The Paleozoic Era: Continents Reassemble and Life Gets Complex

The Paleozoic Era

The Paleozoic Era comprises of Cambrian to Ordovician Periods which time span is 542-251 Ma.

The Early Paleozoic Era  (Cambrian–Ordovician Periods, 542–444 Ma) 


Land and sea in the early Paleozoic Era.
At the beginning of the Paleozoic Era, Pannotia broke up, yielding smaller continents including Laurentia (composed of North America and Greenland), Gondwana (South America, Africa, Antarctica, India, and Australia), Baltica (Europe), and Siberia (figure above a). New passivemargin basins formed along the edges of these new continents. In addition, sea level rose, so that vast areas of continental interiors were flooded with shallow seas called epicontinental seas (figure above b). These regions are now cratonic platforms. In many places, water depths in epicontinental seas reached only a few meters, creating a well-lit marine environment in which life abounded. Deposition in these seas, therefore, yielded layers of fossiliferous sediment. Sea level, however, did not stay high for the entire early Paleozoic Era; regressions and transgressions took place, the former marked by unconformities and the latter by accumulations of sediment. The layer cake of strata in the Grand Canyon is rock formed from such sediment. The geologically peaceful world of the early Paleozoic Era in Laurentia abruptly came to a close in the Middle Ordovician Period, for at this time its eastern margin rammed into a volcanic island arc and other crustal fragments. The resulting collision, called the Taconic orogeny, deformed and metamorphosed strata of the continent’s margin and produced a mountain range in what is now the eastern part of the Appalachians (figure above c). 

Life evolution

A museum diorama illustrates what early Paleozoic marine organisms may have looked like.
The fossil record indicates that soon after the Cambrian began, life underwent remarkable diversification. This event, which paleontologists refer to as the Cambrian explosion, took several million years. What caused this event? No one can say for sure, but considering that it occurred roughly at the time a supercontinent broke up, it may have had something to do with the production of new ecological niches and the isolation of populations that resulted when small continents formed and drifted apart. The first animals to appear in the Cambrian Period had simple tube- or cone-shaped shells, but soon thereafter, the shells became more complex. Shells on other organisms may have evolved as a means of protection against predation by organisms such as conodonts, small, eel-like organisms with hard parts that resemble teeth. By the end of the Cambrian, 
trilobites were grazing the sea floor. Trilobites shared the environment with mollusks, brachiopods, nautiloids, gastropods, graptolites, and echinoderms (figure above). Thus, a complex food chain arose, which included plankton, bottom feeders, and at the top, predators. Many of the organisms crawled over or swam around reefs composed of mounds of sponges with mineral skeletons. The Ordovician Period saw the first crinoids and the first vertebrate animals, jawless fish. At the end of the Ordovician, mass extinction took place, perhaps because of the brief glaciation and associated sea-level lowering of the time.

The Middle Paleozoic Era  (Silurian–Devonian Periods, 444–359 Ma) 


Paleogeography and fossils of Silurian and Devonian time .
As the world entered the Silurian Period, global climate warmed, sea level rose, and the continents flooded once again. In some places, where water in the epicontinental seas was clear and could exchange with water from  the oceans, huge reef complexes grew, forming a layer of  fossiliferous limestone on the continents. Also, distinct orogenies took place, yielding new mountain belts during the middle Paleozoic Era. For example, collisions on the eastern side of  Laurentia during Silurian and Devonian time produced the Caledonian orogen (affecting eastern Greenland, western Scandinavia, and Scotland) and the Acadian orogen in the region that is now the Appalachians (figure above a). Throughout much of the middle Paleozoic, the western margin of North America continued to be a passive-margin basin. Finally, in the Late Devonian, the quiet environment of the west-coast passive margin ceased, possibly because of a collision with an island arc. This event, known as the Antler orogeny, was the first of many orogenies to affect the western margin of the continent. The Caledonian, Acadian, and Antler orogenies all shed deltas of sediment onto the continents; these deposits formed thick successions of red beds, such as those visible today in the Catskill Mountains (figure above b). 

