11.1.3 11.14: The evolution of jaws and fins gave rise to the vast diversity of vertebrate species.

The earliest vertebrates were fish-like animals that lived more than 500 million years ago and did not have jaws. Two kinds of japhelaness vertebrates still exist: the lampreys (with 41 species) and the hagfishes (with 43 species). Most lamprey species do not feed as adults, instead living off energy stores built up by filter feeding in their larval stage. Some lamprey species, however, are parasitic and attach to other fishes by creating suction with a circular oral disk that is studded with sharp spines. First, the spines scrape an opening through the scales and skin. Then, a protein produced by the lamprey keeps the wound from closing, and the lamprey feeds on the blood and body fluids that seep out. Hagfishes feed on dead animals, using two spiny dental plates that they embed in their prey.

Lampreys and hagfishes are about as basic as a vertebrate can be. They have a simple, tube-like shape with no fins or limbs. And because they do not have jaws, they cannot seize prey or chew food before swallowing it. The evolution of fins and jaws is what set the stage for the explosion of diversity in the vertebrates.

Fishes swim the same way that lampreys and hagfishes swim—that is, by bending the body from side to side to create an S-shaped wave that moves from head to tail. A typical fish has seven fins serving different purposes: to drive the fish forward, to minimize rolling from side to side, and for steering and stopping. The combined effects of the seven fins allow the fish to swim rapidly in a straight line when it is in open water or to weave its way through a dense stand of plants or corals.

The evolution of fins was paralleled by the evolution of jaws, because the two structures work together. Fins get you to the organism you are going to eat, and jaws capture and kill it (FIGURE 11-23).

The development of jaws and fins allowed great diversification in vertebrate body shapes and composition. There are three categories of jawed fish. Cartilaginous fishes, with about 880 species, include the sharks and rays (FIGURE 11-24). Cartilaginous fishes are characterized by a skeleton made entirely of cartilage, a solid but slightly flexible connective tissue—the same tissue that gives your nose and ears their shape.

Ray-finned fishes have a rigid skeleton made from bone, which, like cartilage, is a solid connective tissue consisting of specialized cells and an extracellular material (matrix) that the cells secrete. Bone, however, is much less flexible than cartilage because its extracellular matrix is mineralized by crystals of calcium phosphate. Ray-finned fishes have a mouth at the narrow tip, or apex, of the body, and their fins, made from webs of skin, are supported with hardened rays made of bone.

Ray-finned fishes are the largest group of jawed fishes and the most diverse group of vertebrates, with about 27,000 species. This group includes almost everything you think of as “fish,” from salmon to goldfish.

In addition to a bony skeleton, an important evolutionary development in the ray-finned fishes was the swim bladder, a gas-filled organ that keeps the fish from sinking. This bladder is believed to be homologous to the lungs of land-dwelling (terrestrial) vertebrates, with both having evolved from simple air sacs connected to the gut. Cartilaginous fishes, which have no swim bladder, must constantly move through the water or they will sink.

The third group of jawed fishes, the lobe-finned fishes, are represented by just eight species. These fishes have sturdy pelvic and pectoral fins on the underside of their body (see FIGURE 11-25), which, unlike other fins, have a central appendage containing numerous bones and muscles that connect the fins to the body. As we see in the next section, lobe-fins were useful in initiating the move onto land.