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 million years ago to the present. It is the eon during which abundant animal and plant life has proliferated, diversified and colonised 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 colonisation and evolution of insects, chelicerates, myriapods and tetrapods; and the development of modern flora dominated by vascular plants. During this interval, plate tectonic processes assembled the continents into Pangaea, the most recent supercontinent, before its later fragmentation into the present-day continental landmasses.
Etymology The term "Phanerozoic" was coined in 1930 by the American geologist George Halcott Chadwick (1876–1953), deriving from the Ancient Greek words (), meaning "visible"; and (), 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.
Proterozoic–Phanerozoic boundary
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.
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.
Paleozoic Era
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.
Cambrian Period
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.
Ordovician Period
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 colonise the flood plains and riparian zones, giving rise to primitive land plants.
By the end of the Ordovician, Gondwana had moved from the equator to the South Pole, and Laurentia had collided with Baltica, closing the Iapetus Ocean. The glaciation of Gondwana resulted in a major drop in sea level, killing off all life that had established along its coast. Glaciation caused an icehouse Earth, leading to the Ordovician–Silurian extinction, during which 60% of marine invertebrates and 25% of families became extinct. Though one of the deadliest mass extinctions in earth's history, the O–S extinction did not cause profound ecological changes between the periods.
Silurian Period
The Silurian spans from 444 million to 419 million years ago, which saw a warming from an icehouse Earth. This period saw the mass diversification of fish, as jawless fish became more numerous, and early jawed fish and freshwater species appeared in the fossil record. Arthropods remained abundant, and some groups, such as eurypterids, became apex predators in the ocean. Fully terrestrial life established itself on land, including early fungi, arachnids, hexapods and myriapods. The evolution of vascular plants (mainly spore-producing ferns such as Cooksonia) allowed land plants to gain a foothold further inland as well. During this time, there were four continents: Gondwana (Africa, South America, Australia, Antarctica, India), Laurentia (North America with parts of Europe), Baltica (the rest of Europe), and Siberia (Northern Asia).
Devonian Period
The Devonian spans from 419 million to 359 million years ago. Also informally known as the "Age of the Fish", the Devonian features a huge diversification in fish such as the jawless conodonts and ostracoderms, as well as jawed fish such as the armoured placoderms (e.g. Dunkleosteus), the spiny acanthodians and early bony fish. The Devonian also saw the primitive appearance of modern fish groups such as chondricthyans (cartilaginous fish) and osteichthyans (bony fish), the latter of which include two clades — the actinopterygians (ray-finned fish) and sarcopterygians (lobe-finned fish). One lineage of sarcopterygians, Rhipidistia, evolved the first four-limbed vertebrates, which would eventually become tetrapods. On land, plant groups diversified after the Silurian-Devonian Terrestrial Revolution; the first woody ferns and the earliest seed plants evolved during this period. By the Middle Devonian, shrub-like forests existed: lycophytes, horsetails and progymnosperm. This greening event also allowed the diversification of arthropods as they took advantage of the new habitat. Near the end of the Devonian, 70% of all species became extinct in a sequence of mass extinction events, collectively known as the Late Devonian extinction.
Carboniferous Period
The Carboniferous spans from 359 million to 299 million years ago. Tropical swamps dominated the Earth, and the large amounts of trees sequestered much of the carbon that became coal deposits (hence the name Carboniferous and the term "coal forest"). About 90% of all coal beds were deposited in the Carboniferous and Permian periods, which represent just 2% of the Earth's geologic history. The high oxygen levels caused by these wetland rainforests allowed arthropods, normally limited in size by their respiratory systems, to proliferate and increase in size. Tetrapods also diversified during the Carboniferous as semiaquatic amphibians such as the temnospondyls, and one lineage developed extraembryonic membranes that allowed their eggs to survive outside of the water. These tetrapods, the amniotes, included the first sauropsids (which evolved the reptiles, dinosaurs and birds) and synapsids (the ancestors of mammals). Throughout the Carboniferous, there was a cooling pattern, which eventually led to the glaciation of Gondwana as much of it was situated around the South Pole. This event was known as the Permo-Carboniferous Glaciation and resulted in a major loss of coal forests, known as the Carboniferous rainforest collapse.
Permian Period
The Permian spans from 299 million to 251 million years ago and was the last period of the Paleozoic era. At its beginning, all landmasses came together to form the supercontinent Pangaea, surrounded by one expansive ocean called Panthalassa. The Earth was relatively dry compared to the Carboniferous, with harsh seasons, as the climate of the interior of Pangaea was not moderated by large bodies of water. Amniotes still flourished and diversified in the new dry climate, particularly synapsids such as Dimetrodon, Edaphosaurus and therapsids, which gave rise to the ancestors of modern mammals. The first conifers evolved during this period, then dominated the terrestrial landscape. The Permian ended with at least one mass extinction, an event sometimes known as "the Great Dying", caused by large floods of lava (the Siberian Traps in Russia and the Emeishan Traps in China). This extinction was the largest in Earth's history and led to the loss of 95% of all species of life.
Mesozoic Era
The Mesozoic ranges from 252 million to 66 million years ago. Also referred to as the Age of Reptiles, Age of Dinosaurs or Age of Conifers, the Mesozoic featured the first time the sauropsids ascended to ecological dominance over the synapsids, as well as the diversification of many modern ray-finned fish, insects, molluscs (particularly the coleoids), tetrapods and plants. The Mesozoic is subdivided into three periods: the Triassic, Jurassic and Cretaceous.
Triassic Period
The Triassic ranges from 252 million to 201 million years ago. The Triassic is mostly a transitional recovery period between the desolate aftermath of the Permian Extinction and the lush Jurassic Period. It has three major epochs: Early Triassic, Middle Triassic, and Late Triassic.
The Early Triassic lasted between 252 million to 247 million years ago, and was a hot and arid epoch in the aftermath of the Permian Extinction. Many tetrapods during this epoch represented a disaster fauna, a group of survivor animals with low diversity and cosmopolitanism (wide geographic ranges). Temnospondyli recovered first and evolved into large aquatic predators during the Triassic. Other reptiles also diversified rapidly, with aquatic reptiles such as ichthyosaurs and sauropterygians proliferating in the seas. On land, the first true archosaurs appeared, including pseudosuchians (crocodile relatives) and avemetatarsalians (bird/dinosaur relatives).