Chondrichthyes are fish with the
following characteristics: a skeleton made of cartilage,
jaws, paired fins, and paired nostrils. Chondrichthyes
are further divided into two subclasses: Holocephaii and
The subclass Holocephaii includes
fishes known as chimaeras. They are characterized by the
fusion of the upper jaw and cranium (the part of the
skull that encloses the brain), one pair of external gill
openings, and no scales.
The subclass Elasmobranchii includes sharks and
batoids. Elasmobranchs are characterized by cylindrical
or flattened bodies, five to seven pairs of gill slits,
an upper jaw not fused to the cranium, and placoid scales.
Elasmobranchs are grouped into two
superorders: Batoidea (rays and their relatives) and
Batoids include stingrays,
electric rays, skates, guitarfish, and sawfish. They are
characterized by a dorso-ventrally flattened body with
expanded pectoral fins fused to the head. All batoids
have five pairs of ventral gill slits. There are four
orders and about 470 species of batoids.
Selachians include all sharks. They are
characterized by a fusiform body and five to seven pairs
of lateral gill slits. There are eight orders and about
350 species of selachians.
Orders and families
For a list of families in their orders, see the appendix.
Since cartilage rapidly
disintegrates, sharks are seldom preserved as fossils.
The fossil record of sharks consists mainly of teeth and
spines from their fins. The earliest evidence of the
ancestors of modern sharks are isolated spines, teeth,
and scales that appeared 350 to 400 million years ago in
the Devonian Period, known as the "Age of
Fishes." Most of the modern-day shark families had already
evolved 1 00 million years ago, when dinosaurs lived on
Unlike other animals, sharks have changed very
little since. The most recently-evolved families of sharks are
the sphyrnids (hammerhead sharks) and
the carcharhinids (requiem sharks).
One extinct shark known today from its enormous
fossil teeth, Carcharodon megalodon lived in the Tertiary
period, 1 0 to 70 million years ago.
The Chondrichthyes did not give rise to the bony
fishes, but they arose from a common ancestor.
Scientific Classification |
Habitat and Distribution |
Physical Characteristics |
Diet and Eating Habits
Anatomy and Physiology |