Information AboutMiocene |
| CATEGORIES ABOUT MIOCENE | |
| miocenemiocene | |
| neogene | |
|
The Miocene boundaries are not set at an easily identified worldwide event but rather at regional boundaries between the warmer Oligocene and the cooler Pliocene. SUBDIVISIONS The Miocene Faunal Stage s from youngest to oldest are typically named according to the International Commission On Stratigraphy : These subdivisions within the Miocene are defined by the relative abundance of different species of calcareous nanofossils ( Calcite platelets shed by brown single-celled algae) and Foraminifera (single-celled protists with diagnostic shells). Two subdivisions each form the Early, Middle and Late Miocene. In most of North America , faunal stages are defined according to the land mammal fauna ( NALMA ). They overlap the borders of the Miocene and Oligocene/Pliocene: California n sites, which are derived from the former Farallon Plate , provide another sequence which also overlaps with the epoch boundaries : Yet other systems are used to describe the Miocene stratigraphy of Japan , Australia and New Zealand . PALEOGEOGRAPHY Continent s continued to Drift toward their present positions. Of the modern geologic features, only the land bridge between South America and North America was absent, although South America was approaching the western subduction zone in the Pacific Ocean , causing both the rise of the Andes and a southward extension of the Meso-American peninsula. Mountain building took place in Western North America and Europe . Both continental and marine Miocene deposits are common worldwide with marine outcrops common near modern shorelines. Well studied continental exposures occur in the American Great Plains and in Argentina . India continued to collide with Asia , creating more mountain ranges. The Tethys Seaway continued to shrink and then disappeared as Africa collided with Eurasia in the Turkish - Arabia n region between 19 and 12 mya. Subsequent uplift of mountains in the western Mediterranean region and a global fall in sea levels combined to cause a temporary drying up of the Mediterranean Sea (known as the Messinian Salinity Crisis ) near the end of the Miocene. The global trend was one towards increasing aridity caused primarily by global cooling reducing the ability of the atmosphere to absorb moisture. Uplift of East Africa in the Late Miocene was partly responsible for the shrinking of tropical rain forests in that region, and Australia got drier as it entered a zone of low rainfall in the Late Miocene. LIFE Flora Grasslands underwent a major expansion; forests fell victim to a generally cooler and drier climate overall. Grasses also diversified greatly, Co-evolving with large herbivores and grazers, including Ruminant s. Between 7 and 6 million years ago, there occurred a sudden expansion of grasses which were able to assimilate Carbon Dioxide more efficiently but were also richer in Silica , causing a worldwide extinction of large herbivores.Stanley, S.M. ''Earth system history''. New York 1999, 525-6. Fauna Both marine and continental Fauna were fairly modern, although marine mammals were less numerous. Only in isolated South America and Australia did widely divergent fauna exist. Mammals were also modern, with recognizable Wolves , raccoons, Horse s, Beaver , Deer , Camel s, and Whale s. Recognizable Crow s, Duck s, Auk s, Grouse s and Owl s appear in the Miocene. By the epoch's end, all or almost all modern bird Families are believed to have been present; the few post-Miocene bird fossils which cannot be placed in the evolutionary tree with full confidence are simply too badly preserved instead of too equivocal in character. Marine birds reached their highest diversity ever in the course of this epoch. Brown Alga e, called Kelp , proliferate, supporting new species of sea life, including Otter s, Fish and various Invertebrate s. The cetaceans diversified, and some modern genera appeared, such as the Sperm Whales . The Pinniped s, which appeared near the end of the Oligocene, became more aquatic. Perhaps most important were the 100 or so species of Ape s that lived during this time. They occupied much of the Old World and ranged in size, diet, and anatomy. Due to scanty fossil evidence it is unclear which ape or apes contributed to the modern Hominoid clade, but molecular evidence indicates this ape lived from between 15 to 12 million years ago. In the oceans, modern Shark s appeared at this time. Cetacean s, such as Dolphin s, Whale s, and Porpoise s evolved. Their ancestors the Archaeoceti , however, were becoming less common and eventually became extinct. OCEANS East Antarctica had some glaciers during the early Miocene (23-15 million years ago). Oceans cooled, and about 15 million years ago ago the ice cap in the southern hemisphere started to grow to its present form. The Greenland ice cap developed later, in Pliocene time, about 3 million years ago. MIDDLE MIOCENE DISRUPTION See Also: Middle Miocene disruption FOOTNOTES SEE ALSO
REFERENCES
EXTERNAL LINKS |
|
|