|by Marcia Malory|
Fish can be divided into two groups: the jawless fish (Agnatha) and the jawed fish.
A clade is a group of organisms that share a common ancestor.
The name Gnathostomata is Greek for "jaw mouth".
Tetrapods are also gnathostomes.
Jaws are believed to have developed from the two front gill arches of now-extinct species of jawless fish. Gill arches are arches that support the gills. They are made of cartilage or bone.
Originally, jaws were probably used to help pump water over the gills, so that fish could breathe more efficiently.
As time passed, jaws became important for feeding.
Jaws allowed fish to open their mouths wider, so that they could catch larger prey. They also enabled fish to move their teeth up and down, so that they could bite down on their prey.
The first jawed fish are thought to have appeared on Earth toward the end of the Silurian period, which lasted from about 400 million to about 450 million years ago.
Jawed fish have more well-developed spinal columns than lampreys - jawless fish that are also considered vertebrates.
Every jawed fish has an anal fin, which lies on the underside of the fish behind the fish's anus and in front of its tail, a caudal fin (tail fin) and from one to three dorsal fins (fins on its back).
A jawed fish also has two sets of paired fins - a pair of pectoral fins and a pair of pelvic fins.
A pectoral fin can be found on each side of the fish's body.
The pelvic fins, which are sometimes known as ventral fins, can be found below the pectoral fins, closer to the fish's abdomen.
Kinds of Jawed Fish
The earliest jawed fish were the acanthodians (Acanthodii) and the placoderms (Placodermi).
The acanthodians were covered in spines and are sometimes called spiny sharks.
The placoderms had bony armor over their heads and the front sections of their bodies.
Both the acanthodians and the placoderms are now extinct.
Sharks and rays are cartilaginous fish.
Most fish are bony fish.
The majority of fish on Earth today are ray-finned fish.
Lobed-finned fish have fins with fleshy lobes at their bases.
These lobes contain bones that are arranged the same way as the bones in the limbs of tetrapods.
Coelacanths and lungfish are the only living lobe-finned fish.
Tetrapods and lobe-finned fish make up the clade Sarcopterygii.