The shark order Hexanchiformes (in a broad sense, not only comprising the cow sharks, but also including the frilled sharks, Chlamydoselachidae) is often considered the most primitive of extant sharks, since they share some features with Paleozoic and early-Mesozoic shark groups as the Cladoselachiformes.
The Hexanchiformes order (encompassing cow sharks, frilled sharks, and Chlamydoselachidae) is often considered the most ancient among existing sharks because it shares characteristics with Paleozoic and early-Mesozoic shark groups such as Cladoselachiformes.