Definitions

cotylosaur

Permian

[pur-mee-uhn]

The Permian is a geologic period and system that extends from 299.0 ± 0.8 Ma to 251.0 ± 0.4 Ma (million years before the present) . It is the last period of the Paleozoic Era. The Permian period was named after the kingdom of Permia, Russia by Scottish geologist Roderick Murchison in 1841 (not the city of Perm, as commonly misconstrued).

Subdivisions

The subdivisions of the Permian Period are listed below in chronological order, from oldest to youngest, with ranges and key events. Additional age/stage equivalents or subdivisions are given in parentheses.

Cisuralian epoch

The Cisuralian (Rotliegendes) epoch lasted from .

Asselian
The Asselian (Krumaian/Uskalikian/Surenian/Wolfcamp) age lasted from .

Sakmarian
The Sakmarian (Sterlitamakian/Tastubian/Leonard/Wolfcamp/) age lasted from .

Artinskian
The Artinskian (Baigendzinian/Aktastinian/) age lasted from .

Kungurian

The Kungurian (Irenian/Filippovian/Leonard) age lasted from .

Guadalupian Epoch

The Guadalupian is also known as the Zechstein or Middle Permian. This epoch saw coral reefs flourishing in shallow seas, and on land the replacement of the Pelycosaurs by early Therapsids. It comprises three periods: the Roadian Wordian and Capitanian ().

During the Roadian, the low diversity caseid fauna that characterized the Kungurian age is supplanted (and for the most part replaced) by a rich range of early therapsids. It is not unlikely that these early therapsids may have had the beginnings of metabolic development towards the mammalian condition. In any case, these animals quickly radiated into an extraordinary variety of large and small terrestrial herbivores and carnivores. The Early Permian ectothermic families died out early during, or perhaps prior to, this time.

The therapsids belonged to several distinct (albeit related) lineages, none with clear antecedents. These include the modest-sized Biarmosuchidae, relatively long-limbed lightly-built hunters of small game (a kind of therapsid dog perhaps), representing a persisting primitive lineage from which the other groups may have developed, the huge carnivorous eotitanosuchians (essentially biarmosuchids grown large), the bizarre estemmenosuchids, herbivores that seem to have frequented a marshy environment, and possessing strange bony head growths, not unlike antlers, and the large brithopodids, representing another carnivorous lineage, more heavily built than the biarmosuchids. Note that apart from the estemennosuchids, which replaced the cotolyhunchines as great lumbering herbivores, all these animals were carnivores. As with the Early Permian pelycosaur-dominated fauna, this was a primitive ecosystem with a preponderance of meat-eaters over herbivores.

Although the Roadian age was ruled by large and small primitive therapsids, these were also accompanied by a rich fauna of stem tetrapods and unspecialized reptiles.

The tetrapods were mostly large semi-aquatic fish-eaters, superficially crocodile-like in appearance, although there were also a selection of smaller aquatic and fully terrestrial types. Apart from the aquatic batrachosaurs, all belong to the Order Temnospondyli.

Various other types of stem tetrapods and reptiles are well-represented in the Belebei-Mezen Cotylosaur Complex, which is difficult to correlate stratigraphically because of a paucity of shared faunas. It can be assumed however that numerous small lizard-like insectivorous Anapsida were an important part of the ecosystem. The Pelycosaurs may (or may not) be represented by a single femur, Phreatosaurus aenigmaticum Efremov (1954), which Efremov assigns to the family Phraetosuchidae (probably an artificial group based on scrappy postcrania), but Olson, 1962 argues is really a member of the Family Caseidae, a group that is well represented from the Artinskian to the Wordian ages.

Lopingian epoch

The Lopingian climate was hot and arid. The epoch, which stretched from , saw one of the largest deserts of all time, in the middle of the supercontinent Pangaea. Monsoons probably prevailed in the coasts or a little further inland. There were probably fewer glaciers than the previous epoch. The beginning of the epoch saw the extinction of the dominating Dinocephalians therapsids and many remaining pelycosaurs. They were replaced by smaller therapsids, the Gorgonopsians, Therocephalians, and the cynodonts. Other animals, such as the Pareiasaurs replaced the herbivorous dinocephalians, while many animals such as tetrapods, insects, etc., were abundant on land as small to medium sized creatures. In the seas, life was abundant.

This all came to a sudden halt when a mass extinction of unknown cause ended the Lopingian, and of course the Paleozoic Era. This was the Permian-Triassic extinction event, the greatest mass extinction in the history of earth. The Cretaceous–Tertiary extinction event pales in comparison with this event. About 95% of all species in the sea died out, as did more than 75% of all land species. By the time the extinction stopped, land and sea were virtually empty of life, although survivors have come to fill the empty void. The era of Paleozoic was over, and the Mesozoic era had begun. The epoch is followed by Early Triassic Epoch.

The Lopingian epoch consists of two ages, the Wuchiapingian (Wujiapingian, Djulfian, Dzhulfian, Longtanian, Rustlerian, Saladoan, Kazanian, or Castile), from , and the Changhsingian (also known as Changxingian, Dorashamian, Dewey Lake, or Tatarian), from .

