Holocephali

Holocephali Temporal range: 416–0 Ma PreЄ Є O S D C P T J K Pg N Devonian–Recent
Hydrolagus colliei
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Chondrichthyes
Subclass:	Holocephali
Orders
Chimaeriformes †Petalodontiformes †Iniopterygiformes †Eugeneodontida

The subclass Holocephali ("complete heads") is a taxon of cartilaginous fish, of which the order Chimaeriformes is the only surviving group. This group includes the rat fishes (e.g., Chimaera), and elephant fishes (Callorhynchus). They preserve today some features of elasmobranch life in Paleaozoic times, though in other respects they are aberrant. They live close to the bottom and feed on molluscs and other invertebrates. The tail is long and thin and they move by sweeping movements of the large pectoral fins. The erectile spine in front of the dorsal fin is sometimes poisonous. 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.

The fossil record of the Holocephali starts during the Devonian period. The record is extensive, but most fossils are teeth, and the body forms of numerous species are not known, or at best poorly understood. Some experts further group the orders Petalodontiformes, Iniopterygiformes, and Eugeneodontida into the taxon "Paraselachimorpha", and treat it as a sister group to Chimaeriformes. However, as almost all members of Paraselachimorpha are poorly understood, most experts suspect this taxon to be either paraphyletic or a wastebasket taxon.

Taxonomy according to Joseph Nelson, 2006

Subclass Holocephali †Superorder Paraselachimorpha †Order Orodontiformes †Order Petalodontiformes †Order Helodontiformes †Order Iniopterygiformes †Order Debeeriiformes †Order Eugeneodontiformes Superorder Holocephalimorpha †Order Psammodontiformes* †Order Copodontiformes †Order Squalorajiformes †Order Chondrenchelyiformes †Order Menaspiformes †Order Coliodontiformes Order Chimaeriformes (chimaeras) † Extinct * position uncertain