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Intro | Precambrian Eon | Paleozoic Era | Mesozoic Era | Cenozoic Era

Paleozoic Era: (543-248 mya)

Cambrian | Ordovician | Silurian | Devonian | Carboniferous | Permian

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Permian Period (290-248 mya)

A great merger of northern and southern landmasses creates the supercontinent Pangaea. Stretching from pole to pole and surrounded almost entirely by a single world ocean called Panthalassa, Pangaea's dry interior fluctuates in temperature far more than its coastal zones, which are moderated by nearby water.

On land, reptiles continue to evolve. Mammal-like reptiles, precursors to the true mammals, appear and quickly radiate. Seed-producing gymnosperms, which include the conifers and the cycads, begin to replace lycophytes as the dominant plant group.

In the oceans, brachiopods, ammonoids, crinoids, bony fishes, and sharks still thrive, but rugose corals, tabulate corals, and trilobite populations are on the wane. A catastrophic extinction near the close of the period -- several times worse than any other -- nearly ends the entirety of life on land and in the seas.

280 mya: Cycads

With a stout trunk and leafy crown, cycads provide vegetation and canopy cover to animals. Cycad fossils -- leaves, stems, cones, and seeds -- are found on every continent, suggesting these gymnosperms thrived in diverse climates over time. Modern species exist only in tropical regions, however.

280 mya: Pangaea supercontinent forms

By the early Permian, Earth's major land masses -- Gondwana, Laurussia, and Siberia -- fuse with smaller continents to form the supercontinent Pangaea. Pangaea, which means "all the Earth," stretches nearly from pole to pole. Where plates converge, crust folds and mountains form. Pangaea is surrounded almost entirely by a massive ocean, Panthalassa. A secondary body of water, the Tethys Ocean, cuts into the east of Pangaea near the equator.

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Aside from altering the visual landscape, mountain building greatly influences climate. Pangaea's vast interior becomes increasingly dry. Shielded from the moderating effects of coastal waters, temperatures fluctuate wildly from season to season.

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280 mya: Drying trend begins

The interior of Pangaea begins to dry.

275 mya: Mammal-like reptiles

Mammal-like reptiles are cold-blooded, but they spawn true, warm-blooded mammalian successors. They are the dominant land vertebrates during the Permian, but despite their early success, only two families will survive into the next period.

End Permian extinction

Date:

250 mya

Intensity:

1

Affected:

Approximately 90 percent of all species, including nearly 57 percent of marine families and nearly 70 percent of land vertebrate families go extinct

Hypotheses:

Volcanic activity, glaciation, sea-level changes, changes in ocean chemistry, global warming, meteor impact

Summary:

In the most devastating of all extinctions, an estimated 90 percent of all species are eliminated over two pulses, spaced about 8 million years apart.

All remaining trilobites, all graptolites, and nearly all echinoderms are killed off. The brachiopods, dominant through most of the Paleozoic era, will be only a minor marine invertebrate group in the Mesozoic and Cenozoic eras. Marine predators such as sharks and coelacanths are hit hard, and coral reefs will require at least 10 million years to recover. Among the land animals, two-thirds of amphibian families, all large herbivores, and most reptiles die out. Terrestrial plants also suffer greatly.

The extinction might be the result of a combination of factors, including massive glaciation, changes in sea level, and either an oversupply of carbon dioxide or an undersupply of oxygen in the oceans. Some of these conditions might have been triggered by volcanic eruptions in Siberia, or perhaps even by a meteor impact.



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The mother of all extinctions (250 mya)

Marine communities are at their most diverse, and flora and fauna radiate on land throughout the Permian period. The end Permian extinction is the closest that life has come to complete annihilation in the past 600 million years, if not the entire history of Earth.

  • In the oceans, approximately 57 percent of all marine families die out, which is about 400 families. On the species level, 90-95 percent may have perished. That means as few as 12,500 out of 250,000 marine species may have survived.
  • On land, more than 70 percent of all vertebrate families are wiped out, including approximately 75 percent of all reptiles and approximately 67 percent of the amphibians. Most plant life disappears, as does nearly 33 percent of insect orders, marking the only time insects have ever suffered mass extinction.

The extinction generates a complete reorganization of both marine and terrestrial life. Dominant groups that vanish leave several ecological roles vacant. Many of the groups that fill these roles in the subsequent period persist through the Mesozoic and, in some cases, to the present day.

The cause of this extinction is still largely unresolved. Some hypotheses suggest that slow but progressive changes in climate or sea level were responsible. Other hypotheses point to quick-hitting, catastrophic events such as extraordinary volcanic activity. Still others suggest a combination of factors may have contributed to the mass killing.

In 2001, a new hypothesis was put forward. Scientists found traces of iridium in rocks dated to the time of the extinction. Iridium is scarce in Earth's crust but plentiful in asteroids, and the scientists think an asteroid as large as seven miles across might have slammed into Earth, releasing at least a million times more energy than the strongest recorded earthquake. But until more conclusive evidence like a large impact crater is found, this hypothesis will remain, like others, an intriguing but unproven explanation.

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-> Learn more about the Mesozoic Era

Intro | Precambrian Eon | Paleozoic Era | Mesozoic Era | Cenozoic Era

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