Table 12.1: (continued)Era Millions of Years Ago Major Forms of Life
500First fish on EarthThe eras of the Phanerozoic Eon are separated by events called mass extinctions. A mass
extinction occurs when large numbers of organisms become extinct in a short amount of
time. Between the Paleozoic and the Mesozoic, nearly 95% of all species on Earth died off.
The cause or causes of this extinction are still being debated.
Between the Mesozoic and the Cenozoic, about 50% of all animal species on Earth died off.
This mass extinction, 65 million years ago, is the one in which the dinosaurs became extinct.
Although there are other hypotheses, most scientists think that this mass extinction took
place when a giant meteorite struck Earth with the energy of the most powerful nuclear
weapon. The impact kicked up a massive dust cloud. When the particles rained back onto
the surface they heated the atmosphere until it became as hot as a kitchen oven, roasting
animals. Dust that remained in the atmosphere blocked sunlight for a year or more, causing
a deep freeze and ending photosynthesis. Sulfur from the impact mixed with water in the
atmosphere to form acid rain, which dissolved the shells of the tiny marine plankton that
formthebaseofthefoodchain. Withlittlefoodbeingproducedbylandplantsandplankton,
animals starved. Carbon dioxide was also released from the impact and eventually caused
global warming. Life forms could not survive the dramatic temperature swings.
Earth’s climate changed numerous times during the Phanerozoic Eon. Just before the be-
ginning of the Phanerozoic, much of the Earth was cold and covered withglaciers(Figure
12.21). As the Phanerozoic began, however, the climate was changing to a warm andtrop-
icalone (Figure12.22). The glaciers were replaced with tropical seas. This allowed the
Cambrian Explosion of many new life forms on Earth. During the Phanerozoic, Earth’s
climate has gone through at least 4 major cycles between times of cold glaciers and times of
warm tropical seas. Some organisms survived environmental changes in the climate; others
became extinct when the climate changed beyond their capacity to cope with it.