middle Paleozoic fern Glossopteris(Fig. 68),named from the Greek word
meaning “featherlike,” and whose fossil leaf impressions actually look like
feathers, is found in coal beds on the southern continents and India. However,
the plant is suspiciously absent on the northern continents. This suggests the
existence of two large continents, one located in the Southern Hemisphere
and another in the Northern Hemisphere, separated by a large open sea.
Matches between mountains in Canada, Scotland, and Norway indicated their
assembly into the northern supercontinent Laurasia during this time.
At the end of the Ordovician,glaciation reached its peak. Ice sheets radi-
ated outward from a center in North Africa.Around 430 million years ago, the
ice sheets largely disappeared.As Gondwana continued drifting southward, the
farther south it went, the smaller the ice sheets became. When the center of
the continent neared the South Pole, the winters in the interior became
colder.Yet the land warmed sufficiently during the summer to melt the ice.
Meanwhile,the southern glaciated edge of Gondwana moved northward into
warmer seas, and the glaciers soon departed.
THE IAPETUS SEA
During the late Precambrian and early Cambrian, a proto–Atlantic Ocean
called the Iapetus opened between divided continents, forming extensive
Cambrian inland seas. The inundation submerged most of the surrounding
ancient North American continent called Laurentia and the ancient European
continent called Baltica. The Iapetus Sea was similar in size to the North
Atlantic and occupied the same general location about 500 million years ago
(Fig. 69). A continuous coastline running from Georgia to Newfoundland
Figure 68Fossil
Glossopterisleaves
helped prove the theory of
continent drift.ORDOVICIAN VERTEBRATES