The Paleozoic Era (from approximately 541 to 242 million years before present) is bracketed by two of the most important events in the history of animal life. At its beginning, multicelled animals underwent a dramatic "explosion" in diversity, and almost all living animal phyla appeared within a few millions of years. At the other end of the Paleozoic, the largest mass extinction in history wiped out approximately 90 percent of all marine animal species. The causes of both these events are still not fully understood and the subject of much research and controversy. Roughly halfway in between, animals, fungi, and plants alike colonized the land, the insects took to the air.The beginning of the Paleozoic is the definitional start of the Edenic Period, a time span in evolutionary biology that demarcates the initial time taken for the computation of background extinction rates of biota; the Edenic Period is taken to end with the ascent of modern man (e.g. approximately 50,000 to 10,000 years before present).
The Paleozoic took up over half of the Phanerozoic, approximately 300 million years. During the Paleozoic there were six major continental land masses; each of these consisted of different parts of the modern continents. For instance, at the beginning of the Paleozoic, today's western coast of North America ran east-west along the equator, while Africa was at the South Pole. These Paleozoic continents experienced tremendous mountain building along their margins, and numerous incursions and retreats of shallow seas across their interiors. Large limestone outcrops, like the one shown above, are evidence of these periodic incursions of continental seas.
Many Paleozoic rocks are economically important. For example, much of the limestone quarried for building and industrial purposes, as well as the coal deposits of western Europe and the eastern USA, were formed during the Paleozoic.
A favorite exercise in introductory paleontology courses is to invent mnemonics for the periods of the Paleozoic. These mnemonics all stand for the seven time periods into which the Paleozoic is divided: Cambrian, Ordovician, Silurian, Devonian, Mississippian, Pennsylvanian, Permian. Note that European geologists lump the Mississippian and Pennsylvanian into a single time period, the Carboniferous.
Most of these names derive from locations where rocks of these ages were first studied. Cambria was the Latin name for Wales, and the Ordovices and Silures were two Welsh Celtic tribes. The Devonian is named for Devonshire, England. The Mississippian is named for the upper Mississippi River valley, NOT the state of Mississippi, which has very few rocks of this age. The Pennsylvanian is named for Pennsylvania, surprisingly enough, while the Permian was described from rocks in the region of Perm, a town in the Ural Mountains of Russia. The exception to this rule is the Carboniferous Period; its name means "coal-bearing," and this period is in fact the time when extensive coal beds were formed around the world.
Two great animal faunas dominated the seas during the Paleozoic. The "Cambrian fauna" typified the Cambrian oceans; although members of most phyla were present during the Cambrian, the seas were dominated by trilobites, inarticulate brachiopods, monoplacophoran molluscs, hyolithids, "small shelly fossils" of uncertain systematic posiiton, and archaeocyathids. Although all of these except the archaeocyathids survived past the Cambrian, their diversity declined after the Ordovician. Later Paleozoic seas were dominated by crinoid and blastoid echinoderms, articulate brachiopods, graptolites, and tabulate and rugose corals. By the end of the Ordovician, life was no longer confined to the seas. Plants had begun to colonize the land, closely followed in the Silurian by invertebrates, and in the Late Devonian by vertebrates. The early tetrapods of this time were amphibian-like animals that eventually gave rise to the reptiles and synapsids by the end of the Paleozoic. One of the earliest terrestrial tetrapod faunas known in the world is from Joggins, Nova Scotia.
Land plants evolved rapidly into the vacant niches afforded them on land. By the end of the Devonian, forests of progymnosperms, such as Archaeopteris dominated the landscape. By the end of the Paleozoic, cycads, glossopterids, primitive conifers, and ferns were spreading across the landscape.
The Permian extinction, 244 million years ago, devastated the marine biota: tabulate and rugose corals, blastoid echinoderms, graptolites, and most crinoids died out, as did the last of the trilobites. Articulate brachiopods and one lineage of crinoids survived, but never again dominated the marine environment.
The Paleozoic Era is bracketed by the times of global super-continents. The Cambrian opened with the breakup of the world-continent Rodinia and closed with the formation of Pangea, as the Earth's continents came together once again.
References and further reading
- Peter Gray. 1890. The Succession of Plant Life upon the Earth. Transactions and journal of proceedings, Dumfriesshire and Galloway Natural History and Antiquarian Society, Dumfriesshire, Scotland.
- S.Sahney and M.J.Benton (2008) "Recovery from the most profound mass extinction of all time" (PDF). Proceedings of the Royal Society: Biological 275 (1636): 759–65.
- University of California Museum of Paleontology
- E.O. Wilson. 2005. The Future of Life. Alfred A. Knopf. New York, New York, USA