it was somehow magical, whereas “modern” effects are so realistic (or trying to be) that they lose that magic and wonder.

It’s an interesting discussion about balancing credibility with suspension of disbelief, and defining the boundaries of entertainment and visualization. The above image shows similar scenes designed respectively by Willis O’Brien and Harryhausen - head over to Mark Witton’s post for a detailed comparison.