The Paleozoic Era

Diversification of Plant and Animal Life

Britannica Educational Publishing, John P Rafferty

• Publisher: Britannica Educational Publishing
• Year: 2010
• ISBN:
  9781615301966
• Topic: Ciencias físicas
• Subtopic: Geología y ciencias de la Tierra

Characterized by a surge in biodiversity and evolutionary development, the Paleozoic Era ushered in the beginnings of life as we know it. Within these pages, readers will discover the fossil and geologic evidence from this time that reveals a dynamic planet, where new species of plants and animals were constantly emerging and continents were breaking apart and reforming. While many of the species from this era are now extinct and the continents are relatively stable, remnants of this era still continue to shed much-needed light on the conditions of Earth, both past and present.

CHAPTER 1
AN OVERVIEW OF PALEOZOIC TIME

Spanning nearly 300 million years, the Paleozoic Era was one of the defining intervals shaping life on Earth. It was characterized by the sudden emergence of multitudes of new life-forms as well as the migration of several of Earth’s continents from the Southern Hemisphere to the Northern Hemisphere. By the end of the era, all of Earth’s major landmasses were merged into one. The Paleozoic was a time in which many bizarre creatures thrived. Long, tubular Hallucigenia floated through ancient seas along with trilobites and armoured fishes, while early amphibians first crawled onto solid ground. These were just a few of the notable members of Paleozoic faunas. However, life on Earth was subjected to three of the most severe extinctions of all time.

The Paleozoic Era was a major interval of geologic time that began 542 million years ago with the Cambrian explosion, an extraordinary diversification of marine animals, and ended 251 million years ago with the end-Permian extinction, the greatest extinction event in Earth history. The major divisions of the Paleozoic Era, from oldest to youngest, are the Cambrian (542 to 488.3 million years ago), Ordovician (488.3 to 443.7 million years ago), Silurian (443.7 to 416 million years ago), Devonian (416 to 359.2 million years ago), Carboniferous (359.2 to 299 million years ago), and Permian (299 to 251 million years ago) periods. The Paleozoic takes its name from the Greek word for ancient life.

PALEOZOIC LIFE

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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. 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. About 542 million years ago, at the onset of the Cambrian Period, intense diversification resulted in more than 35 new animal phyla. However, new discoveries show that the “explosion” started roughly 575 million years ago, near the end of the Proterozoic Eon, with the Ediacara fauna. 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.

An early Silurian coral-stromatoporoid community.

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.

The lone tree with horizontal grooves in the right foreground is a jointed sphenopsid (Calamites). The large trees with scar patterns are lycopsids. Courtesy of the Department Library Services, American Museum of Natural History, neg. #333983

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: waterborne spores were replaced by seeds in plants of seed-fern origin, and shell-less eggs were replaced by amniote eggs with protective shells in animals of reptilian origin. 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 (small colonial animals that produce a skeletal framework of calcium carbonate), 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.

PALEOZOIC GEOGRAPHY

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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, a craton primarily made up of present-day North America and Greenland, was rotated 90° clockwise from its present orientation and sat astride the paleoequator during Cambrian times. Laurentia (a name derived from Quebec’s portion of the Canadian Shield) 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. To the east of Laurentia, the Siberian craton was positioned just south of the paleoequator between Laurentia and the western coast of Gondwana. Until the late Carboniferous Period, Siberia was rotated 180° from its present orientation.

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.

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 paleo-equator from southern cool temperate latitudes into northern warm latitudes during the Paleozoic Era. 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.

Devonian volcanics formed Ancient Wall. Once the floor of a shallow ocean, it now juts up to a peak in Canada’s Jasper National Park. Jonathan S. Blair/National Geographic/Getty Images

Paleozoic rocks are widely distributed on all continents. Most are of sedimentary origin, and many show evidence of deposition in or near shallow oceans. Among the more useful guide fossils for correlation are trilobites (an extinct group of aquatic arthropods), for Cambrian through Ordovician strata; graptolites (small, colonial, planktonic animals), for rocks dated from Ordovician through Silurian times; conodonts (primitive chordates with tooth-shaped fossil remains), for Ordovician to Permian rocks; ammonoids (widely distributed extinct mollusks resembling the modern pearly nautilus), for Devonian through Cretaceous strata; and fusulinids (single-celled amoeba-like organisms with complex shells), for rocks dating from the Carboniferous through the Permian Period.

CHAPTER 2
THE CAMBRIAN PERIOD

The earliest time division of the Paleozoic Era was the Cambrian Period, an interval extending from about 542 to 488.3 million years ago. The Cambrian Period is divided into four stratigraphic series: Series 1 (542 to 521 million years ago), Series 2 (521 to 510 million years ago), Series 3 (510 to 501 million years ago), and the Furongian Series (499 to 488.3 million years ago).

