# Holocephali

Holocephali ("complete heads"), sometimes given the name Euchondrocephali ("true cartilage heads"), is a subclass of cartilaginous fish in the class Chondrichthyes. The earliest fossils are of teeth and come from the Devonian period. The only living holocephalans are the chimaeras (Chimaeriformes), though during the late Paleozoic (Carboniferous and Permian) Holocephali was much more diverse, including an array of forms including those considerably different from modern Chimaeriformes, including shark-like predatory forms and slow, durophagous fish.
Chimaeras, also known as rat fish, or ghost sharks, include three living families and a little over 50 species of surviving holocephalans. These fishes move by using sweeping movements of their large pectoral fins. They are deep sea fish with slender tails, living close to the seabed to feed on benthic invertebrates. They lack a stomach, their food moving directly into the intestine.

## Characteristics

Members of this taxon preserve today some features of elasmobranch life in Paleozoic 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 venomous. 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 only surviving members of the group are the rabbit fish (Chimaera), and the elephant fishes (Callorhinchus).

## Evolution

The fossil record of the Holocephali starts during the Devonian period. The record is extensive, but most fossils are of 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.
Lund & Grogan (1997) coined the subclass Euchondrocephali to refer to the total group of holocephalians, i.e. all fish more closely related to living holocephalians than to living elasmobranchs such as sharks and rays. Under this classification scheme, "Holocephali" would have a much more restricted definition. Other authors have used Holocephali in a broad sense covering all fish more closely related to chimaeras than to sharks and rays.
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.
While historically considered closely related to elasmobranchs, recent studies have found members of the shark-like "symmoriiformes", which may be paraphyletic, to be early diverging relatives of Holocephali, (with some of these studies choosing to exclude them from Holocephali proper).
* Taxonomy according to Joseph Nelson, 2006
* Subclass Holocephali
* †Superorder Paraselachimorpha
* †Order Orodontiformes
* †Order Petalodontiformes
* †Order Helodontiformes
* †Order Iniopterygiformes
* †Order Debeeriiformes
* †Order Eugeneodontida
* Superorder Holocephalimorpha
* †Order Psammodontiformes*
* †Order Copodontiformes
* †Order Squalorajiformes
* †Order Chondrenchelyiformes
* †Order Menaspiformes
* †Order Cochliodontiformes
* Order Chimaeriformes (chimaeras)
* Incertae sedis
* †Order Symmoriiformes
* † Extinct
* * position uncertain