Paleozoic Age
Phanerozoic Australia is divided at the Tasman Line into two parts. Those are a western terrane of exposed Precambrian blocks and fold belts overlain by thin Phanerozoic basins and an eastern terrane of exposed Phanerozoic fold belts and basins. The story of the earliest Paleozoic animals is one of life in the sea. Presumably simple fungi and related forms existed in freshwater environments , but the fossil record provides no evidence of these modes of life.
Paleozoic Era: Facts & Information
The terrestrial environment of the early Paleozoic was barren of the simplest of life-forms. The Cambrian explosion was a sharp and sudden increase in the rate of evolution. The biota rapidly diversified throughout the Cambrian and Ordovician periods as life-forms adapted to virtually all marine environments.
In numbers of described marine species, fossils of trilobites dominate Cambrian rocks, whereas brachiopods lamp shells predominate in strata from the Ordovician through the Permian Period. Several different kinds of organisms adapted independently to life on land, primarily during the middle Paleozoic. Leafless vascular plants psilophytes and invertebrate animals centipede-like arthropods were both established on land at least by Silurian time.
Vertebrate animals made the transition to land via the evolution of amphibians from air-breathing crossopterygian fish during Devonian times. Further conquest of the land became possible during the Carboniferous Period , when plants and animals evolved solutions to overcome their dependence on moist environments for reproduction: Flight was first achieved also during the Carboniferous Period as insects evolved wings. The Permian extinction , at the end of the Paleozoic Era, eliminated such major invertebrate groups as the blastoids an extinct group of echinoderms related to the modern starfish and sea lilies , fusulinids , and trilobites.
Other major groups, which included the ammonoids , brachiopods , bryozoans moss animals , corals, and crinoids cuplike echinoderms with five or more feathery arms , were severely decimated but managed to survive. It has been estimated that as many as 95 percent of the marine invertebrate species perished during the late Permian Period. Extinction rates were much lower among vertebrates, both aquatic and terrestrial, and among plants. Causes of this extinction event remain unclear, but they may be related to the changing climate and exceptionally low sea levels of the time.
Although of lesser magnitude, other important Paleozoic mass extinctions occurred at the end of the Ordovician Period and during the late Devonian Period. On a global scale, the Paleozoic was a time of continental assembly. The majority of Cambrian landmasses were gathered together to form Gondwana , a supercontinent made up of the present-day continents of Africa , South America , Australia , and Antarctica and the Indian subcontinent.
It extended from the northern tropics to the southern polar regions. With the exception of three major cratons landmasses forming the stable interiors of continents not part of the initial configuration of Gondwana, the remainder of Earth was covered by the global Panthalassic Ocean.
Laurentia was separated from Gondwana by the Iapetus Ocean. The smaller Baltica craton was positioned within the Iapetus Ocean, lying to the south of Laurentia and just off the northern margin of Gondwana. Baltica was made up of much of Scandinavia and western Europe.
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To the east of Laurentia, the Siberian craton was positioned just south of the paleoequator between Laurentia and the western coast of Gondwana. While a portion of Gondwana was positioned at or near the South Pole , there is no evidence of glaciation during Cambrian time. While little is known about the finer details of the Cambrian climate, geologic evidence shows that the margins of all continents were flooded by shallow seas. It is in the rock formed within these shallow seas that the greatest explosion of life ever recorded occurred.
By Ordovician time, part of Gondwana had begun to move over the South Pole. The distribution of extensive glacial deposits, which formed later in the Paleozoic, has been used to track the movement of parts of Gondwana over and around the South Pole.
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Siberia, Baltica, and Laurentia also moved to new locations during the course of the Paleozoic. Siberia , essentially the large Asian portion of present-day Russia , was a separate continent during the early and middle Paleozoic, when it moved from equatorial to northern temperate latitudes. Baltica moved across the paleoequator from southern cool temperate latitudes into northern warm latitudes during the Paleozoic. It collided with and joined Laurentia during the early Devonian Period.
