Temporal range: Early Devonian-Recent
Dmitry Obruchev (1900 – 1970), Russian Paleontologist in 1953.
Chimaeras are cartilaginous fish in the order Chimaeriformes, known informally as ghost sharks, ratfish (not to be confused with the rattails), spookfish (not to be confused with the true spookfish of the family Opisthoproctidae), or rabbitfishes (not to be confused with the true rabbitfishes of the family Siganidae).
They may be the "oldest and most enigmatic groups of fishes alive today". At one time a "diverse and abundant" group (based on the fossil record), their closest living relatives are sharks, though in evolutionary terms they branched off from sharks nearly 400 million years ago and have remained isolated ever since. Today they are largely confined to deep water.
Description and habits 
Chimaeras live in temperate ocean floors down to 2,600 metres (8,500 ft) depth, with few occurring at depths shallower than 200 metres (660 ft). Exceptions include the members of the genus Callorhinchus, the rabbit fish and the spotted ratfish, which locally/periodically can be found at relatively shallow depths. Consequently, these are also among the few species from the Chimaera order that are kept in public aquaria. They have elongated, soft bodies, with a bulky head and a single gill-opening. They grow up to 150 centimetres (4.9 ft) in length, although this includes the lengthy tail found in some species. In many species, the snout is modified into an elongated sensory organ.
Like other members of the class Chondrichthyes, chimaeras have a skeleton constructed of cartilage. Their skin is smooth and largely covered by placoid scales, and their color can range from black to brownish gray. For defense, most chimaeras have a venomous spine located in front of the dorsal fin.
Chimaeras resemble sharks in some ways: they employ claspers for internal fertilization of females and they lay eggs with leathery cases. However, unlike sharks, male chimaeras also have retractable sexual appendages on the forehead (a type of tentaculum) and in front of the pelvic fins. The females lay eggs in spindle-shaped leathery cases.
They also differ from sharks in that their upper jaws are fused with their skulls and they have separate anal and urogenital openings. They lack sharks' many sharp and replaceable teeth, having instead just three pairs of large permanent grinding tooth plates.They have gill cover or operculum like bony fishes. Chimaera are the only vertebrates to retain traces of a third pair of limbs.
In some classifications the chimaeras are included (as subclass Holocephali) in the class Chondrichthyes of cartilaginous fishes; in other systems this distinction may be raised to the level of class. Chimaeras also have some characteristics of bony fishes.
A renewed effort to explore deep water and to undertake taxonomic analysis of specimens in museum collections led to a boom during the first decade of the 21st century in the number of new species identified. There are more than fifty extant species in six genera and three families (an additional three genera and two families are only known from fossils):
- Genus Callorhinchus (Lacépède, 1798)
- Genus †Edaphodon-prehistoric
- †Edaphodon agassizi (Buckland, 1835)
- †Edaphodon antwerpiensis (Leriche, 1926)
- †Edaphodon bucklandi (Agassiz, 1843)
- †Edaphodon eyrensis (Long, 1985)
- †Edaphodon hesperis (Shun, 2010)
- †Edaphodon kawai (Consoli, 2006)
- †Edaphodon laqueatus (Leidy, 1873)
- †Edaphodon leptognathus - (has not been formally classified) (Agassiz)
- †Edaphodon minor
- †Edaphodon mirabilis - (has not been formally classified)
- †Edaphodon mirificus (Leidy, 1856)
- †Edaphodon sedgwicki
- †Edaphodon smocki (Cope)
- †Edaphodon stenobryus (Cope)
- †Edaphodon tripartitus (Cope)
- Genus Chimaera (Linnaeus, 1758)
- Chimaera argiloba (Last, W. T. White & Pogonoski, 2008) (Whitefin chimaera)
- Chimaera bahamaensis (Kemper, Ebert, Didier & Compagno, 2010) (Bahamas ghost shark)
- Chimaera cubana (Howell-Rivero, 1936) (Cuban chimaera)
- Chimaera fulva (Didier, Last & W. T. White, 2008) (Southern Chimaera)
- Chimaera jordani (S. Tanaka (I), 1905) (Jordan's Chimaera)
- Chimaera lignaria (Didier, 2002) (Carpenter's chimaera)
