Cladogram of Chordates

What can we learn by studying the cladogram of chordates?

The hard body structures of many chordates fossilize well, so there is an excellent fossil record of chordate evolutionary history. The cladogram of chordates presents current hypotheses about relationships among chordate groups. It also shows at which points important vertebrate features, such as jaws and limbs, evolved. The cladogram of chordates is shown in Figure 26–5.

The circles (nodes) in the cladogram represent the appearance of certain adaptive features during chordate evolution. Each time a new adaptation evolved in chordate ancestors, a major adaptive radiation occurred. One notable adaptation, for example, was the development of jaws, which jump-started the adaptive radiation of jawed fishes—now the most diverse chordate group. Other important adaptations include the development of true bone and paired appendages. Refer to the geologic time scale in Chapter 19 as you read about the evolutionary history of chordates.

Nonvertebrate Chordates Two chordate groups lack backbones. These nonvertebrate chordates are tunicates and lancelets. Fossil evidence from the Cambrian Period suggests that the ancestors of living nonvertebrate chordates diverged from the ancestors of vertebrates more than 550 million years ago.

Adult tunicates (subphylum Urochordata) look more like sponges than us. They have neither a notochord nor a tail. But their larval forms have all the key chordate characteristics. The small, fishlike lancelets (subphylum Cephalochordata) live on the sandy ocean bottom. DOL•46–DOL•47