Chimaeras are cartilaginous fishes in the order Chimaeriformes, known informally as ghost sharks, ratfish (not to be confused with the rattails), spookfish (not to be confused with the true spookfish of the family Opisthoproctidae), or rabbitfishes (not to be confused with the true rabbitfishes of the family Siganidae).At one time, a "diverse and abundant" group (based on the fossil record), their closest living relatives are sharks, though in evolutionary terms, they branched off from sharks nearly 400 million years ago and have remained isolated ever since.