Chimaeras, also known as rat fish, or ghost sharks, include three living families and a little over 50 species of surviving holocephalans.

Members of this taxon preserve today some features of elasmobranch life in Paleozoic times, though in other respects they are aberrant.

There is no stomach (that is, the gut is simplified and the 'stomach' is merged with the intestine), and the mouth is a small aperture surrounded by lips, giving the head a parrot-like appearance.

[5] Other authors have used Holocephali in a broad sense covering all fish more closely related to chimaeras than to sharks and rays.

[6] Based on genetic research, it is estimated the Holocephali split from the Elasmobranchii (the branch of chondrichthyans containing true sharks and rays) about 421 million years ago.