Extinctions, Mass

Ordovician Mass Extinction

The first mass extinction took place at the end of the Ordovician time period 438 million years ago (mya). Life forms in the Ordovician period were restricted to the seas, and the Ordovician mass extinction is one of the most devastating extinctions in geological history, with more than one quarter of Earth's marine species being eliminated. Numerous species of the trilobites and brachio-pods were eliminated, while graptolites and cono-donts were also seriously affected. Over 100 marine invertebrate families were destroyed in the mass extinction. Scientists attribute the cause of the mass extinction at the end of the Ordovician to glaciations and the consequent drop in sea levels. As a result of the drop in sea levels, there becomes a shortage of inhabitable space on the continental shelves. Along with this shortage, the sea temperature dropped as a result of glaciations, also affecting life forms.

Devonian Mass Extinction

During the late Devonian period, a second mass extinction swept the earth around 360 mya. The reefs faced near elimination, as did several other marine animals. Creatures affected included bra-chiopods, trilobites, and conodonts, as well as the placoderm armored fish. Land records for this time are less clear, and it is possible there was a major extinction among land flora.

Permian Mass Extinction

The Permian mass extinction is the largest mass extinction in geological history. This extinction ended the Permian period, 250 mya, and marked the transition between the Paleozoic and Mesozoic eras. The effect of the mass extinction reached across both land and sea, wiping out over 90% of all species on Earth. Over one half of the marine [Page 504]species perished, including the rugose and tabulate corals, fusulinids, and several echinoderms. Brachiopods also suffered, ending their dominance in population. There was also a regionally vital extinction among plant life. The last of the trilo-bites were eliminated during the Permian mass extinction. Over one half of the vertebrate families were lost. The exact cause of the mass extinction is debated, with speculative causes including major volcanic eruptions, glaciations, and changes in sea level. Some scientists attribute the mass extinction to the collision of the Gondwana and Laurasia land masses to create the land form Pangea. Another theory includes a massive comet or asteroid impacting Earth, which would have also contributed to the mass extinction.

Triassic Mass Extinction

The Triassic mass extinction happened 50 million years after the previous mass extinction, about 200 mya. By this time, a number of land and marine life forms had repopulated the earth, and reptiles had evolved into crocodile-like animals and a few mammal-like reptiles. Most of these animals were destroyed in the mass extinction, including conodonts, and others, such as the calcareous sponges and shelled ammonites, were nearly wiped out. The cause of this mass extinction is believed to be the result of a combination of catastrophes happening over the span of 100,000 years or less. These events include an estimated 4-mile-wide meteor impacting Earth near Quebec, creating a 70-mile-wide crater; eruptions of lava flow underneath the Amazon river valley; and a dramatic change in climate. Together these events were sufficient enough to create a mass extinction, allowing the dinosaurs to emerge.

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