Geology Reference
In-Depth Information
Cooling tropical waters
The first major mass extinction that scientists know about happened approximately 445
million years ago toward the end of the Ordovician period (in the Paleozoic era). At the
time, life was lived in the oceans; no evidence indicates that land plants or animals exis-
ted yet. Scientists think the expansive marine environment was affected by a cooling cli-
mate and abrupt changes in sea level as extensive glaciers grew over the continents of
the South Pole. More than 100 families of marine organisms, primarily those living in
tropical regions near the equator, went extinct. This totaled more than 50 percent of the
living families of that period.
Scientists conclude from the evidence for glaciation that the colder climate at the poles
meant conditions in the tropics were also cooler, leaving the warm water-adapted or-
ganisms nowhere to go. The amount of water locked up as ice over the South Pole may
also have dramatically lowered sea levels all over the planet, reducing the habitat for un-
dersea organisms.
Reducing carbon dioxide levels
At the end of the Devonian period (also in the Paleozoic era), around 370 million years
ago, another extinction event affected marine life. This event seems to have affected
reef-building organisms living in shallow marine environments, as well as some groups
of early land plants.
