History of the Earth Part 2: Phanerozoic Eon – Paleozoic Era

In the previous tutorial we paused our story of Earth’s history at the end of the Proterozoic eon, and the Cambrian explosion. The first era of the Phanerozoic Eon is the Paleozoic Era, which lasted from 541 to 252 million years ago. It was a time of great change, and many new types of organisms evolved during the Paleozoic.

The Burgess shale provides a detailed look into the strange creatures of the time, like Hallucigenia. Nature experimented with a wide variety of different body designs during the Cambrian, many of which went extinct. A factor that influenced the rapid radiation of animals was the lack of predators during and immediately after animals first evolved, which allowed for large populations since they were not hindered by predation.

Life began in the oceans, and by the Early Phanerozoic, was beginning to invade the barren lands, pioneered by the first vascular plants, which were later followed by lungfish and other creatures looking for new food sources. The colonization of land by vascular plants had a large impact on the climate by absorbing CO2 from the atmosphere, which cooled the Earth so much that it caused a mass extinction event. Land plants laid the foundation for brand new ecosystems, as other organisms moved into forests looking for food and shelter, the most consequential being a population of lungfish that evolved leg-like fins.

These new animals, amphibians, were the first vertebrates to walk on land. All vertebrates, from dinosaurs to humans, share a common ancestor with amphibians. Before long, the land was colonized by amphibians, which evolved into reptiles near the end of the Paleozoic.

The main innovation that marks the evolution of amphibians to reptiles was the rise of the amniote egg. It allowed reptiles to reproduce on dry land, whereas amphibians must return to water for this part of their life cycle. The Paleozoic was a time of great mountain building along Eastern North America.

North America collided with three separate land masses, causing three separate orogenies, or mountain building events. The first orogeny is called the Taconic Orogeny, occurring 450 million years ago. The second is called the Acadian Orogeny, at 380 million.

And the third and final orogeny, called the Alleghanian Orogeny, occurred 300 million years ago and was responsible for forming the Appalachian Mountains. These mountains were around the size of the Himalayas when they first formed and have extensively eroded over the past 300 million years, leaving behind the resistant stumps of a once great mountain range. The Alleghanian orogeny concluded with the assembly of the supercontinent Pangea at the end of the Paleozoic.

An important geologic period toward the end of the Paleozoic Era is called the Carboniferous Period. A geologic period is a more specific classification of geologic time compared to an era. The Carboniferous Period is named for the large deposits of coal that were formed during this time, deposits that span the entire globe.

Coal forms when plant tissues are rapidly buried in anoxic conditions in coastal and inland swamps. Rapid burial of organic material requires rapid fluctuations in sea level, which are caused by the formation and melting of glaciers, so without Carboniferous glaciers, the Industrial Revolution may not have occurred! In summary, the Paleozoic Era was a time of dramatic transformation, for both the Earth and its inhabitants, so it seems only fitting that the era would go out with a bang.

Let’s move forward and find out precisely what happened to prompt the next era of the Phanerozoic Eon.