By the early Cenozoic era (65 mya), the positions of the continents resembled those of today, but Australia was still attached to Antarctica, and the Atlantic Ocean was much narrower. The Cenozoic era was characterized by an extensive radiation of mammals, but other groups were also undergoing important changes. Flowering plants diversified extensively and came to dominate world forests, except in cool regions. The Cenozoic era is divided into two periods, the Tertiary and the Quaternary.
the tertiary (65-1.8 mya). During the Tertiary period, Australia began its northward drift. By 20 mya it had nearly reached its current position. The early Tertiary was a hot/ humid time, during which vegetation belts shifted latitudi-nally. The Tropics were probably too hot for rainforests, and were clothed in low-stature vegetation instead. In the middle of the Tertiary, however, Earth's climate became considerably drier and cooler. Many lineages of flowering plants evolved herbaceous (nonwoody) forms; grasslands spread over much of Earth.
By the beginning of the Cenozoic era, invertebrate faunas resembled those of today. It is among the vertebrates that evolutionary changes during the Tertiary period were most rapid. Living groups of reptiles, including snakes and lizards, underwent extensive radiations during this period, as did birds and mammals. Three waves of mammals dispersed from Asia to North America about 55 mya. Rodents, marsupials, primates, and hoofed mammals appeared in North America for the first time.
the quaternary (1.8 mya to present). The current geological period, the Quaternary, is subdivided into two epochs, the Pleistocene and the Holocene (also known as the Recent). The Pleistocene epoch was a time of drastic cooling and climatic fluctuations. During four major and about 20 minor episodes of glaciation, massive glaciers spread across the continents, and animal and plant populations shifted toward the equator. The last of these glaciers retreated from temperate latitudes less than 15,000 years ago; this retreat marked the beginning of the Holocene epoch. Organisms of the Holocene are still adjusting to these changes. Many high-latitude ecological communities have occupied their current locations for no more than a few thousand years.
Interestingly, few species became extinct during these climatic fluctuations. However, the Pleistocene was the time of hominid evolution and radiation, resulting in the species Homo sapiens—modern humans (see Chapter 34). Many large bird and mammal species became extinct in Australia and in the Americas when H. sapiens arrived on these continents about 40,000 and 15,000 years ago, respectively. Human hunting may have caused these extinctions, although existing evidence does not convince all paleontologists.
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