key concept 32.4 Life on Land Contributed to Vertebrate Diversification

The evolution of lunglike sacs in fishes set the stage for the vertebrate invasion of the land. Some early ray-finned fishes probably used those sacs to supplement their gills when oxygen levels in the water were low, as many groups of ray-finned fishes do today. But with their unjointed fins, those fishes could only flop around when out of water. Changes in the structure of the fins first allowed lobe-limbed vertebrates to support themselves better in shallow water, and later, to move better on land.

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  • Most modern amphibians are confined to moist environments because they rapidly lose water when their skin is exposed to air.

  • Birds are a specialized group of theropods and are the only group of dinosaurs to survive the mass extinction event at the end of the Cretaceous.

  • Feathers are modified scales, which probably evolved first for insulation and then for flight.

  • Mammals increased dramatically in number, diversity, and size after non-avian dinosaurs disappeared during the mass extinction at the end of the Cretaceous.

  • Mammals are distinguished by having sweat glands, mammary glands, hair, and a four-chambered heart.

  • Movements of the continents affected the diversification of eutherians.

  • Modifications of the amniote egg allowed the embryo to develop inside its mother’s body