Vertebrate Evolution

Most biologists think that vertebrates originated about 560 million years ago, shortly after the first chordates appear in the fossil record. The oldest known vertebrate fossils are those of tadpolelike jawless fishes. They appear in the fossil record about 560 million years ago. Figure 39-2 shows an artist's reconstruction of one of these fishes. Jawless fishes were the only vertebrates for more than 50 million years. The survivors became the ancestors of today's jawless fishes.

Origin of Jaws and Paired Fins

Almost 450 million years ago, the first fishes with jaws and paired fins appeared. Paired fins increased fishes' stability and maneuverability, and jaws allowed them to seize and manipulate prey. Jaws are thought to have evolved from the first pair of gill arches, the skeletal elements that support the pharynx. Figure 39-3 shows three possible stages in this transformation. Modern fishes—the sharks and rays and the bony fishes—make their first appearance in the fossil record about 400 million years ago.

figure 39-2

Early jawless fishes, such as this Pharyngolepis, lacked paired fins and probably fed on small invertebrates. Most species of early jawless fishes were less than 15 cm (6 in.) in length.

figure 39-2

Early jawless fishes, such as this Pharyngolepis, lacked paired fins and probably fed on small invertebrates. Most species of early jawless fishes were less than 15 cm (6 in.) in length.

figure 39-3

Jaws are thought to have developed from the anterior gill arches of early jawless fishes. These figures represent hypothesized stages of evolution.

Skull

Skull

Gill arch' Gill slit

Jaws are thought to have developed from the anterior gill arches of early jawless fishes. These figures represent hypothesized stages of evolution.

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