TellUs: "The Earth Story" Series, Mass Extinctions & Ice Ages

The Earth’s Story Series: Palaeozoic Era

Over the remaining ‘Earth Story Series’ blogs I will upload, the next three era’s entire timespans happen within the Phanerozoic eon, or “the time of visible life”. This timespan is a lot shorter than the previous eons discussed in previous blogs. This Phanerozoic eon encompasses the Palaeozoic, Mesozoic and Cenozoic era’s (the last two will be discussed in future blogs), spanning  over approximately 550 million years, just over a tenth of the Earth’s history. However the pace of change is fast with a major event occurring frequently, hence the need for division within deep time. This is defined by the changing of an eon (the three mentioned above) or a period. 

Today I will be sharing the first era that appears within the time of visible life, the Palaeozoic era, occurring around the 550 million years ago mark. 

Palaeozoic Era

Firstly all five major divisions of life, the five kingdoms, have evolved. The journey begins in the Palaeozoic Cambrian period with the “Cambrian explosion”, made famous by the extortionate diversity of marine life founded in the Burgess Shale rock type, high up in the Canadian Rocky Mountains. Animals with hard skeletons or shells appear for the first time in oceans. Their remains are seen today as fossils and their appearance is why this section of deep time is known as the sign of visible life. Reef-building corals and sponges start to appear and so do the first vertebrates! Teetering into the Silurian period a major mass extinction occurs, coinciding with an ice age.

However not long into the Silurian period plants and animals begin to emerge out of the oceans and colonise the land, joining fungi, bacteria and algae. Finally all kingdoms of life have expanded beyond their birthplace, the oceans. This is a pinnacle step for evolution! Life is now flourishing on land, fresh water and in the oceans. As a result the rate of photosynthesis has increased, along with other important processes, that allow the maintenance of high levels of oxygen in the air and ozone layer. 

Moving through the Devonian period, life is thriving in shallow tropical seas and predators are now present. On land the first vertebrate animals appear. Plants, similar to mosses and ferns have taken to swampy areas, some developing into larger trees and forming the first forests! Marking the boundary between the Devonian and Carboniferous periods, another mass extinction occurs. This time as a prolonged series of extinctions that eliminate approximately 70% of all species! The causes are still somewhat unclear, however it is believed to have coincided again with a colder climate.

The Carboniferous period see the first winged insects (damn it) and amphibians. Warmth returns and these early forests flourish in the humid, swampy conditions. When they die, many of the trees and plants are preserved within the swamps and are slowly transformed into the largest coal deposits that are found today! This process however reduces carbon dioxide in the atmosphere, cooling the climate. This is a likely cause for a series of major glaciations that occur later on and that last for tens of millions of years!

The Palaeozoic era ends with the most catastrophic mass extinction ever seen, with estimates of 95% of all marine species and 70% of all land species becoming extinct. It’s known that most oceans lost their oxygen at this time, which would have been deadly. Again the causes of this aren’t totally clear, although a loss of oxygen within oceans is a sign that any oxygen that might have been dissolved at the waters surface is no longer being transported to their depths. This indicates that the movement of deep ocean currents have been interrupted. Funnily enough this event coincides with periods of extensive volcanic activity, in what is now Siberia, marking the coming together of the Earth’s major landmasses to form the single continent Pangaea. Fun fact: Pangaea means ‘whole Earth’!

Map of the Earth showing the shape & distribution of its continents 255 million years ago