Chondrichthyes comprises the jawed, cartilaginous fish with a
flexible skeleton made of cartilage rather than bone. Since appearing
on earth some 450 million years ago, some members of class Chondrichthyes,
notable the sharks, have been and remain among the fiercest and
of predators. Their success that has not only assured their survival,
but has required their minimal evolutionary adaptation over the
eons to assure survival. Sharks have a good sense of smell but
poor eyesight. To compensate for the poor eye site, they have
a lateral line system of sensory organs running down the side
of the body that can detect very small changes in water pressure
as would be made by a fish swimming nearby. Skates and rays are
other examples of cartilaginous fish that consists of about 1,000
Lamniformes contains the extinct Megalodon (Carcharodon megalodon)
which like all extinct sharks is only known by the fossil teeth.
Shark fossils are extremely rare, in general, because cartilage
fossilizes very poorly. Their teeth, however, are made of a bone-like
material coated with hard enamel and they fossilize very well.
(sharks, rays and skates)
(rays and skates)
(common rays and skates)
Holocephali (chimaera) - families Callorhynchidae, Rhinochimaeridae
and Chimaeridae that are closely related to sharks and are
also called ghost sharks.