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Temporal range: Early Devonian-Present
They may be the "oldest and most enigmatic groups of fishes alive today". 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.
At one time they were a diverse and abundant group, as shown by the fossil record. They are now found mainly to deep water. There are fifty living species in six genera and three families. Studying the evolution of these species has been problematic because there are few good fossils. DNA sequences have become the preferred approach to understanding speciation.
They do not have shark-like skin or teeth. They have smooth scales on their bodies, and three pairs of grinding tooth plates. They have a poisonous spine on their backs between the head and the dorsal fin.
Otherwise, they are rather like other Chondrichthyes. They have skeletons are made of cartilage. They use claspers for internal fertilization of females. They lay eggs with leathery cases. They use electroreception to find their prey, which is also common in rays.
- "Chimaeriformes". FishBase. Ed. Ranier Froese and Daniel Pauly. November 2014 version. N.p.: FishBase, 2014.
- 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. http://www.sciencedaily.com/releases/2009/09/090922095816.htm.
- Inoue J.G. et al 2010. Evolutionary origin and phylogeny of the modern holocephalans (Chondrichthyes: Chimaeriformes): a mitogenomic perspective. Molecular Biology and Evolution. 27 (11): 2576-2586.
- T.H. Bullock (2012). Electroreceptors and other specialized receptors in lower vertrebrates. Springer. p. 125. . https://books.google.com/books?id=wq3tCAAAQBAJ&pg=PA125.