"Arnaz called Mark's attention to the creature. To most folks, the fish might have been little more than a curiosity, but Mark was a marine biologist with a recent PhD from the University of California at Berkeley, resulting from his study of mantis shrimps in Indonesia. He recognized it immediately as a coelacanth, a "living fossil" whose body plan hadn't changed appreciably in hundreds of millions of years." (emphasis mine)
First off let me apologize, the spelling of the fish's name is "coelacanth."
Secondly, your point (and the point of several others) appears to be valid. Some species appear to evolve and others not. This is really a generalization of my original argument. I have not seen a satisfactory explanation for this observation. Unless perhaps evolution is extremely rare. In which case it would seem unlikely to provide a satisfactory explanation for the great diversity of life we observe.
For further reference, here's a link to the NOVA episode on the "living fossil":
http://www.pbs.org/wgbh/nova/fish/