Phanerozoic

The Phanerozoic is the current and latest of the four geologic eons in the Earth's geologic time scale. It began about 539 million years ago and continues today. This eon is special because animals and plants became common and lived in many places on Earth. It started in the Cambrian period, when animals first developed hard shells that we can find as fossils.

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 made in 1930 by an American scientist named George Halcott Chadwick. It comes from ancient Greek words for "visible" and "life". The name was chosen because people once thought life only began in the Cambrian period, the first part of this time era. They thought this because they couldn’t find many fossils from earlier times. But later, scientists found signs of complex life from an earlier time called the Ediacaran. Today, we know simple life was already in the oceans even earlier, during the Archean time.

Proterozoic–Phanerozoic boundary

The Proterozoic–Phanerozoic boundary happened 538.8 million years ago. This is when many animals first got hard shells. Hard shells make it easier to find fossils. Before this time, during the Ediacaran period, there were complex animals with soft bodies. But they did not leave many clear traces.

This boundary shows a big change in life on Earth. It marks the shift from mostly still organisms to more active, moving 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. These eras are split into 12 periods.

The Paleozoic era saw the evolution of important animal groups, including arthropods, molluscs and chordates. Chordates include fish, amphibians and animals that could live on land, called amniotes (synapsids and sauropsids).

The Mesozoic era featured the evolution of crocodilians, turtles, dinosaurs (including birds), lepidosaurs (lizards and snakes) and mammals.

The Cenozoic era began after many large animals, such as non-avian dinosaurs, pterosaurs and marine reptiles, were no longer around. This era saw many new birds and mammals develop. 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 special way. This growth followed a pattern that helps us understand how plants and animals increased over time. The growth happened because more species appeared and their communities became more complex.

Climate

During the Phanerozoic, changes in climate were mostly due to the amount of carbon dioxide in the air. Sometimes, carbon dioxide levels and temperatures did not change together, especially during cold periods. Since the Devonian period, big quick changes in carbon dioxide levels were rare. Global temperatures were helped to stay balanced by processes in the phosphorus cycle. When more phosphorus entered the ocean, it helped plants and animals grow. This growth then removed phosphorus from the water. This kept the amount of carbon taken from the air and ocean fairly steady. Very large temperature changes were often linked to times when many plants and animals died out.