- Prehistoric Sharks
The order Hexanchiformes contains two families. The Frilled sharks, or Chlamydoselachidae, and the Cow sharks, or Hexanchidae.
These sharks can be found all over the world in tropical, temperate and boreal waters. They prefer deep, cold habitats mostly in the tropics. However, they can be found inshore in temperate temperatures as well.
Hexanchiformes typically have 6 or 7 gill slits. Their vertebral column extends all the way to the caudal fin’s long dorsal lobe. They have one spineless dorsal fin over or behind the pelvic fins.
Carcharhiniformes, also known as the common name Ground Sharks, is the largest order of sharks, with over 270 species. These species are the most diverse group of sharks. Members of this order are characterized by the presence of a nictitating membrane over the eye, two dorsal fins, an anal fin, and five gill slits. Most sharks in this order are small and harmless, but this order also includes some of the largest and most dangerous sharks to humans.
The families in the order Carcharhiniformes are expected to be revised; recent DNA studies show that some of the conventional groups are not monophyletic. These sharks are found worldwide. Some of these sharks are bottom-dwelling, and some large, oceanic sharks. Some in cold oceans, and others in tropical seas from intertidal zone to pelagic open ocean. Some sharks are poor swimmers, and others excel in swimming.
Reproduction and behavior vary greatly among families and species within this order.