Hexanchiformes is the order consisting of the most primitive types of sharks, and numbering just six extant species. Fossil sharks that were apparently very similar to modern sevengill species are known from Jurassic specimens.

Hexanchiformes is the order consisting of the most primitive types of sharks, and numbering just six extant species. Fossil sharks that were apparently very similar to modern sevengill species are known from Jurassic specimens.