Although now represented by a single (or possibly two) living species, as a group the coelacanths were once very successful with several genera and species that left an abundant fossil record from the Devonian to the end of the Cretaceous period, at which point they apparently suffered a nearly complete extinction, and past which point no fossils are known. It is often claimed that the coelacanth has remained unchanged for millions of years but in fact the living species and even genus are unknown from the fossil record. However some of the extinct species, particularly those of the last known fossil coelacanth, the Cretaceous genus Macropoma, closely resemble the living species.In other words, the coelacanths today are not the same critters found in the fossil record.
Good point, it is the genus that lives on, not the species.
Modern coelecanths, on the other hand, average around 4-1/2 feet long, and they can grow to be nearly six feet long, or almost five times the size of the largest ancient specimens. Keep that in mind next time you look at side-by-side photos of an ancient coelecanth and a modern one - the photos may not reveal that the modern specimen is (at least) three times the size of the ancient one.