Oceans

Sea levels in the Permian remained generally low, and near-shore environments were limited by the collection of almost all major landmasses into a single continent -- Pangaea. One continent, even a very large one, has a smaller shoreline than six to eight smaller ones with the same total area. This could have in part caused the widespread extinctions of marine species at the end of the period by severely reducing shallow coastal areas preferred by many marine organisms.

Paleogeography

During the Permian, all the Earth's major land masses except portions of East Asia were collected into a single supercontinent known as Pangaea. Pangaea straddled the equator and extended toward the poles, with a corresponding effect on ocean currents in the single great ocean ("Panthalassa", the "universal sea"), and the Paleo-Tethys Ocean, a large ocean that was between Asia and Gondwana. The Cimmeria continent rifted away from Gondwana and drifted north to Laurasia, causing the Paleo-Tethys to shrink. A new ocean was growing on its southern end, the Tethys Ocean, an ocean that would dominate much of the Mesozoic Era. Large continental landmasses create climates with extreme variations of heat and cold ("continental climate") and monsoon conditions with highly seasonal rainfall patterns. Deserts seem to have been widespread on Pangaea. Such dry conditions favored gymnosperms, plants with seeds enclosed in a protective cover, over plants such as ferns that disperse spores. The first modern trees (conifers, ginkgos and cycads) appeared in the Permian.

Three general areas are especially noted for their extensive Permian deposits - the Ural Mountains (where Perm itself is located), China, and the southwest of North America, where the Permian Basin in the U.S. state of Texas is so named because it has one of the thickest deposits of Permian rocks in the world.

Climate

As the Permian opened, the Earth was still in the grip of an ice age, so the polar regions were covered with deep layers of ice. Glaciers continued to cover much of Gondwanaland, as they had during the late Carboniferous.

The Permian Period, at the end of the Paleozoic era, marked a great changes in the Earth's climate and appearance. Towards the middle of the period the climate became warmer and milder, the glaciers receded, and the continental interiors became drier. Much of the interior of Pangaea was probably arid, with great seasonal fluctuations (wet and dry seasons), because of the lack of the moderating effect of nearby bodies of water. This drying tendency continued through to the late Permian, along with alternating warming and cooling periods.

Life

Marine biota

Permian marine deposits are rich in fossil mollusks, echinoderms, and brachiopods. Fossilized shells of two kinds of invertebrates are widely used to identify Permian strata and correlate them between sites: fusulinids, a kind of shelled amoeba-like protist that is one of the foraminiferans, and ammonoids, shelled cephalopods that are distant relatives of the modern nautilus. By the close of the Permian, trilobites and a host of other marine groups became extinct

Terrestrial biota

Terrestrial life in the Permian included diverse plants, fungi, arthropods, and various types of tetrapods. The period saw a massive desert covering the interior of the Pangaea. The warm zone spread in the northern hemisphere, where extensive dry desert appeared. The rock formed at that time were stained red by iron oxides, the result of intense heating by the sun of a surface devoid of vegetation cover. A number of older types of plants and animals died out or became marginal elements.

The Permian began with the Carboniferous flora still flourishing. About the middle of the Permian there was a major transition in vegetation. The swamp-loving lycopod trees of the Carboniferous, such as Lepidodendron and Sigillaria, were replaced by the more advanced conifers, which were better adapted to the changing climatic conditions. The Permian saw the radiation of many important conifer groups, including the ancestors of many present-day families. Lycopods and swamp forests still dominated the South China continent because it was an isolated continent and it sat near or at the equator. Oxygen levels were probably high there. The ginkgos and cycads also appeared during this period. Rich forests were present in many areas, with a diverse mix of plant groups.

Insects of the Permian

By the Pennsylvanian and well into the Permian, by far the most successful were primitive relatives of cockroaches. Six fast legs, two well developed folding wings, fairly good eyes, long, well developed antennae (olfactory), an omnivorous digestive system, a receptacle for storing sperm, a chitin skeleton that could support and protect, as well as form of gizzard and efficient mouth parts, gave it formidable advantages over other herbivorous animals. About 90% of insects were cockroach-like insects ("Blattopterans").

The dragonflies Odonata were the dominant aerial predator and probably dominated terrestrial insect predation as well. True Odonata appeared in the Permian and all are amphibious. Their prototypes are the oldest winged fossils, go back to the Devonian, and are different from other wings in every way. Their prototypes may have had the beginnings of many modern attributes even by late Carboniferous and it is possible that they even captured small vertebrates, for some species had a wing span of 71 cm. A number of important new insect groups appeared at this time, including the Coleoptera (beetles) and Diptera (flies).

Reptile and amphibian fauna

Early Permian terrestrial faunas were dominated by pelycosaurs and amphibians, the middle Permian by primitive therapsids such as the dinocephalia, and the late Permian by more advanced therapsids such as gorgonopsians and dicynodonts. Towards the very end of the Permian the first archosaurs appeared, a group that would give rise to the dinosaurs in the following period. Also appearing at the end of the Permian were the first cynodonts, which would go on to evolve into mammals during the Triassic. Another group of therapsids, the therocephalians (such as Trochosaurus), arose in the Middle Permian. There were no aerial vertebrates.

