Phanerozoic

During the Phanerozoic the biodiversity shows a steady but not monotonic increase from near zero to several thousands of genera.

The Phanerozoic (occasionally Phanaerozoic) eon is the period of geologic time that spans from the appearance of abundant, macroscopic, hard-shelled fossils, roughly 542 million years ago (mya), to the present time.

Preceding the Phanerozoic is the Precambrian, an informal name for the entire preceding span of the history of Earth, beginning with the Earth's formation around 4.5 billion years ago. The Precambrian ends with the onset of the Cambrian period, the first period of the Phanerozoic. The Cambrian is the age of the first major flowering of multicellular eukaryotes, the Cambrian explosion, when a majority of the skeleton-bearing phyla of animals appear in the fossil record (Mayr 2001).

The Phaenerozoic eon is divided into three eras: The Paleozoic, Mesozoic, and Cenozoic. Earlier stages provide the foundation for later stages, and for the modern world seen today. In the older literature, the term Phanerozoic is generally used as a label for the time period of interest to paleontologists. The term seems to be falling into disuse in more modern literature.

The name Phanerozoic name derives from the Greek meaning, visible life, referring to the large size of organisms since the Cambrian explosion. The time span of the Phanerozoic includes the rapid emergence of a number of animal phyla, the emergence of terrestrial plants, the development of complex plants, the appearance of fish, the emergence of terrestrial animals, and the development of modern faunas.

Phanerozoic eon (542 mya - present)
Paleozoic era	Mesozoic era	Cenozoic era

Beginning point

The Phanerozoic beginning point is usually set around 542 or 543 mya. However, the exact time of the boundary between the Phanerozoic and the Precambrian is slightly uncertain. In the nineteenth century, the boundary was set at the first abundant metazoan fossils. But several hundred taxa of Precambrian metazoa have been identified since systematic study of those forms started in the 1950s.

Most geologists and paleontologists would probably set the Precambrian-Phanerozoic boundary either at the classic point where the first trilobites and archaeocyatha appear; at the first appearance of a complex feeding burrow called Trichophycus pedum: or at the first appearance of a group of small, generally disarticulated, armored forms termed, "the small shelly fauna." The three different dividing points are within a few million years of each other.

Eras

The Phaenerozoic eon is divided into three basic divisions, the Paleozoic era, the Mesozoic era, and the Cenozoic era.

The Paleozoic, meaning "old animals," spanned from roughly 542 mya to roughly 251 mya (ICS 2004). It is subdivided into six geologic periods (from oldest to youngest): Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian.

The start of the Paleozoic, between roughly 542 mya and 530 mya, is a time that presents a major enigma for Darwinian evolution. This is the phenomenon known as the Cambrian explosion, when there was a sudden geological appearance of a large number of body plans. Indeed, all of the major body plans (phyla) of complex, multi-cellular, macroscopic organisms that exist today appeared in this time period, and since then no fundamentally new body plan has come into existence (Mayr 2001). Among the phyla that were first evident in the Cambrian were the brachiopods, mollusks, arthropods (trilobites), and echinoderms. Mayr (2001) concludes that the apparent explosion of new phyla in the early Cambrian was possibly due to the skeletonization of a variety of soft-bodies organisms that already existed in the Precambrian.

The Mesozoic, meaning "middle animals," extended from about 251 million years ago (mya) to 65 mya. The Mesozoic era is separated into three geologic periods: Triassic, Jurassic, and Cretaceous. It is often called the "Age of the Dinosaurs," after the dominant fauna of the era. Many diverse animal species appeared during this time, including birds and mammals.

Mesozoic era (251 - 65 mya)
Triassic	Jurassic	Cretaceous

The Cenozoic, meaning "new life," is the most recent of the three classic geological eras of the geologic time scale. It covers the 65.5 million years since the Cretaceous-Tertiary extinction event at the end of the Cretaceous The Cenozoic era is ongoing.

The Cenozoic is divided into two periods, the Paleogene and Neogene, and they are in turn divided into epochs. The Paleogene consists of the Paleocene, Eocene, and Oligocene epochs, and the Neogene consists of the Miocene, Pliocene, Pleistocene, and Holocene epochs, the last of which is ongoing.

Cenozoic era (65-0 mya)
Paleogene	Neogene      Quaternary

Paleogeography

Map of Pangaea, which began to form near the end of the Paleozoic

During the Phanerozoic, continents drifted about, eventually collected into a single landmass known as Pangea, and then split up into the current continental landmasses.

Geologically, the Phanerozoic eon is theorized by some scientists to have begun shortly after the breakup of a hypothesized supercontinent at the end of a global ice age. Throughout the early Paleozoic era, the Earth's landmass was broken up into a substantial number of relatively small continents. Toward the end of the Paleozoic era, the continents gathered together into the supercontinent Pangea, which included most of the Earth's land area.

The Mesozoic era featured the dramatic rifting of the supercontinent Pangea, which gradually split into a northern continent, Laurasia, and a southern continent, Gondwana. By the end of the era, the continents had rifted into nearly their present form. Laurasia became North America and Eurasia, while Gondwana split into South America, Africa, Australia, Antarctica, and the Indian subcontinent.

Geologically, the Cenozoic is the era when continents moved into their current positions. Australia-New Guinea split from Gondwana. Antarctica moved into its current position over the South Pole. The Atlantic Ocean widened and, later in the era, South America became attached to North America.

Phanerozoic climate

500 million years of climate change

500 million years of changes in carbon dioxide concentrations

During the Phanerozoic, the Earth's climate varied between conditions that support large-scale continental glaciation and those which are extensively tropical and lack permanent ice caps even at the poles. The difference in global mean temperatures between a fully glacial earth and ice free Earth is estimated at approximately 10 °C, though far larger changes would be observed at high latitudes and smaller ones at low latitudes.

One key requirement for the development of large scale ice sheets is the arrangement of continental land masses at or near the poles. With plate tectonics constantly rearranging the continents, it can also shape long-term climate evolution. However, the presence of land masses at the poles is not sufficient to guarantee glaciations. Evidence exists of past warm periods in Earth's climate when polar land masses similar to Antarctica were home to deciduous forests rather than ice sheets.

The establishment of CO₂-consuming (and oxygen-producing) photosythesizing organisms in the Precambrian led to the production of an atmosphere like today's, though for most of this period it was much higher in CO₂ than today. Similarly, the Earth's average temperature was also frequently higher than at present. It is clear that the preindustrial atmosphere, with only 280 ppm CO₂, is not far from the lowest ever occurring since the rise of macroscopic life.

Superimposed on the long-term evolution between hot and cold climates have been many short-term fluctuations in climate similar to, and sometimes more severe than, the varying glacial and interglacial states of the present ice age. Some of the most severe fluctuations, such as the Paleocene-Eocene Thermal Maximum, may be related to rapid increases in atmospheric carbon dioxide due to the collapse of natural methane reservoirs in the oceans. Severe climate changes also seem to have occurred during the course of the Cretaceous-Tertiary, Permian-Triassic, and Ordovician-Silurian extinction events; however, it is unclear to what degree these changes caused the extinctions rather than merely responding to other processes that may have been more directly responsible for the extinctions.

References

ISBN links support NWE through referral fees

• International Commission on Stratigraphy (ICS), Homepage. Retrieved January 2, 2007.
• Mayr, E. 2001. What Evolution Is. New York: Basic Books. ISBN 0465044263
• Veizer, J., Y. Godderis, and L. M. Francois. 2000. Evidence for decoupling of atmospheric CO2 and global climate during the Phanerozoic eon. Nature 408:698–701.

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