Lichens, which are a symbiotic relationship between algae and fungi
whereby the two mutually live off each other, probably took the first tentative
steps onto dry land. Following the lichens were mosses and liverworts. Fungi also
had a symbiotic relationship with the roots of plants when they first evolved, aid-
ing vegetation with the uptake of nutrients and receiving carbohydrates in
return. Bacteria also played an important role in the fixation of nitrogen, an abun-
dant soil gas and an important nutrient for plant life. In the nitrogen cycle, den-
itrifying bacteria convert the dissolved nitrate back into elemental nitrogen.
Otherwise, all nitrogen in the atmosphere would have long ago disappeared.
THE ORDOVICIAN ICE AGE
Species that rapidly evolved during the early Cambrian advanced significantly
inthe warm Ordovician seas. The warming was largely because the atmos-
phere held as much as 16 times today’s carbon dioxide content, enough to heat
the climate to tropical levels even though the Sun was 4 percent dimmer than
at present.The average global temperature was about 18 degrees Celsius, some
8 degrees hotter than today. Corals, which require warm waters, began build-
ing extensive carbonate reefs. In addition, the first fish appeared in the ocean.
The existence of freshwater jawless fish on the continents suggests that lakes
and streams were inhabited by red and green algae.
Plants began to invade the land and extend to all parts of the world dur-
ing the late Ordovician about 450 million years ago. The early land plants
absorbed large quantities of atmospheric carbon dioxide. Rapid burial under
anaerobic conditions deposited the organic carbon into the geologic column,
where it converted into coal. Plants also aided the weathering process by
leaching minerals from the rocks. Carbonate rocks, such as limestone
deposited by shelly organisms from the Cambrian onward (Fig.67), locked up
massive amounts of carbon dioxide.
The withdrawal of substantial amounts of carbon dioxide from the
atmosphereweakened the greenhouse effect.The resulting climate cooling ini-
tiated in large part by the plant invasion spawned a major ice age at the end of
the Ordovician about 440 million years ago. At the time, the southern edge of
Gondwana was just over the South Pole, where an ice sheet grew to about 80
percent the size of present-day Antarctica. The glaciations of the late Ordovi-
cian and the glacial epochs of the middle and late Carboniferous about 330
million and 290 million years ago might have been influenced by a reduction
of atmospheric carbon dioxide to roughly one-quarter of its present value.
Atmospheric scientists have amassed information on global geochemical
cycles to ascertain the cause of such a radical change in the carbon dioxide con-
tent of the atmosphere. Data from deep-sea cores showed that carbon dioxide
ORDOVICIAN VERTEBRATES