The subclass Holocephali ("complete heads") is a taxon of cartilaginous fish
Chondrichthyes or cartilaginous fishes are jawed fish with paired fins, paired nares, scales, two-chambered hearts, and skeletons made of cartilage rather than bone...

, of which the order
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...

 Chimaeriformes is the only surviving group.

Holocephali has an extensive fossil record that starts during the Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...

 period. However, 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
Order Petalodontiformes is a group of extinct marine cartilaginous fish related to modern day chimaera found in what is now the United States of America and Europe. With a very few exceptions, they are known entirely from teeth...

, Iniopterygiformes
Iniopterygiformes is an extinct order of chimaera-like cartilaginous fish that lived from the Devonian to Carboniferous periods . Fossils of them have been found in Montana, Indiana, Illinois, and Nebraska...

, and Eugeneodontida
Eugeneodontida is an extinct and poorly known order of bizarre sharks. They possessed a unique "tooth-whorl" on the symphysis of the lower jaw as well as pectoral fins supported by long radials. The palatoquadrate was either fused to the skull or reduced...

 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
A group of taxa is said to be paraphyletic if the group consists of all the descendants of a hypothetical closest common ancestor minus one or more monophyletic groups of descendants...

 or a wastebasket taxon.It include the rat fishes (chimaera), rabbit-fishes (hydrolagus)& 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.there is an erectile spine infront of the dorsal fin, sometimes poisonous.there is no stomach and the mouth is a small aperture surrounded by lips,giving the head a parrot-like appearance.
The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.