Life evolution

Life on Earth underwent radical changes in the middle Paleozoic Era. In the sea, new species of trilobites,  gastropods, crinoids, and bivalves replaced species that had disappeared during the mass extinction at the end of the Ordovician Period. On land, vascular plants with woody tissues, seeds, and veins (for transporting water and food) rooted for the first time. With the evolution of veins and wood, plants could grow much larger, and by the Late Devonian Period the land surface hosted swampy forests with tree-sized relatives of club mosses and ferns. Also at this time, spiders, scorpions, insects, and crustaceans began to exploit both dry-land and freshwater habitats, and jawed fish such as sharks and bony fish began to cruise the oceans. Finally, at the very end of the Devonian Period, the first amphibians crawled out onto land and inhaled air with lungs (figure above c).

The Late Paleozoic Era  (Carboniferous–Permian Periods, 359–251 Ma)


Paleogeography at the end of the Paleozoic Era.
The climate cooled significantly in the late Paleozoic. Seas gradually retreated from the continents, so that during the Carboniferous Period, regions that had hosted the limestone-forming reefs of epicontinental seas now became coastal areas and river deltas in which sand, shale, and organic debris accumulated. In fact, during the Carboniferous Period, Laurentia lay near the equator, so it enjoyed tropical and semitropical conditions that favored lush growth in swamps. This growth left thick piles of plant debris that transformed into coal after burial. Much of Gondwana and Siberia, in contrast, lay at high latitudes, and by the Permian Period became covered by ice sheets. The late Paleozoic Era also saw a succession of continental collisions, culminating in the formation of a single supercontinent, Pangaea (figure above a). The largest collision occurred during Carboniferous and Permian time, when Gondwana rammed into Laurentia and Baltica, causing the Alleghanian orogeny of North America (figure above b). 

 Features of the Appalachian Mountains in the eastern United States.
During this event, the final stage in the development of the Appalachians, eastern North America rammed against northwestern Africa, and what is now the Gulf Coast region of North America squashed against the northern margin of South America. A vast mountain belt grew, in which deformation generated huge faults and folds. We now see the eroded remnants of rocks deformed during this event in the Appalachian and Ouachita Mountains. Along the  continental side of the range, a wide band of deformation called the Appalachian fold-thrust belt formed (figure above). Movement on the faults displaced strata and resulted in the formation of large folds. At depth, the thrust faults merged with a near-horizontal sliding surface, called a detachment, just above the Precambrian basement. Stresses generated during the Alleghanian orogeny were so strong that preexisting faults in the continental crust clear across North America became active again. The movement produced uplifts and sediment-filled basins in the Midwest and in the region of the present-day Rocky Mountains. Geologists refer to the late Paleozoic uplifts of the Rocky Mountain region as the Ancestral Rockies.
The assembly of Pangaea involved a number of other collisions around the world as well. Notably, Africa collided with southern Europe to form the Hercynian orogen. Also, a rift or small ocean in Russia closed, leading to the uplift of the Ural Mountains, and parts of China along with other fragments of Asia attached to southern Siberia. 

Life evolution

A museum diorama of a Carboniferous coal swamps includes a giant dragonfly, with a wingspan of about 1 m.
The insect photo gives series of its size relative to human.
The fossil record indicates that during the late Paleozoic Era, plants and animals continued to evolve toward more familiar forms. In coal swamps, fixed-wing insects including huge dragonflies flew through a tangle of ferns, club mosses, and scouring rushes, and by the end of the Carboniferous Period insects such as the cockroach, with foldable wings, appeared (figure above). Forests containing gymnosperms (“naked seed” plants such as conifers) and cycads (trees with a palm-like stalk and fern-like fronds) became widespread in the Permian Period. Amphibians and, later, reptiles populated the land. The appearance of reptiles marked the evolution of a radically new component in animal reproduction: eggs with a protective covering. Such eggs permitted reptiles to reproduce without returning to the water. The late Paleozoic Era came to a close with a major mass extinction event, during which over 95% of marine species disappeared. Why this event occurred remains a subject of debate. According to one hypothesis, the terminal Permian mass extinction occurred as a result of an episode of extraordinary volcanic activity in the region that is now Siberia; basalt sheets extruded during the event are known as the “Siberian traps.” Eruptions could have clouded the atmosphere, acidified the oceans, and disrupted the food chain.
Credits: Stephen Marshak (Essentials of Geology)
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