Paleozoic Era

The Paleozoic era is the earliest of three geologic eras of the Phanerozoic eon. This era spanned from roughly 542 million years ago to roughly 251 million years ago. The Paleozoic era is subdivided into six geologic periods: the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian. The Paleozoic covers the time from the first appearance of abundant, hard-shelled fossils to the time when the continents were beginning to be dominated by large reptiles and modern plants. The oldest geological period was classically set at the first appearance of creatures known as trilobites and archeocyathids. The youngest geological period marks a major extinction event 300 million years ago, known as the Permian extinction.

At the start of the era, all life was confined to bacteria, algae, sponges, and a variety of enigmatic forms known collectively as the Ediacaran fauna. The Cambrian explosion resulted in an exponential increase of lifeforms. There is some evidence that simple life may already have invaded the land at the start of the Paleozoic, but substantial plants and animals did not take to the land until the Silurian and did not thrive until the Devonian.

Although primitive vertebrates are known near the start of the Paleozoic, invertebrates were the dominant lifeforms until the mid-Paleozoic. Fish populations exploded in the Devonian. During the late Paleozoic, great forests of primitive plants thrived on land, forming the great coal beds of Europe and eastern North America. By the end of the era, the first large, sophisticated reptiles and the first modern plants had developed.

The Paleozoic era began shortly after the breakup of a supercontinent called Pannotia and at the end of a global ice age. During the early Paleozoic, the Earth’s landmass was broken up into a number of relatively small continents. Toward the end of the era, the continents gathered together into a supercontinent called Pangaea, which included most of the Earth’s land area. The early Cambrian climate was probably moderate at first, becoming warmer over the course of the Cambrian, as the second-greatest sustained sealevel rise in the Phanerozoic got underway. Gondwana moved south with considerable speed. By the Ordovician period, most of West Gondwana (Africa and South America) lay directly over the South Pole. The early Paleozoic climate was also strongly zonal. The climate became warmer, but the continental shelf marine environment became steadily colder. The early Paleozoic ended rather abruptly with the short, but apparently severe, late Ordovician ice age. This cold spell caused the second-greatest mass extinction of Phanerozoic time.

The middle Paleozoic was a time of considerable stability. Sea levels had dropped coincident with the ice age, but slowly recovered over the course of the Silurian and Devonian. The slow merger of Baltica and Laurentia and the northward movement of bits and pieces of Gondwana created numerous new regions of relatively warm, shallow seafloor. The far southern continental margins of Antarctica and West Gondwana became increasingly less barren. The Devonian period (410 to 360 million years ago) resulted in diversifiaction of life on the land, including the first terrestrial vertebrates, amphibians, and forests of trees. In the waters, fish continued their diversification with the rise of the lobe-finned and ray-finned fish. The Devonian ended with a series of turnover pulses that killed off much of middle Paleozoic vertebrate life, without noticeably reducing species diversity overall.

Global cooling tied to Gondwanan glaciation has been proposed as the cause of the Devonian extinction, as it was also suspected of causing the terminal Ordovician extinction. Rocks in parts of Gondwana suggest a glacial event. The forms of marine life most affected by the extinction were the warm-water to tropical species. The late Paleozoic consisted of the Carboniferous period (360 to 286 million years ago), also known as the Mississippian period. The period began with a spike in atmospheric oxygen, while carbon dioxide plummeted. This destabilized the climate and led to multiple ice age events during the Carboniferous. The supercontinent of Pangaea was assembled during this time, causing the uplift of seafloor as continental land masses collided to build the Appalachian and other mountains.

This created huge arid inland areas subject to temperature extremes. The Permian period spanned the time interval from 286 to 245 million years ago. During the Permian, the assembly of Pangaea was completed and a whole host of new groups of organisms evolved. The Permian ended in the greatest of the mass extinctions, where over 90 percent of all species were extinguished. With the assembly of Pangaea and resulting mountain building, many of the shallow seas retreated from the continents. The Permian saw the spread of conifers and cycads, two groups that would dominate the floras of the world until the Cretaceous period with the rise of the flowering plants. The end of the Permian, also the end of the Paleozoic era, was marked by the greatest extinction of the Phanerozoic eon. During the Permian extinction event, over 95 percent of marine species went extinct, while 70 percent of terrestrial taxonomic families suffered the same fate.

The fusulinid foraminiferans went completely extinct, as did the trilobites. The majority of extinctions seem to have occurred at low paleolatitudes, possibly suggesting some event involving the ocean. The exact cause of the terminal Permian extinction remains unknown; however, many theories have been hypothesized. Regardless, this event proved to be a massive and severe crisis for life. Many groups of organisms went extinct at that time. Surviving groups diversified during the Triassic period, and gradually, a more modern world developed.