Phanerozoic

The Phanerozoic is the current and the latest of the four geologic eons in the Earth's geologic time scale, covering the time period from 538.8 million years ago to the present. It is the eon during which abundant animal and plant life has proliferated, diversified and colonized various niches on the Earth's surface, beginning with the Cambrian period when animals first developed hard shells that can be clearly preserved in the fossil record. The time before the Phanerozoic, collectively called the Precambrian, is now divided into the Hadean, Archaean and Proterozoic eons.

The time span of the Phanerozoic starts with the sudden appearance of fossilised evidence of a number of animal phyla; the evolution of those phyla into diverse forms; the evolution of plants; the evolution of fish, arthropods and molluscs; the terrestrial colonization and evolution of insects, chelicerates, myriapods and tetrapods; and the development of modern flora dominated by vascular plants. During this time span, tectonic forces which move the continents had collected them into a single landmass known as Pangaea (the most recent supercontinent), which then separated into the current continental landmasses.

The term "Phanerozoic" was coined in 1930 by the American geologist George Halcott Chadwick (1876–1953), deriving from the Ancient Greek words φανερός (phanerós), meaning "visible"; and ζωή (zōḗ), meaning "life". This is because it was once believed that life began in the Cambrian, the first period of this eon, due to the lack of Precambrian fossil record back then. However, trace fossils of booming complex life from the Ediacaran period (Avalon explosion) of the preceding Proterozoic eon have since been discovered, and the modern scientific consensus now agrees that complex life (in the form of placozoans and primitive sponges such as Otavia) has existed at least since the Tonian period and the earliest known life forms (in the form of simple prokaryotic microbial mats) started in the ocean floor during the earlier Archean eon.

The Proterozoic–Phanerozoic boundary is at 538.8 million years ago. In the 19th century, the boundary was set at time of appearance of the first abundant animal (metazoan) fossils, but trace fossils of several hundred groups (taxa) of complex soft-bodied metazoa from the preceding Ediacaran period of the Proterozoic eon, known as the Avalon Explosion, have been identified since the systematic study of those forms started in the 1950s. The transition from the largely sessile Precambrian biota to the active mobile Cambrian biota occurred early in 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.

The Paleozoic is a time in Earth's history when active complex life forms evolved, took their first foothold on dry land, and when the forerunners of all multicellular life on Earth began to diversify. There are six periods in the Paleozoic era: Cambrian, Ordovician, Silurian, Devonian, Carboniferous and Permian.

The Cambrian is the first period of the Paleozoic Era and ran from 539 million to 485 million years ago. The Cambrian sparked a rapid expansion in the diversity of animals, in an event known as the Cambrian explosion, during which the greatest number of animal body plans evolved in a single period in the history of Earth. Complex algae evolved, and the fauna was dominated by armoured arthropods (such as trilobites and radiodontids) and to a lesser extent shelled cephalopods (such as orthocones). Almost all phyla of marine animals evolved in this period. During this time, the super-continent Pannotia began to break up, most of which later recombined into the super-continent Gondwana.

The Ordovician spans from 485 million to 444 million years ago. The Ordovician was a time in Earth's history in which many groups still prevalent today evolved or diversified, such as primitive nautiloids, vertebrates (then only jawless fish) and corals. This process is known as the Great Ordovician Biodiversification Event or GOBE. Trilobites began to be replaced by articulate brachiopods, and crinoids also became an increasingly important part of the fauna. The first arthropods crept ashore to colonise Gondwana, a continent empty of animal life. A group of freshwater green algae, the streptophytes, also survived being washed ashore and began to colonize the flood plains and riparian zones, giving rise to primitive land plants.