The beginnings of such mountainous regions as the Appalachians , Caledonides, and Urals resulted from the Paleozoic collision of the lithospheric plates. By the end of the Paleozoic, continued tectonic plate movements had forced these cratons together to form the supercontinent of Pangea.
Large areas of all continents were episodically inundated by shallow seas, with the greatest inundations occurring during the Ordovician and early Carboniferous Mississippian periods. By the Cisuralian , both oxygen and carbon dioxide had recovered to more normal levels. On the other hand, the assembly of Pangea created huge arid inland areas subject to temperature extremes.
The Lopingian is associated with falling sea levels, increased carbon dioxide and general climatic deterioration, culminating in the devastation of the end-Permian extinction. As one might expect from such a vast interval of time, there are a great many Paleozoic sites to choose from. Rather than attempt the impossible task of describing the scars left by My of geological time, we thought we would briefly summarize the ten Paleozoic sites which, in our judgment, had left the greatest mark on paleontology.
That, at least, is what we thought. As it turned out, after going through the agonizing job of paring down the list, we found that we could not get much below twelve or fifteen sites.
Paleozoic geography
Rather than make some kind of difficult, rational choice, we have simply hacked off the Permian and about half the Carboniferous, as well as randomly discarding some of the many Devonian sites. Some of these sites are covered in detail elsewhere on this site which is, of course, one of the ten indispensible sites of Holocene time. Accordingly, without wasting a single electron or pixel more on vain regrets:. Terreneuvian of South China. This site is discussed at Chenjiang. The English spellings are somewhat variable, "Chenjiang" being another popular variant.
It might be better to referred to as the Maotianshan Shale. This is less accurate it is more properly the Qiongzhusi Formation , but seems least likely to be misspelled by ignorant foreigners, such as ourselves. The Chengjiang fossils are dated at Mya, or perhaps a bit younger, corresponding most nearly to the Botomian Age in our system [1].
Outcrops of the Qiongzhusi occur in scattered locations south of Kunming in eastern Yunnan Province, Chengjiang County, near the towns of Chengjiang and Ercai.
Additional sites have now been opened further south. Of all the sites mentioned here, Chengjiang is geologically the oldest and historically the youngest. The fossil potential of the region was discovered by Dr. Hou Xianguang in Many of the fossils have been recovered -- and many lost forever -- in connection with phosphate mines in the area. The incredible soft-tissue preservation of the fossils here seems to have resulted from rapid burial, complete sediment anoxia, and replacement of organic remains with pyrite or phosphates -- nothing magical, except the absolutely unreasonable number of such sites, of varying ages, in Yunnan Province.
The faunal list from Chengjiang is a virtually complete census of the major metazoan taxa of the time, and includes our personal favorite of all early chordates, Haikouella. There seems to be little selectivity. There are now Chengjinag fossil images all over the web.
However, many of the Chengjiang organisms remain undescribed, simply for lack of competent describers, and new specimens are being discovered at an extraordinary rate. Middle Cambrian of Canada. The Burgess Shale is slightly younger than Chengjiang. The closest major town is Banff, about 90 km to the east. The site was discovered by Charles Walcott of the Smithsonian Institution in , and the Walcott Quarry is named after him. The deposits are deepwater, benthic sediments, but the fauna probably represent a reef community swept off the reef and buried in an anoxic bottom by a mudslide.
The Burgess is actually far less spectacular than Chengjiang, but it attained great fame ironically, just at the time that Chengjiang was starting to produce large quantities of fossils due in part to Jay Gould's book, Wonderful Life.
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The Burgess Shale's influence on paleontology has been, in part, due to the fact that Gould chose this book to set out some of his most interesting and controversial ideas about evolution, and in a manner readable by almost everyone. Thus even the tiniest change in Proterozoic conditions might have resulted in an entirely different modern fauna.
His proof was the diversity of phyla in the Shale, hinting at an enormous initial diversity in the Cambrian Explosion which was quickly pruned away, largely by happenstance.