The actual meaning for living fossil, is any creature that has been on earth for many millions of years and has not changed at all. In this respect one would expect to find tuataras, crocodiles, sharks, coelocanths and cockroaches in the group, but it turns out that all of these animals have had tremendous variations and modifications during the millions of years that they have been on earth. So stating that a tuatara of today is the exact same as one 140 mya would be false.
Recent studies have shown that tuataras, like sharks, have gone through some rather large changes in their evolution showing many advanced features compared to their nearest extinct counterparts.
In truth the only true living fossils would have been the deinosaurs in Jurassic Parktm and that folks, is science fiction.