Phylogenetics
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.)
Read more about this topic: Chimaera