Evolution
Fishes were the first vertebrates to evolve. They did not arise directly from tunicates or lancelets, but fishes and non vertebrate chordates probably did evolve from common invertebrate ancestors. During the course of their evolution, fishes underwent several important changes. The evolution of jaws and the evolution of paired fins were important developments during the rise of fishes.
The First Fishes
The earliest fish to appear in the fossil record were odd-looking, jawless creatures whose bodies were armored with bony plates. They lived in the oceans about 510 milllion years ago. Fishes kept this armored, jawless body plan for 100 million years.
The Age of Fishes
About 410 million years ago, fishes underwent a major adaptive radiation. The species to emerge from the radiation ruled the seas during the devonian period, which is often called the ages of fishes. Some of these fishes were jawless species that had very little armor. These jawless fishes were the ancestors of modern hag fishes and lampreys.
The Arrival of Jaws & Paired Fins
Still other ancient fishes kept their boney armor and possessed a feeding adaption that would revolutionize vertebrate evolution; These fish had jaws. Jaws are an extremely useful adaptation. Jawless fishes are limited to eating small particles of food that they filter out of the water or suck up like a vacuum cleaner. Because jaws can hold teeth and muscles, jaws make it possible vertebrates to nibble on plants and munch on other animals. Animals with jaws can eat a much wider variety of food. They can also defend themselves by biting.
The evolution of jaws in early fishes accompanied the evolution of paired pectoral (anterior) and pelvic (posterior) fins. Paired fins gave fishes more control of body movement. In addition, tail fins and powerful muscles gave fishes greater thrust when swimming. The combination of accuracy and speed enabled fishes to move in new and varied patterns. This ability, helped fishes use their jaws in complex ways.
The Rise of Modern Fishes
Although the early jawed fishes soon disappeared, they left behind two major groups that continued to evolve and still survive today. One group- the ancestors of modern sharks and rays-evolved a skeleton made of strong, resilient cartilage. The other group evolved skeletons made of true bone. A sub group of bony fishes, called lobe-finned fishes, had fleshy fins from which the limbs of chordates later evolved.