The Permian period saw the development of a fully terrestrial fauna and the appearance of the first large herbivores and carnivores. It was the high tide of the anapsides in the form of the massive Pareiasaurs and host of smaller, generally lizzard-like groups. A group of small reptiles, the diapsids started to abound. These were the ancestors to most modern reptiles and the ruling dinosaurs as well as pterosaurs and crocodiles.

Thriving also, were the early ancestors to mammals, the synapsida, which included some large reptiles such as Dimetrodon. Reptiles grew to dominance among vertebrates, because their special adaptations enabled them to flourish in the drier climate. Permian amphibians consisted of temnospondyli, lepospondyli and batrachosaurs.

Permian-Triassic extinction event

The Permian ended with the most extensive extinction event recorded in paleontology: the Permian-Triassic extinction event. 90% to 95% of marine species became extinct, as well as 70% of all land organisms. On an individual level, perhaps as many as 99.5% of separate organisms died as a result of the event.

There is also significant evidence that massive flood basalt eruptions from magma output lasting thousands of years in what is now the Siberian Traps contributed to environmental stress leading to mass extinction. The reduced coastal habitat and highly increased aridity probably also contributed. Based on the amount of lava estimated to have been produced during this period, the worst case scenario is an expulsion of enough carbon dioxide from the eruptions to raise world temperatures five degrees Celsius, not enough to kill off 95% of life.

Another hypothesis involves ocean venting of hydrogen sulfide gas. Portions of deep ocean will periodically lose all of its dissolved oxygen allowing bacteria that live without oxygen to flourish and produce hydrogen sulfide gas. If enough hydrogen sulfide accumulates in an anoxic zone, the gas can rise into the atmosphere.

Oxidizing gases in the atmosphere would destroy the toxic gas, but the hydrogen sulfide would soon consume all of the atmospheric gas available to change it. Hydrogen sulfide levels would increase dramatically over a few hundred years.

Modeling of such an event indicates that the gas would destroy ozone in the upper atmosphere allowing ultraviolet radiation to kill off species that had survived the toxic gas (Kump, et al, 2005). Of course, there are species that can metabolize hydrogen sulfide.

Another hypothesis builds on the flood basalt eruption theory. Five degrees Celsius would not be enough increase in world temperatures to explain the death of 95% of life. But such warming could slowly raise ocean temperatures until frozen methane reservoirs below the ocean floor near coastlines (a current target for a new energy source) melted, expelling enough methane, among the most potent greenhouse gases, into the atmosphere to raise world temperatures an additional five degrees Celsius. The frozen methane hypothesis helps explain the increase in carbon-12 levels midway into the Permian-Triassic boundary layer. It also helps explain why the first phase of the layer's extinctions was land-based, the second was marine-based (and starting right after the increase in C-12 levels), and the third land-based again.

An even more speculative hypothesis is that intense radiation from a nearby supernova was responsible for the extinctions.

Trilobites, which had thrived since Cambrian times, finally became extinct before the end of the Permian.

In 2006, a group of American scientists from Ohio State University reported evidence for a possible huge meteorite crater (Wilkes Land crater) with a diameter of around 500 kilometers in Antarctica. The crater is located at a depth of 1.6 kilometers beneath the ice of Wilkes Land in eastern Antarctica. The scientists speculate that this impact may have caused the Permian-Triassic extinction event, although its age is bracketed only between 100 million and 500 million years ago. They also speculate that it may have contributed in some way to the separation of Australia from the Antarctic landmass, which were both part of a supercontinent called Gondwana. Levels of iridium and quartz fracturing in the Permian-Triassic layer do not approach those of the Cretaceous-Tertiary boundary layer. Given that a far greater proportion of species and individual organisms became extinct during the former, doubt is cast on the significance of a meteor impact in creating the latter. Further doubt has been cast on this theory based on fossils in Greenland showing the extinction to have been gradual, lasting about eighty thousand years, with three distinct phases.

The warm zone spread in the northern hemisphere, where extensive dry desert appeared. The rock formed at that time were stained red by iron oxides, the result of intense heating by the sun of a surface devoid of vegetation cover. The old types of plants and animals died out.

Many scientists believe that the Permian-Triassic extinction event was caused by a combination of some or all of the hypotheses above and other factors; the formation of Pangaea decreased the number of coastal habitats and may have contributed to the extinction of many clades.

See also

Notes

References

  • Ogg, Jim; June, 2004, Overview of Global Boundary Stratotype Sections and Points (GSSP's) http://www.stratigraphy.org/gssp.htm Accessed April 30, 2006.
  • Kump, L.R., A. Pavlov, and M.A. Arthur (2005). "Massive release of hydrogen sulfide to the surface ocean and atmosphere during intervals of oceanic anoxia". Geology 33 (May): 397–400.

External links

Search another word or see cotylosauron Dictionary | Thesaurus |Spanish
Copyright © 2014 Dictionary.com, LLC. All rights reserved.
  • Please Login or Sign Up to use the Recent Searches feature