Holocephali

Holocephali ("complete heads") is a subclass of cartilaginous fish, of which the order Chimaeriformes is the only surviving group. 

Holocephali have a long fossil record that starts during the Devonian period. However, most fossils are teeth, and the body forms of numerous species are not known, or, at best, poorly understood. Sequence analysis on living chimaera suggests the whole group originated  420 million years ago in the Silurian period. 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.

 References 

Category:Cartilaginous fish