Mass Extinction at the end of the Ordovician, start of Silurian, 445 mya

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At the end of the Ordovician and the start of the Silurian, about 445 million years ago, there was a great extinction event. This event is sometimes identified as the first of the six great extinction events, but there were other major extinction events that were probably more severe. At any rate, benthic fauna (animals dwelling on the sea floor) died off in incredible numbers, with trilobites and brachiopods suffering tremendous losses in diversity. Graptolites suffered as well, and conodont species also died off, although the conodont mortality seems to have happened at the end of the extinction event, perhaps as the ice melted and the seas warmed and began rising.

This First Great Extinction (which may not have been as severe as any of the earlier extinctions listed below) is called the Ordovician-Silurian (or merely the “Ordovician”) extinction event. This seems to have destroyed over 100 taxonomic families in two stages separated by nearly a million years between. It’s clear that there was a major ice age, possibly triggered by the growth of mountains and the spread of rock-eroding mosses and lichens on the land, combined with the continental drift of Gondwana over the polar south, allowing massive glaciation. A sharp decrease in CO2 (probably related to erosion of silcate rocks like granite), and perhaps volcanic activity, may have been part of the mix of environmental causes for this extinction event.

Here are some major extinction events that occurred before the “first great extinction” at the end of the Ordovician:

3.8 to 4 billion years ago, there was a period of frequent asteroid collisions with earth, called the Late Heavy Bombardment. There now evidence that life had formed before this (based on isotopes found in certain traces of material embedded in crystals that formed around the time). While life so early in Earth’s history must have been extremely simple, and possibly not very diverse, there would have been massive death as these meteorites smacked into the planet and re-melted the crust in many areas.

2.7-2.5 billion years ago. The Oxygen Catastrophe, or the Great Oxidization. As photosynthesis began, Oxygen levels in the atmosphere increased by four orders of magnitude (10,000 times more than had previously been present, from concentrations of 0.002 percent up to nearly 20 percent), and this killed off many organisms that required a low oxygen environment.

2.2 billion years ago, 710 million years ago, and 640 million years ago. Snowball Earth. These periods saw glaciers on land right up to near the equator, suggesting a very cold climate, lower sea levels, and widespread glacial and ice cap covers for land and sea. This surely must have caused a severe extinction event, perhaps leaving life only at the hydrothermal vents, but the fossil records of life before and after these events are so slight that it is difficult to say anything definitive about these extinction events.

517, 502, and 488 million years ago. The Cambrian Period saw at least four major extinction events, and one or two of these may have been even more severe than the “first” great extinction between the Ordovician and Silurian Periods. The Botomian Extinction Event (517 mya) and the Dresbachian Extinction Event (502 mya) may exceed even the famous end of the Cretaceous asteroid extinction event in terms of destructive power. There was also a terrible extinction event at the end of the Cambrian and start of the Ordovician at about 488 million years ago. The causes of these mass extinction events are not fully understood. Asteroid collisions are also plausible suspects, but evidence for ice ages and dramatic reductions of oxygen content in shallow seas are the leading candidates.

 

Links about the Ordovician-Silurian extinction event:

1. The Trilobite Extinctions page suggests that the Ordovician extinction event exterminated 27% of all families, 57% of all genera, and 60% or more of all species.

2. Berkeley faculty Berry, Quinby-Hunt, and Wilde have a 1995 article on the impact of late Ordovician glaciation-deglaciation on marine life.

3. BBC Nature has a fine little summary of the Ordovician-Silurian extinction event with a lovely painting of massive glaciers.

4. The Wikipedia article on this extinction event is reasonably accurate and not too far out-of-date.

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