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.