Unit Five. Evolution of Animal Life
20. History of the Vertebrates
The relatively warm, moist climates of the early Cenozoic era (65 M.Y.A. to present) have gradually given way to today’s colder and drier climate. The first half of the Cenozoic was very warm, with junglelike forests at the poles. With the extinction of the dinosaurs and many other organisms and a change in climate, new forms of life were able to invade new habitats. Mammals diversified from earlier, small nocturnal forms to many new forms. Most present-day orders of mammals appeared at this time, a period of great diversity.
About 40 million years ago, the climate began to cool, ice caps formed at the poles, and the world entered into an ice age. As glaciation in Antarctica and the Northern Hemisphere became fully established by about 13 million years ago, regional climates cooled dramatically. A series of glaciations followed, the most recent ending about 10,000 years ago. Many very large mammals evolved during the ice ages, including mastodons, mammoths, saber-toothed tigers, and enormous cave bears (table 20.1).
TABLE 20.1. SOME GROUPS OF EXTINCT MAMMALS
The Antarctic, Arctic, and Greenland ice masses that formed as a result of these glaciations have made the world’s climate cooler near the poles, warmer near the equator, and drier in the middle latitudes than ever before. In general, forests covered most of the land area of continents, except for Antarctica, until about 15 million years ago, when the forests began to recede rapidly and modern plant communities appeared. During the past several million years, the formation of extensive deserts in northern Africa, the Middle East, and India made migration between Africa and Asia very difficult for organisms of tropical forests. Overall, the Cenozoic era has been characterized by sharp differences in habitat, even within small areas, and the regional evolution of distinct groups of plants and animals. Although there has been an overall decline in mammalian species, these factors have facilitated the rapid formation of many other new species.
Key Learning Outcome 20.3. We live in the Cenozoic era, the Age of Mammals. Many large mammals common in the ice ages are now extinct.