Rocks formed or deposited during this time are assigned to the Cambrian System, which was named in 1835 by English geologist Adam Sedgwick for successions of slaty rocks in southern Wales and southwestern England. These rocks contain the earliest record of abundant and varied life-forms. The corresponding period and system names are derived from Cambria, the Roman name for Wales. As originally described, the Cambrian System was overlain by the Silurian System, which was named, also in 1835, by Scottish geologist Roderick I. Murchison. Subsequent disagreement between Sedgwick and Murchison over the definition and placement of the Cambrian-Silurian boundary led to a bitter controversy that involved many British geologists. The problem persisted until after the deaths of both Sedgwick and Murchison in the 1870s and the eventual adoption of an intervening system, the Ordovician, which was proposed in 1879 by English geologist Charles Lapworth.

The Cambrian world differed greatly from that of the present, but it was also quite different from the preceding Proterozoic Eon (2.5 billion to 542 million years ago) in terms of climate, geography, and life. Average global temperatures during much of the Neoproterozoic Era (1 billion to 542 million years ago) were cooler (around 12°C [54°F]) than the average global temperatures (around 14°C [57°F]) of the present day, whereas the global temperature of Cambrian times averaged 22°C (72°F). Low temperatures during the Neoproterozoic helped to sustain a series of worldwide events known as the Sturtian (748 to 713 million years ago), Marinoan (650 to 600 million years ago), and Gaskiers (595 to 565 million years ago) glaciations. Climate studies suggest that Cambrian temperatures were the norm for most of the Phanerozoic Eon (the last 542 million years), and these were exceeded only by a brief increase during the Permian Period near the end of the Paleozoic Era. Cooler periods, similar to the average global temperature of the present day, occurred during the end of the Ordovician, Late Carboniferous, Early Permian, Late Jurassic, and Early Cretaceous periods, as well as near the end of the Oligocene Epoch.

Just prior to the beginning of the Neoproterozoic Era, Earth experienced a period of continental suturing that organized all of the major landmasses into the huge supercontinent of Rodinia.

Rodinia was fully assembled by one billion years ago and rivaled Pangaea (a supercontinent that formed later during the Phanerozoic Eon) in size. Before the beginning of the Cambrian, Rodinia split in half, resulting in the creation of the Pacific Ocean west of what would become North America. By the middle and later parts of the Cambrian, continued rifting had sent the paleocontinents of Laurentia (made up of present-day North America and Greenland), Baltica (made up of present-day western Europe and Scandinavia), and Siberia on their separate ways. In addition, new collisional events led to the formation of Gondwana, a supercontinent composed of what would become Australia, Antarctica, India, Africa, and South America.

The tectonic events involved in the breakup of Rodinia also modified the ocean basins, forcing their expansion and flooding portions of many continents. The melting of the Varanger glaciers during the Neoproterozoic Era also played a role in the flooding of continents. This episode represented one of the largest and most persistent rises in sea level of the Phanerozoic Eon. Though the extent of continental flooding varied, for most continents sea level reached its maximum by the middle and later parts of the Cambrian time. This flooding, combined with the elevated Cambrian temperatures and changes in Earth’s geography, led to increased rates of erosion that altered ocean chemistry. The most notable result was an increase in the oxygen content of seawater, which helped set the stage for the rise and later diversification of life—an event that has come to be known as the “Cambrian explosion.”

THE CAMBRIAN ENVIRONMENT

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Known from rocks and fossils surrounding three of the period’s major landmasses, the Cambrian was a time of volcanism across Gondwana and several other continents, ice-free polar regions, and warmer conditions than today. Scientists contend that global sea levels dropped and rose relatively suddenly during this period that created as well as destroyed many biological communities.

PALEOGEOGRAPHY

The geography of the Cambrian world differed greatly from that of the present day. The geographic reconstruction is based on integrated geologic and biological evidence. Fossils in continental-shelf deposits indicate the presence of at least three major faunal provinces (or biogeographical regions) during much of the Cambrian Period.

The most distinct faunal province surrounded the continent of Laurentia. Paleomagnetic evidence indicates that Laurentia was located over the paleoequator during most or all of Cambrian time. This geographic interpretation is supported by the presence of thick, warm-water carbonate-platform deposits that accumulated in a broad belt encircling the continent. These carbonates are commonly flanked on the inner shelf by lagoonal shale and nearshore sandstone deposits. On the outer shelf, the carbonates commonly grade into laminated mudstone and shale that accumulated in deeper water. At times, two almost mutually exclusive ecosystems are separated by temperature and salinity barriers in the shallow water on the carbonate platforms. Inner restricted-shelf deposits were characterized by sparse low-diversity communities that tended to be highly endemic (confined to a particular region). Outer open-shelf deposits are characterized by high-diversity ecosystems that were widely distributed around the continent. Fossils are usually most abundant and most diverse near the outer margins of the carbonate platform. Because Laurentia remained nearly intact structurally, it is ideal for studying the relationships between Cambrian environments and communities of organisms around a low-latitude Cambrian continent.

Another Cambrian faunal province surrounded the small continent of Baltica, which was located in middle to high southern latitudes. Cambrian shelf deposits of Baltica are relatively thin, rarely exceeding 250 metres (820 feet) in thickn...