Earth's Carboniferous Period

The Carboniferous Period followed the Devonian mass extinction and lasted from around 359 million to 299 million years ago. This is a period in Earth's history that saw a proliferation of life on land as well as a drastic change in the atmosphere which would have large effects on many forms of life.

It is during the course of the Carboniferous Period that the supercontinent of Pangaea began to come together. This continental collision had the effect of pushing up many mountain ranges that still exist today such as the Appalachians and the Ural mountains. By the late Carboniferous, the supercontinent of Pangaea was not quite fused into one solid mass, but was rather shaped like a cheerio. The great ocean of Panthalassa surrounded the outside of the supercontinent and another large ocean called the Paleo-Tethys occupied the space in the interior. By now many of the shallow seas that had formed the backbone of many marine ecosystems in the Silurian and Devonian Periods were beginning to drain off the continents due to a lack of rain or feeding waterways. The climate of the early Carboniferous was generally more tropical than in other eons, which allowed evolution to run wild in both the oceans and on land. However, glaciation at the poles caused drier and colder climes in regions away from the equator, which would eventually spread into most land regions once Pangaea fused entirely.

The Carboniferous was a time of vast forests. The majority of the coal that was used during the Industrial Revolution was formed by the compaction of millions of years of dead Carboniferous vegetation. Ferns were becoming more widespread and diverse, and by the end of the eon, the first primitive conifer trees were appearing on the continents. Bark-bearing trees began to form during this period, which allowed forests to exist in swampier areas. Bark prevented excess water from entering the tissues of the trees and prevented rotting. The success of these forests allowed the atmosphere of the earth to change dramatically. Oxygen content in the Carboniferous was as high as 35% as compared to the 21% content of today's atmosphere.

Fish continued to diversify during this period, and the primitive tetrapods that hauled themselves out of the water during the Devonian were beginning to make more marked strides on land. Sharks began to replace the armored placoderms of the Devonian as the top predators of the Carboniferous seas. Giant rhizodont fish reaching 7 meters in length were also fearsome predators during this time period.

Arthropods began to flourish on land during the Carboniferous. This is partially due to the increase in terrestrial ecosystems as well as the increase in oxygen in the atmosphere. Insects do not have lungs, and their size is restricted by the amount of oxygen in the atmosphere that they can diffuse through their skin. The high oxygen content caused by the vast forests on the continents allowed many new insects to evolve, such as dragonflies, millipedes, scorpions, and spiders. These insects also could reach much larger sizes that they can today due to the high oxygen content in the atmosphere. Dragonflies with a meter-long wingspan and spiders the size of dinner plates were quite common in the Carboniferous swamp forests. The largest terrestrial arthropod ever to live on the earth, a millipede up to two meters in length, lived during the Carboniferous Period. In general, the mass of decaying plant matter fostered an explosion of insect diversity.

Amphibians made a very important step during the Carboniferous Period with respect to their eggs. Amphibians were restricted to the water during the late Devonian Period and early Carboniferous due to the nature of their skin and eggs. Amphibian skin is very thin and requires constant moisture in order to prevent it from drying out and splitting. Furthermore, amphibian eggs are soft and their young have gills, not lungs. These qualities restrict amphibians close to water during their life cycles. However, certain groups of amphibian tetrapods began to evolve during the Carboniferous. Their once fish-like bodies now became flatter and their limbs became shorter to allow for easier and more efficient land predation. Also, some amphibians began to develop amniotic eggs for reproduction. These eggs contain a hard outer shell and can be laid on land, where the animal can look after them. The rising importance of amniotic eggs led to the evolution of the first reptiles on earth.

As the supercontinent of Pangaea began to fuse, the wet conditions needed for the vast tropical forests began to diminish. The subsequent drop in oxygen levels in the atmosphere spelled the end for the era of the giant insects. In coming times when the Earth would enter a much more seasonal, dry climate, the reptiles would become the most adaptable organisms on land.