Geology Reference
In-Depth Information
Only slightly less dramatic than the earlier extinction event, the Late Devonian extinc-
tion saw almost 50 percent of the existing families disappear. The fact that organisms in
shallow marine waters as well as on land were affected has led scientists to hypothesize
that atmospheric conditions, such as changes in carbon dioxide levels, played a large
role in this event.
The early plants themselves may have altered atmospheric levels of carbon dioxide
through photosynthesis. Less carbon dioxide leads to cooler global climate conditions,
which may then have affected marine life in warm, shallow sea ecosystems.
The Great Dying
A mass extinction event that marks the transition between the Paleozoic and Mesozoic
eras, about 250 million years ago, is called
The Great Dying,
the
Permian-Triassic event,
the
Permo-Triassic extinction,
or the
End-Permian extinction.
At this time, more than 96
percent of species in the oceans and 70 percent of the species on land (including some
plants) perished. The End-Permian extinction is the only extinction event in earth's his-
tory to affect insects, resulting in a loss of 33 percent of the insect species of the time.
Scientists are not certain what caused the End-Permian extinction. This event appears
to have occurred over a few million years, leading scientists to rule out an impact as the
primary cause.
At the time of this extinction, the supercontinent of Pangaea (see Chapter 19)was form-
ing, which may have changed ocean circulation patterns and temperatures more quickly
than species could adapt to. But some scientists argue that by the time of the extinc-
tions, the landmasses had already moved and shouldn't have further changed the mar-
ine environments in any important way.
This mass extinction was most severe in the oceans, leading some scientists to conclude
that global water conditions must have experienced a disruptive change. One explana-
tion may be that the oceans became
anoxic:
lacking in dissolved oxygen.
Oxygen levels in the ocean are maintained by the circulation of surface wa-
ters, which cool near the poles, sink (taking oxygen-rich water into the deep sea),
and move back toward the equator. This circulation of water due to changes in
temperature and salinity (the amount of salt it contains) is today called the
ther-
mohaline ocean conveyor.
