Large Coal Forests

LARGE COAL FORESTS During the second half of the Paleozoic, continents rose and sea levels fell. This led to the departure of the inland sea, which was replaced by large swamps. About 315 million years ago, vast forests grew in large swamps. This area forms a vast tropical belt that runs through the supercontinent Pangaea, which straddles the equator. In the middle Paleozoic, terrestrial flora was abundant and varied. The most significant evolutionary step was the development of vascular rods to drain water to the extremities of plants. Early clubs of mosses, ferns, and horsetails were the first plants to utilize this aquatic vascular system. Early complex land plants diverged into two large groups. One gave rise to lycopods. Others gave birth to gymnosperms, which are the ancestors of many modern land plants. Gymnosperms, including cycads, ginkgos, and conifers, originated in the Permian and gave birth to seeds that lacked a fruit cover. Eventually, as evolution continued to develop, huge coal forests spread across the continent. The forest includes thick stands of seed ferns and true trees, which are gymnosperms with acorns and woody trunks. About 370 million years ago, forests transformed the planet with green, lush vegetation. Lycopods mastered ancient swamps. They soar 130 feet tall and become the first trees to develop true roots and leaves, which are generally small. The branches are arranged in a spiral. Spores are attached to modified leaves that become primitive cones. Their trunks consist mostly of bark and, for the most part, have no branches along the trees, looking like a forest of telephone poles. Only towards the end of their lives do lycopods grow small crowns of limbs as they prepare for reproduction. Making a living among these trees are giant insects, huge millipedes, walking fish, and primitive amphibians. The ancestors of dragonflies with a wingspan of 3 feet, mayflies the size of swallows, and other monstrous insects ruled the skies 300 million years ago. In one of the longest battles on Earth, plants and insects have been fighting each other for more than 300 million years. Some of the fiercest wars were staged in the tropics. The hungry insect fiercely attacks vegetation, which protects itself with several defensive weapons, including chemical warfare. The great success of insects may be due to the abundance of atmospheric oxygen. During the Carboniferous, oxygen levels may have reached 35 percent of Earth's atmosphere compared to 21 percent today. Growing up is an insect's way of utilizing oxygen-rich air. Higher oxygen levels may have given early amphibians a chance to develop their lungs and become permanent on land. However, by 245 million years ago, the heyday of oxygen was over. The rate dropped to 15 percent, which may have contributed to the massive extinction at the end of the Permian. Over millions of years, lycopods underwent changes in sea level and climate that alternately dried up and flooded swamps. Then about 310 million years ago, the tropical climate became drier and most of the marshland disappeared entirely. Climate change triggered a wave of extinctions that wiped out nearly all lycopods as early as the Permian 280 million years ago. Today, they exist only as small grass-like plants in the tropics. At the end of the Carboniferous, as the climate grew wetter and swamps reappeared, weedy plants called tree ferns (Fig. 110) dominated the Paleozoic wetlands. The second most diverse group of living plants are true ferns. They range from Devonian to the present. However, they are very widespread in the Mesozoic and prosper well in mild climates even at higher latitudes. Instead, today they are confined to the tropics. Some ancient ferns reached the height of present-day trees. Permian seed fern Glossopteris is very significant. Its fossil leaves were prevalent on the continent that formed Gondwana but less so on the continent that comprised Laurasia.This suggests that these two landmasses were in separate parts of the world divided by the Tethys Sea.This body of water is wide in the east and narrow in the west, where land bridges aid the migration of plants and animals from one continent to another. Terrestrial fossils are not nearly as numerous as those from the sea, mainly because terrestrial species do not fossilize well and fossil-containing sediments undergo erosion. However, some environments such as swamps and swamps provide many plant and animal fossils. Well-preserved carbonized plant material is usually found among easily separated sedimentary layers (Fig. 111). Animals are also buried in large ancient coal swamps, where their bones are preserved and fossilized.