The Phanerozoic is one of the four eons represented on the geologic time scale. It is the current eon, the shortest. All previous eons are grouped under the Precambrian supereon. It was once believed that life began at the start of the Phanerozoic, but we now know that this is untrue. Despite that, life became much more abundant and complex during this eon.
The Phanerozoic is subdivided into three eras, which are further divided into periods and epochs. The boundaries between these periods of time are determined by significant events evidenced in the fossil record, like mass extinctions.
The Paleozoic era is the first era of the Phanerozoic, and the longest. The Paleozoic is noted by the emergence of diverse animal and plant life, as well as significant climate and geological change.
The Cambrian Period is the first period of the Paleozoic. Most major groups of animals appeared in the fossil record during this period. This event is known as the Cambrian explosion, because of the short amount of time in which this diversity appeared - just 40 million years.
The Ordovician is the next period of the Paleozoic. Life would continue to diversify in the world’s seas. Some invertebrates would begin moving on to land.
Glaciers forming on Gondwana, a supercontinent settled over the south pole, would cause a change in sea level, and begin an ice age. This change resulted in an extinction event that devastated marine environments, eliminating nearly 85% of marine species.
The Silurian is the third period of the Paleozoic. The animal world rebounded from the extinctions for most of the period. Plants also began to colonize land, originating with freshwater algae that adapted to life in drying pools. Invertebrates would follow en masse.
The Devonian is the fourth period of the Paleozoic. It is marked by the evolutionary radiation of fish, hence its nickname, the age of fish. The first trees would appear toward the end of the period. The ancestors of tetrapods, four-legged vertebrates, began adapting to walking on land.
An extinction event occurred during the end of the Devonian period, affecting jawless fish, trilobites, ammonites, and more. The causes of this event are still unknown.
The Carboniferous is the fifth period of the Paleozoic, named for the coal deposits that formed due to its swamps. In the US, it can be further split into the Mississippian and the Pennsylvanian. Seed plants appeared during this period, and animals would continue to evolve on land, including tetrapods like amphibians. Reptiles appear for the first time.
Global climate change caused the collapse of the world’s rainforests. This caused some amphibian species to go extinct as their habitats shrunk and changed. Reptiles fared better, thanks to adaptations that allow them to survive in drier habitats.
The Permian is the last of the Paleozoic’s periods. Reptiles would rise to dominance in the dry climates of the supercontinent Pangaea. Amphibians would decline, but not disappear entirely. The ancestors of mammals, turtles, lizards, and archosaurs (like birds and crocodiles) would all appear during this period.
The end of the Paleozoic era is marked by a massive extinction event known as the Permian-Triassic extinction, or the Great Dying. 95% of marine life, and 75% of terrestrial life, died out due to volcanic eruptions poisoning the atmosphere. It is the largest extinction event in Earth’s history.
The Mesozoic is the second proper era of the timescale. It is also called the Age of Reptiles, as they and other sauropsids were the dominant animals at the time. Dinosaurs are the most famous creatures from this era, but mammals, pterosaurs, and flowering plants would appear as well.
The Triassic is the first period of the Mesozoic. The climate in Pangaea was very dry, especially at its interior. Therapsids and archosaurs managed to thrive in these conditions, and despite the massive extinction. Dinosaurs first appeared during this time, as did frogs, true mammals, and gingko trees.
The Triassic ends as it began, with a major extinction event. 34% of marine genera went extinct, including an entire class of jawless fish. Many archosaurs, therapsids, and large amphibians also went extinct.
The Jurassic is the second period of the Mesozoic. Dinosaurs would flourish in the wake of the extinction, and become the dominant animals. Pangaea had broken up toward the end of the Triassic, and the climates changed, becoming cooler and more humid. Insects became part of plant reproduction by transporting pollen during this period, and the first birds appeared.
The Cretaceous is the third, and last, period of the Mesozoic. Flowering plants appeared at the start of this period. Dinosaurs reached the height of diversification, and birds, crocodiles, and mammals continued to develop. By the end of the period, the continents were roughly in the places they sit today. Some of the largest dinosaurs, pterosaurs, and crocodylomorphs lived during this period.
The K-Pg event is one of the most famous mass extinctions. It is thought that the event was caused by a massive asteroid impact. 75% of all species on earth went extinct. Non-avian dinosaurs, pterosaurs, plesiosaurs, mosasaurs, and ammonites disappeared completely, and many other groups saw massive losses.
The Cenozoic is the current geological era, ongoing today. In the wake of the K-Pg event, birds and mammals began to fill the niches left empty by the dinosaurs. The era became known the Age of Mammals, due to the dominance and diversity of the latter. Mammalian megafauna and the rise of humans are hallmarks of this era.
The Paleogene is first period of the Cenozoic. The world became more recognizable as the continents continued to move toward their present-day positions. Mammals diversified widely, and plants recovered from the changes in climate. Many fish and reptiles would gain recognizable forms.
The Paleogene is marked by a series of global warming and cooling cycles. The thermal Maximum was a period during one such cycle when temperatures rose 5°C worldwide. This caused changes in ocean and atmospheric circulation, which would lead extinctions and turnovers.
The Neogene is the second period of the Cenozoic. Birds and mammals began to adopt familiar, modern forms. North and South America were connected by the Isthmus of Panama, and India would meet the Asian continent, forming the Himalayas. The first hominids appear toward the end of this period in Africa.
The Quaternary is the third, and current, period of the Cenozoic. The period is marked by an ongoing ice age and various glaciations. Giant mammals lived during the last glaciation period, and modern humans developed within the last 315,000 years.