- Chimaera macrospina (Didier, Last & W. T. White, 2008) (Longspine Chimaera)
- Chimaera monstrosa (Linnaeus, 1758) (Rabbit fish)
- Chimaera notafricana (Kemper, Ebert, Compagno & Didier, 2010) Cape Chimaera
- Chimaera obscura (Didier, Last & W. T. White, 2008) (Shortspine Chimaera)
- Chimaera opalescens (Luchetti, Iglésias & Sellos, 2011)
- Chimaera owstoni (S. Tanaka (I), 1905) (Owston's Chimaera)
- Chimaera panthera (Didier, 1998) (Leopard Chimaera)
- Chimaera phantasma (Jordan & Snyder, 1900) (Silver chimaera)
- Genus Hydrolagus (Gill, 1863)
- Hydrolagus affinis (Brito Capello, 1868) (Smalleyed rabbitfish)
- Hydrolagus africanus (Gilchrist, 1922) (African chimaera)
- Hydrolagus alberti Bigelow & Schroeder, 1951
- Hydrolagus alphus Quaranta, Didier, Long & Ebert, 2006
- Hydrolagus barbouri (Garman, 1908) (Ninespot chimaera)
- Hydrolagus bemisi (Didier, 2002) (Pale ghost shark)
- Hydrolagus colliei (Lay & E. T. Bennett, 1839) (Spotted ratfish)
- Hydrolagus deani (H. M. Smith & Radcliffe, 1912) (Philippine chimaera)
- Hydrolagus eidolon (Jordan & Hubbs, 1925)
- Hydrolagus homonycteris Didier, 2008 (Black Ghostshark)
- Hydrolagus lemures (Whitley, 1939) (Blackfin ghostshark)
- Hydrolagus lusitanicus (Moura, Figueiredo, Bordalo-Machado, Almeida & Gordo, 2005)
- Hydrolagus macrophthalmus (de Buen, 1959)
- Hydrolagus marmoratus (Didier, 2008) Marbled Ghostshark
- Hydrolagus matallanasi (Soto & Vooren, 2004) (Striped rabbitfish)
- Hydrolagus mccoskeri (Barnett, Didier, Long & Ebert, 2006) (Galápagos Ghostshark)
- Hydrolagus melanophasma (K. C. James, Ebert, Long & Didier, 2009) (Eastern Pacific Black Ghostshark)
- Hydrolagus mirabilis (Collett, 1904) (Large-eyed rabbitfish)
- Hydrolagus mitsukurii (Jordan & Snyder, 1904) (Spookfish)
- Hydrolagus novaezealandiae (Fowler, 1911) (Dark ghostshark)
- Hydrolagus ogilbyi (Waite, 1898)
- Hydrolagus pallidus (Hardy & Stehmann, 1990)
- Hydrolagus purpurescens (Gilbert, 1905) (Purple chimaera)
- Hydrolagus trolli (Didier & Séret, 2002) (Pointy-nosed blue chimaera)
- Hydrolagus waitei (Fowler, 1907)
- Hydrolagus sp. D/G Giant Black Chimaera
- Hydrolagus sp. F Peruvian Ratfish
- Genus Harriotta (Goode & Bean, 1895)
- Genus Neoharriotta (Bigelow & Schroeder, 1950)
- Genus Rhinochimaera (Garman, 1901)
- Genus Squaloraja (Agassiz, 1836)
- Genus Echinochimaera (Lund, 1977)
The evolution of these species has been problematic given the paucity of good fossils. DNA sequences have become the preferred approach to understanding speciation.
The order appears to have originated about 420 million years ago during the Silurian. The 39 extant species fall into three families - the callorhinchids, rhinochimaerids and chimaerids with the callorhinchids being the most basal clade. The families appear to have diverged during the late Jurassic to early Cretaceous (170-120 MYA.)
See also 
- Froese, Rainer, and Daniel Pauly, eds. (2009). "Chimaeriformes" in FishBase. January 2009 version.
- "Ancient And Bizarre Fish Discovered: New Species Of Ghostshark From California And Baja California". ScienceDaily. September 23, 2009. Retrieved 2009-09-23.
- Tozer, H., & D. D. Dagit (2004). Husbandry of Spotted Ratfish, Hydrolagus colliei., Chapter 33 in: Smith, M., D. Warmolts, D. Thoney, & R. Hueter (editors). Elasmobranch Husbandry Manual: Captive Care of Sharks, Rays, and their Relatives. Ohio Biological Survey, Inc.
- Stevens, J. & Last, P.R. (1998). Paxton, J.R. & Eschmeyer, W.N., ed. Encyclopedia of Fishes. San Diego: Academic Press. p. 69. ISBN 0-12-547665-5.
- Freaky New Ghostshark ID’d Off California Coast, a September 22, 2009 blog post from Wired Science
- American Wildlife, Wm. H, Wise & Co., Inc. New York, 1947. p. 279
- Luchetti, E.A., Iglésias, S.P. & Sellos, D.Y. (2011): Chimaera opalescens n. sp., a new chimaeroid (Chondrichthyes: Holocephali) from the north-eastern Atlantic Ocean. Journal of Fish Biology, 79 (2): 399–417.
- Quaranta et al. (2006). "A new species of chimaeroid, Hydrolagus alphus sp. nov. (Chimaeriformes: Chimaeridae) from the Galapagos Islands". Zootaxa 1377: 33–45.
- Inoue JG, Miya M, Lam K, Tay BH, Danks JA, Bell J, Walker TI, Venkatesh B.(2010). Evolutionary origin and phylogeny of the modern holocephalans (Chondrichthyes: Chimaeriformes): A mitogenomic perspective. Mol. Biol. Evol.