This section introduces two of the earliest groups to branch off the vertebrate tree: the jawless fishes, whose shared features include a specialized suction disk and rasping tongue, and the cartilaginous fishes, which have calcified cartilage instead of true bone.

	Eating without jaws Because lampreys lack true jaws, they cannot bite or chew. To eat, this sea lamprey (Petromyzon marinus) latches onto the side of a fish or mammal with its circular mouth and scrapes through the skin with its rasping "tongue" to feast on blood. Breck P. Kent / Animals Animals

Before Jaws
Jawless fishes were among the earliest vertebrates. Most jawless species went extinct long ago and are known only from the fossil record. But a few survived. Today, the remaining descendants of this ancient lineage are represented by about 40 species of lampreys.

The ancestors of today's lampreys branched off the evolutionary tree after the first vertebrates had evolved, but before the first jawed vertebrates. Thus lampreys have backbones, but no jaws.

An even more primitive group, whose descendants include the modern-day hagfishes, branched off the tree before the lampreys. Hagfishes were once considered close relatives of lampreys, because both lack jaws. But unlike lampreys, hagfishes appeared before the backbone had evolved, so hagfishes do not have backbones and are not included on the vertebrate tree. Hagfishes do have a nerve chord in their back, however, placing them with the vertebrates in the larger chordate group.

Vertebrates that evolved after the lampreys inherited jaws from a common ancestor that arose later. Jaws made many new feeding techniques possible including grasping, biting, and suction feeding—opening up a world of evolutionary opportunities.

	Shark or Ray? The flattened body of this angel shark (Squatina californica) could cause it to be mistaken for one of its close relatives, the rays. Jeffrey L. Rotman / Photo Researchers

From Sharks to Rays
The cartilaginous fishes include about 400 different species of sharks and their close relatives the rays, which are even more numerous with about 600 living species. Rays have wide, flattened bodies and swim by flapping or undulating their winglike pectoral fins.

Rays evolved from early sharks. Some of the last sharks to evolve before rays appeared were the angel sharks. Today's angel sharks are the descendants of a transitional stage between sharks and rays, and they look much like rays.

In addition to lacking true bones, sharks and rays also lack gas bladders. Some compensate by having large, fatty livers for buoyancy. But many angel sharks and rays take advantage of being heavier than water by burying their flat bodies in the sand to ambush prey.

An endless supply of teeth Besides cartilaginous bones, another trait shared by all sharks and rays is the ability to constantly grow new teeth and replace them, as seen in this sand tiger shark (Carcharias taurus). Because shark teeth are shed continually—and these teeth are made of hard enamel, not cartilage—ancient shark teeth are among the most abundant vertebrate fossils in the world. Marty Snyderman / Visuals Unlimited, Inc.

Before True Bones
The bones of sharks, rays and chimaeras are made primarily of cartilage, a tough, flexible protein matrix. But these cartilaginous bones are not soft, because they are coated with hard calcium minerals.

The fossil record indicates that cartilaginous fishes probably arose before bony fishes and they lack many features that evolved later, such as true bone and a buoyant lung or gas bladder. But it would be a mistake to think of them as "primitive." Cartilage has several advantages over true bone, being tougher, lighter and more flexible. We know that some early ancestors of this group actually had some true bone in their skeletons. So it may be that modern cartilaginous fishes actually replaced bone with cartilage - contradicting the idea of cartilaginous fishes as primitive or inferior. Sharks and rays also have several other advanced features, such as internal fertilization, and many give birth to live young. Some sharks even produce placental "milk" to nourish developing fetuses.