Phanerozoic

The Phanerozoic is the current and latest of the four geologic eons in the Earth's geologic time scale. It began about 538.8 million years ago and continues to the present day. This eon is special because it is the time when animals and plants became abundant and filled many places on Earth. It started in the Cambrian period, when animals first developed hard shells that we can find as fossils today.

During the Phanerozoic, many amazing changes happened. Animals, plants, fish, insects, and many other creatures evolved and spread out to live in different places. The landmasses of Earth also moved, coming together into a huge single continent called Pangaea before breaking apart into the continents we know today. This eon includes many important events that shaped the world and the living things that call it home. The time before the Phanerozoic is called the Precambrian, which includes the Hadean, Archaean, and Proterozoic eons.

Etymology

The word "Phanerozoic" was created in 1930 by an American scientist named George Halcott Chadwick. It comes from ancient Greek words meaning "visible" and "life". This name was chosen because people once thought that life only started in the Cambrian period, the first part of this time era. They believed this because they couldn't find many fossils from earlier times. But later, scientists found signs of complex life from an earlier period called the Ediacaran, showing that life existed before the Cambrian. Today, we know that simple life forms were already in the oceans much earlier, during the Archean time.

Proterozoic–Phanerozoic boundary

The Proterozoic–Phanerozoic boundary occurred 538.8 million years ago. It marks the time when many animals first appeared with hard shells, making them easier to find as fossils. Before this, during the Ediacaran period, there were already complex soft-bodied animals, but they did not leave as many clear traces. This boundary shows the start of a big change in life on Earth, from mostly stationary organisms to more active, mobile ones.

Main articles: Proterozoic, metazoan, trace fossils, taxa, Ediacaran, Avalon Explosion, largely sessile Precambrian biota

Eras of the Phanerozoic

The Phanerozoic is divided into three eras: the Paleozoic, Mesozoic and Cenozoic, which are further subdivided into 12 periods. The Paleozoic features the evolution of the three most prominent animal phyla, arthropods, molluscs and chordates, the last of which includes fish, amphibians and the fully terrestrial amniotes (synapsids and sauropsids). The Mesozoic features the evolution of crocodilians, turtles, dinosaurs (including birds), lepidosaurs (lizards and snakes) and mammals. The Cenozoic begins with the extinction of all non-avian dinosaurs, pterosaurs and marine reptiles, and features the great diversification in birds and mammals. Humans appeared and evolved during the most recent part of the Cenozoic.

Biodiversity

During the Phanerozoic, the variety of life on Earth grew in a way that matches a special pattern called the hyperbolic model. This model is often used to understand how human populations grow, and it works well for showing how plants and animals increased over time too. The growth of biodiversity in this period happened because of a connection between how many different species there were and how complex their communities became.

Climate

During the Phanerozoic, the main reason for long-term changes in climate was the amount of carbon dioxide in the air. However, some studies suggest that carbon dioxide levels and temperatures did not always change together, especially during cold times. Since the Devonian period, big swings in carbon dioxide levels—over 2,000 parts per million—were rare in short periods. Global temperatures were kept in check by processes in the phosphorus cycle. When more phosphorus entered the ocean, it helped plants and animals grow more, which then removed phosphorus from the water. This kept the amount of carbon being taken from the air and ocean fairly steady. Big temperature changes of more than 7 °C were often linked to times when many plants and animals died out.