The Phanerozoic (Brit. Phanærozoic) Eon is the current eon in the geologic timescale, and the one during which abundant animal life has existed. It covers roughly 542 million years and goes back to the time when diverse hard-shelled animals first appeared. Its name is derived from the Greek words φαίνω and ζωή, meaning make life appear, since it was once believed that life began in the Cambrian, the first period of this eon. The time before the Phanerozoic, formerly called the Precambrian supereon, is now divided into the Hadean, Archaean and Proterozoic eons.
The exact time of the boundary between the Phanerozoic and the Proterozoic is slightly uncertain. In the 19th Century, the boundary was set at the first abundant metazoan fossils. But several hundred taxa of Proterozoic metazoa have been identified since systematic study of those forms started in the 1950s. Most geologists and paleontologists would probably set the Proterozoic-Phanerozoic boundary either at the classic point where the first trilobites and archaeocyatha appear; at the first appearance of a complex feeding burrow called Treptichnus 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.
The Phanerozoic is divided into three eras: the Paleozoic, Mesozoic, and Cenozoic. In the older literature, the term Phanerozoic is generally used as a label for the time period of interest to paleontologists, but that use of the term seems to be falling into disuse in more modern literature.
The time span of the Phanerozoic includes the rapid emergence of a number of animal phyla; the evolution of these phyla into diverse forms; the emergence of terrestrial plants; the development of complex plants; the evolution of fish; the emergence of terrestrial animals; and the development of modern faunas. During the period covered, continents drifted about, eventually collecting into a single landmass known as Pangaea and then splitting up into the current continental landmasses.
It has been demonstrated that changes in biodiversity through the Phanerozoic correlate much better with hyperbolic model (widely used in demography and macrosociology) than with exponential and logistic models (traditionally used in population biology and extensively applied to fossil biodiversity as well). The latter models imply that changes in diversity are guided by a first-order positive feedback (more ancestors, more descendants) and/or a negative feedback arising from resource limitation. Hyperbolic model implies a second-order positive feedback. The hyperbolic pattern of the world population growth arises from a second-order positive feedback between the population size and the rate of technological growth. The hyperbolic character of biodiversity growth in the Phanerozoic can be similarly accounted for by a feedback between the diversity and community structure complexity. It is suggested that the similarity between the curves of biodiversity and human population probably comes from the fact that both are derived from the interference of the hyperbolic trend with cyclical and stochastic dynamics.
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