At the same time, the rapid spread of forests trapped sediments with their roots, slowing rivers and forming swamps and marshes. The still water became deprived of dissolved oxygen and killed off numerous aquatic animals. This combination of events heralded the end of an era, killing off a huge number of unique animals and plants all over the Earth.
The Carboniferous Period followed this extinction roughly 360 million years ago. On a surface level, it would look as though not much had changed in the forests. Archaeopteris, the dominant trees of the early forests were still around, yet their reign would end soon. For a long time there had been other plants growing in the shadows of the ancient trees, until the middle of the Carboniferous when new actors would take center stage.
The forests had been moving farther away from their waterlogged domains and were starting to diversify. Trees loaded themselves with a molecule called lignin, which formed a massive amount of tree bark. The bark-to-wood ratio was so high that hardly any decomposing organism could digest Carboniferous tree bark. While there were plenty of plant-eating animals on land, their mouthparts were not suited to chew through these tough foods. After their death, these forests left plenty of tissues behind to fossilize, resulting in much of the coal we burn today. At their greatest, the so-called coal forests covered 20 million square kilometers (7.7 million square miles).
The flora of the period was, of course, centered around the swamplands and floodplains that grew inland. Many of the plants at the time were members of a large group known as the pteridophytes, which includes the ferns. They range from the tree-sized individuals in New Zealand and Tasmania to the small commonplace fern species that cover the floor of temperate forests.
Other pteridophytes are the less common but still recognizable clubmosses, quillworts and horsetails. These days they stay tucked in waterlogged or dewy areas to avoid drying out under direct sunlight. Carboniferous pteridophytes on the other hand, were reaching for the sun and by the Late Carboniferous they had become the most common plants in their ecosystems.
In North America, remains of freshwater systems and terrestrial forests were found in areas like Mazon Creek in Illinois, part of a swampy forest over 307 million years old. These sites show us the great variety of flora at the time and the various relationships that had evolved among different organisms. Fossils from the terrestrial part of Mazon Creek, called the Braidwood Beds, show us just how close arthropods were to the plants.
From the Braidwood Beds impressive plant fossils are found that are also seen in many rock beds across the Western Hemisphere. In life, they were among the weirdest plants to have evolved. Some looked like 30 meter (100 foot) tall bottlebrushes while others had sparse leaves and branches while still others were like titanic copies of a modern horsetail.
Some of these include famous plants like the giant quillwort Lepidodendron. They were mostly unbranched, with leaves sitting right at their crown. Its trunk contained very little wood in comparison to its relatives and its bark was capable of photosynthesis. In the fossil record, the trunk sections show up in a raised checkerboard pattern or something similar to crocodile skin. This feature gives the plants their common name — scale trees. Lepidodendron and its relatives reproduced with cone-like organs filled with large spores rather than seeds like most modern plants.
Meanwhile, scientists have discovered fossils in the Carboniferous of the oldest known beetles, as well as millipedes, centipedes, whip spiders, segmented worms and dragonflies. These animals would not look out of place even today, and would get lost in a crowd of their modern descendants. Fossils in these rocks show some of the earliest examples of animals directly eating a living plant, including bite marks and fossil dung.
One fossil shows us that a huge animal had bitten into a cone, only to spit out much of it. The only animal in the coal forests big and strong enough to tackle a cone of this size would be a giant milipede, likely related to the 2.5-meter-long Arthropleura found in Europe and Canada. Although arthopods today are rarely more than a few centimeters, some scientists think the high levels of oxygen in the Carboniferous allowed them to grow to gigantic sizes.
Despite the great diversity of fauna and flora, the period ended with an environmental catastrophe. The event known as Carboniferous Rainforest Collapse was characterized by a change in climate that favored low-growing ferns over bigger trees. The spread of fern prairies fragmented the rainforests, creating separate "islands" of isolated biodiversity when once the same species existed everywhere across the continent.
The progressive drying killed off the species that heavily depended on these fertile ecosystems, ushering a new age for plants and animals. But in those dying forests appeared a new adaptation that allowed plants to survive in a drier world.
This is part three of a five-part series on the evolution of plants.
1: The first conquerors of land
2: Birth of the forests
3: The Age of Coal
4: A tale of flowers and seeds