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 life-forms. 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 life-forms 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 [Page 777]gathered together into a supercontinent called Pangaea, which included most of the Earths land area.

The Early Cambrian climate was probably moderate at first, becoming warmer over the course of the Cambrian, as the second-greatest sustained sea level rise in the Phanerozoic got underway. Gondwana moved south with considerable speed. By the Ordo-vician 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, the amphibians, and the first 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 which 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 ones.

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