Theory of evolution is now well accepted in the science fraternity as the unearthed fossils provide evidence at intermediate stages of evolutionary developments. In case of turtles, if the concerned fossils were not found earlier, it shouldn't have puzzled the scientists...

Finding of fossils is a 'chance' phenomena and it doesnt mean that, if fossils of ancestors of certain creature were not found, it would challenge the theory of evolution. It only would tell you that fossils either didnt exist or not been found yet.