SHARKS IN GENERAL

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Scientific Classification

Class- Chondrichthyes

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 Elasmobranchii.

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.

Superorders

Elasmobranchs are grouped into two superorders: Batoidea (rays and their relatives) and Selachii (sharks).

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.

Fossil record

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 earth.

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.

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