Holocephali

Holocephali; Temporal range: 416–0 Ma PreꞒ Ꞓ O S D C P T J K Pg N Devonian–Recent
	Chimaera monstrosa, a rat fish
	†Helicoprion davisii, a eugeneodontid
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Chondrichthyes
Subclass:	Holocephali; Bonaparte, 1832
Orders
	Chimaeriformes; †Petalodontiformes; †Iniopterygiformes; †Eugeneodontida; †Symmoriiformes?;

Holocephali ("complete heads"), sometimes given the name Euchondrocephali, is a

Chimaeriformes

.

Chimaeriformes, commonly known as chimaeras, rat fish, or ghost sharks, include three living families and a little

durophagous

fish.

Characteristics

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

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).^[2]^[3]

Evolution

The fossil record of the Holocephali starts during the Devonian period.^[4] 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^[who?] 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^[5]

.

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.^[6] Analysis of the 280 million-year-old holocephalian Dwykaselachus demonstrates that early members of the group were more shark-like.^[7]

Taxonomy according to Joseph Nelson, 2006^[8]
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

References

ISSN 0272-4634
.

^ Pough, Janis & Heiser 2013, pp. 99, 101, Table 5-1.

^ Martin, lead section.

^ Pough, Janis & Heiser 2013, pp. 103, 105, Paleozoic Holocephalians.

S2CID 40689320
.

PMID 23825540
.

^ 280 million-year-old fossil reveals evolutionary origins of shark-like fishes

^ Nelson 2006.

Bibliography

Martin, R. Aidan; et al. "Chimaeras — The Neglected Chondrichthyans". Biology of Sharks and Rays. ReefQuest Centre for Shark Research. Retrieved 2016-02-22.

Nelson, Joseph S. (2006). ISBN 978-0-471-25031-9
.

Pough, F. Harvey; Janis, Christine M.; Heiser, John B. (2013). Vertebrate Life (9th,international ed.).
ISBN 978-0-321-78235-9
.

Retrieved from "https://en.wikipedia.org/w/index.php?title=Holocephali&oldid=1230824180"

[1] ISSN 0272-4634
.

[FOOTNOTEPoughJanisHeiser201399,_101Table_5-1-2] Pough, Janis & Heiser 2013, pp. 99, 101, Table 5-1.

[FOOTNOTEMartinlead_section-3] Martin, lead section.

[FOOTNOTEPoughJanisHeiser2013103,_105Paleozoic_Holocephalians-4] Pough, Janis & Heiser 2013, pp. 103, 105, Paleozoic Holocephalians.

[5] S2CID 40689320
.

[6] PMID 23825540
.

[7] 280 million-year-old fossil reveals evolutionary origins of shark-like fishes

[FOOTNOTENelson2006-8] Nelson 2006.

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