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much like a forest of telephone poles. Only near the end of their lives did the
lycopods sprout a small crown of limbs as they prepared for reproduction.
Making their living among these trees were giant insects, huge milli-
pedes, walking fish, and primitive amphibians. The ancestors of the dragonfly
with 3-foot wingspans, swallow-sized mayflies, and other monstrous insects
ruled the skies 300 million years ago. In one of the longest-running battles on
Earth, plants and insects have fought each other for more than 300 million
years. Some of the fiercest wars were staged in the tropical regions. Hungry
insects fiercely attacked vegetation, which protected itself with multiple
defensive weapons, including chemical warfare.
The great success of the insects was probably due to an abundance of
atmospheric oxygen. During the Carboniferous, oxygen levels might have
reached 35 percent of Earth’s atmosphere as compared with today’s 21 per-
cent. Growing big was an insect’s way of taking advantage of the oxygen-rich
air.The higher oxygen levels might have given the early amphibians a chance
to develop their lungs and become permanently established on land. However,
by 245 million years ago, the oxygen heyday was over. Levels dropped to 15
percent, which might have contributed to the massive extinctions at the end
of the Permian.
For millions of years, the lycopods endured changes in sea level and cli-
mate that alternately drained and flooded the swamps. Then about 310 mil-
lion years ago, the climate of the tropics became drier and most swamplands
disappeared entirely. The climate change set off a wave of extinctions that
wiped out virtually all lycopods at the beginning of the Permian 280 million
years ago.Today, they exist only as small grasslike plants in the tropics. Late in
the Carboniferous, as the climate grew wetter and the swamps reemerged,
weedy plants called tree ferns (Fig. 110) dominated the Paleozoic wetlands.
The second most diverse group of living plants were the true ferns.They
ranged from the Devonian to the present. However, they were particularly
widespread in the Mesozoic and prospered well in the mild climates even in
the higher latitudes. In contrast, today they are restricted to the tropics. Some
ancient ferns attained heights of present-day trees. The Permian seed fern
Glossopteriswas especially significant. Its fossil leaves are prevalent on the con-
tinents that formed Gondwana but are lacking on the continents that com-
prised Laurasia.This indicates that these two landmasses were in separate parts
of the world divided by the Tethys Sea. This body of water was wide in the
east and narrow in the west, where land bridges aided the migration of plants
and animals from one continent to the other.
Terrestrial fossils are not nearly as abundant as those of marine origin, pri-
marilybecause land species do not fossilize well and fossil-bearing sediments
are subjected to erosion. However, some environments such as swamps and
marshes provided an abundance of plant and animal fossils. Well-preserved,
147CARBONIFEROUS AMPHIBIANS