Holocephali ("complete heads"), sometimes given the term Euchondrocephali, is a subclass of

cartilaginous fish Chondrichthyes (; ) is a class that contains the cartilaginous fishes that have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or ''bony fishes'', which have skeletons primarily composed of bone tissue. ...

in the

class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used differently ...

Chondrichthyes. The earliest

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

s are of teeth and come from the

Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, wh ...

period. Little is known about these primitive forms, and the only surviving group in the subclass is the

order Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of ...

Chimaeriformes. Chimaeriformes, commonly known as chimaeras, includes the rat fishes in the

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...

Chimaera Chimaeras are cartilaginous fish in the order Chimaeriformes , known informally as ghost sharks, rat fish, spookfish, or rabbit fish; the last three names are not to be confused with rattails, Opisthoproctidae, or Siganidae, respectively. At ...

'', and the elephant fishes in the genus ''

Callorhinchus ''Callorhinchus'', the plough-nosed chimaeras or elephantfish, are the only living genus in the family Callorhinchidae (sometimes spelled Callorhynchidae). A few extinct genera only known from fossil remains are recognized. ''Callorhinchus'' s ...

''. 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 The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning " ...

invertebrates. They lack a stomach, their food moving directly into the intestine. Extinct holocephalans were much more diverse in lifestyles, including

shark Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachi ...

-like predatory forms and slow, durophagous fish.

Characteristics

Members of this taxon preserve today some features of elasmobranch life in

Paleozoic The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon. The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838 by combining the Greek words ''palaiós'' (, "old") and ...

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 (''

''), and the elephant fishes (''

'').

Evolution

The fossil record of the Holocephali starts during the

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 Iniopterygiformes ("Nape Wing Forms") is an extinct order of chimaera-like cartilaginous fish that lived from the Devonian to Carboniferous periods (345–280 million years ago). Fossils of them have been found in Montana, Indiana, Illinois, and ...

, and

Eugeneodontida The Eugeneodontida is an extinct and poorly known order of cartilaginous fishes. They possessed "tooth-whorls" on the symphysis of either the lower or both jaws and pectoral fins supported by long radials. They probably lacked pelvic fins and an ...

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 In taxonomy, a group is paraphyletic if it consists of the group's last common ancestor and most of its descendants, excluding a few monophyletic subgroups. The group is said to be paraphyletic ''with respect to'' the excluded subgroups. In ...

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 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. Analysis of the 280 million-year-old holocephalian '' Dwykaselachus'' demonstrates that early members of the group were more shark-like.280 million-year-old fossil reveals evolutionary origins of shark-like fishes
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References

Bibliography

* * * Extant Early Devonian first appearances Vertebrate subclasses {{paleo-cartilaginous-fish-stub