Earth's Devonian Period

The Devonian Period lasted from around 415 to 360 million years ago and ended, like the Ordovician, with a mass extinction event. It is during this period that diversity of life on land began to explode, particularly with respect to plants.

The Devonian Period saw significant movement of the continents, as the supercontinents of Laurussia and Gondwana began to drift closer together. Eventually this continental drift would cause the fusion of all the major continents into a giant land mass called Pangea. As with the Silurian Period, the sea levels during the Devonian were relatively high, causing flooding of the continents and the formation of shallow seas. The giant ocean Panthalassa still existed and provided a number of diverse, deep-water ecosystems.

Marine life still continued to be dominated by brachiopods and corals, although there were a number of evolutionary explosions in other families of organisms. Fish were becoming incredibly diverse, evolving past their small, shield-headed forms that were common in the Silurian period. Now more familiar families of fish were evolving, including primitive sharks and ray-finned fish. A now extinct family of armored fishes known as placoderms flourished as predators during the Devonian Period. One species of placoderm, the Dunkleosteus, could reach lengths of up to 33 feet, weigh four tons, and had one of the most powerful bites of any creature to live on Earth. These titanic fish illustrate the great leaps made by bony fish during a mere fifty million years. Toward the end of the Devonian, a very special group of fish called lobe-finned fish was evolving. These fish contain powerful fins that allow them to move to a limited extent on land. Present-day mudskippers are lobe-finned fish, and these curious creatures are able to spend part of their time out of water. It would be this peculiar family of fish that would give rise to land-dwelling tetrapods when the lobe fins would evolve into the first limbs.

Perhaps the most fascinating evolutionary explosion was taking place on land. Although land fauna was still in its infancy and restricted to small animals like arthropods, Earth's flora was flourishing. In the early Devonian Period there were small, primitive plants, many of which did not contain vascular tissue. The largest of these would not have exceeded a meter in height. However, by the end of the eon, the continents were covered by vast forests of primitive ferns and lycophytes, the oldest type of vascular plant that still exists today. Also, by the end of the Devonian, the first seed-forming plants appeared which allowed forests to thrive on the once barren badlands of the continents. These seed-producing plants could reach heights of up to thirty meters and allow for canopy ecosystems to develop within the forests. The symbiosis between insects and plants began during the Devonian as well, in which insects helped to propagate plants and replenish soil nutrients.

The creation of vast forests on the continents had a consequence for Earth's atmosphere. Since plants use photosynthesis to convert sunlight and carbon dioxide into sugars and oxygen, the greenhouse gases that were warming the Earth were depleted. As a result, oxygen levels went up considerably in the atmosphere. This would eventually allow more and more animals to move out of the water and onto land. However, the carbon dioxide depletion may have contributed to the mass extinction at the end of the Devonian Period. A drop in global temperature due to a reduced greenhouse effect could have altered ocean currents and storm cycles considerably, leading to the destruction of a number of ecological niches. In addition, possible glaciation of Gondwana as in the Ordovician could have potentially lowered sea levels and altered the climate and ocean currents.