The Coelcanth "breaks" evolution

I've heard from Bill Nye during his debate with Ken Hamm that, and from other people, that if you were able to find a fossil in the incorrect Era, you would disprove evolution. Dr. Hamm brought up the story of the coelcanth found in Africa I believe, and how we had a fossil of it and once thought it was an intermediary species of fish.

I'm an agnostic athiest but I do not fully understand Bill Nyes reasoning nor the correct way to explain the coelcanth.