Ordovician–Silurian extinction events
The Ordovician–Silurian extinction events, occurring approximately 440 to 450 million years ago, represent one of Earth's most significant mass extinctions, with over 85% of species lost. During this period, life was predominantly marine, thriving in warm, shallow seas. The extinction is attributed to drastic environmental changes, primarily a severe global cooling and a rapid decline in sea levels. The leading theory suggests that the supercontinent Gondwana drifted over the South Pole, leading to ice formation that altered weather patterns and oceanic conditions. Other hypotheses include the impact of early land plants on atmospheric carbon dioxide levels and geological activities related to the formation of the Appalachian Mountains.
As ocean temperatures dropped and sea levels fell, many marine species, particularly corals and shellfish, faced extinction. The end of this period saw a warming trend that allowed for ecological recovery and an increase in biodiversity, including the emergence of land-dwelling organisms. While the causes of the cooling event remain debated, various scientific theories attempt to explain this complex phenomenon, highlighting the intricate interactions between geological and biological systems during this critical time in Earth's history.
Ordovician–Silurian extinction events
The Ordovician–Silurian extinction events were a series of catastrophic events that occurred about 440 to 450 million years ago, leading to the second-largest mass extinction in history. At the time, almost all life on Earth was confined to the oceans, especially the warm, shallow waters near the tropics. Then, within the span of a few million years, the planet was hit with a one-two punch that killed more than 85 percent of all species on Earth. The prevailing belief among scientists is that the extinction was caused by a massive global cooling and a sudden drop in sea levels. However, the trigger for these environmental catastrophes remains open to debate. The most commonly accepted theory is that a supercontinent drifted over the South Pole, became covered in ice, and plunged the world into an ice age. Other theories suggest the rise of land-based plants or the birth of the Appalachian Mountain range was responsible for the global cooling. More exotic explanations include a deadly radiation burst from space or the pollution of the ocean with toxic metals.
Background
Scientists divide the 4.5 billion-year history of Earth into chronological categories known as eons, eras, periods, and epochs. Eons are the longest segments of geological time, spanning four intervals from the planet’s formation to the present day. The current eon, the Phanerozoic, began about 541 million years ago when the planet’s oceans seemed to suddenly explode with new life forms. The Phanerozoic is divided into three eras: the Paleozoic, Mesozoic, and Cenozoic. The oldest of these, the Paleozoic, lasted until 251 million years ago when the most deadly extinction event in Earth’s history wiped out more than 90 percent of all life on the planet, including about 96 percent of all marine species.
The Paleozoic is itself divided into six periods: the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian. In the early Ordovician Period, which began about 488 million years ago, all life on Earth lived in the oceans. The planet’s seas were teeming with marine species such as sponges, jawless fish, trilobites, crab-like arthropods, shell-covered brachiopods, and starfish-like echinoderms. Global temperatures were far warmer than in modern times, with sea temperatures near the equator approaching 110 degrees Fahrenheit (43.3 Celsius).
Earth’s surface looked radically different during the Ordovician, with most of what would become Africa, Antarctica, Australia, India, and South America concentrated in a supercontinent called Gondwana. The smaller continent of Laurentia, which included much of modern North America, was located near the equator. The position of these landmasses during the period created large sections of coastlines and shallow seas. As the Ordovician progressed, plate tectonics began to slowly shift the continents across the globe. The Appalachian Mountain range, which today runs along eastern Canada and the United States, began forming on the edge of Laurentia.
Overview
The transition from the Ordovician Period to the Silurian Period occurred 443 million years ago and was marked by the first of five mass extinctions in Earth’s history. This extinction was not a single event, but was a series of two extinction waves that killed off most of the life on the planet. Global temperatures had begun to become more mild during the late Ordovician Period, but sometime between 450 million and 440 million years ago, the planet began to rapidly cool and large glaciers covered much of the southern hemisphere. This change in temperature proved deadly for many species living in the warmer waters of the Ordovician seas. As the temperature cooled, the growing ice sheets absorbed more of the ocean’s water, lowering sea levels around the globe. Because the shallow seas near the equator were home to many marine species, the dropping ocean levels brought about a second wave of extinctions.
Scientists estimate that about 85 percent of all species and 100 biological families of organisms went extinct during the Ordovician–Silurian events. Especially hard hit were species of coral, brachiopods, echinoderms, and the mollusk-like bivalvia—organisms that resemble modern clams and oysters. About one-third of all brachiopod families died out during the extinction. As temperatures warmed during the Silurian, the glaciers receded and sea levels rose, leading to a dramatic rebound in biological diversity. Scientists believe the first creatures to permanently colonize land did so during the Silurian.
Although scientists are confident of the general causes of the Ordovician–Silurian extinction, they remain unsure as to what events triggered the massive global cooling and drop in sea levels in the first place. The most widely accepted explanation is that the continent of Gondwana slowly drifted south to cover the region around the South Pole. Because landmasses lose heat faster than water, this caused large ice caps to form over the continent. The ice-covered landmass altered weather patterns and sea currents, leading to the growth of large glaciers. These glaciers not only locked up the water from the ocean, causing sea levels to fall, they also changed the chemical makeup of the sea water. When the water melted and returned to the sea, its chemical composition was no longer suited to the organisms of the original ocean environment.
Another theory holds that the proliferation of early land plants—simple moss-like plants that first appeared about 470 million years ago—used up much of the carbon dioxide in the atmosphere. Carbon dioxide is a gas that traps heat. With less carbon dioxide in the atmosphere, more heat was able to escape into space, thereby cooling the planet. A similar hypothesis suggests that it was the rocks of the infant Appalachian range that absorbed carbon dioxide from the atmosphere. Without abundant plant cover, the rocks would have been weathered by the elements more quickly. Certain types of silicate rocks can draw carbon dioxide from the air though the process of weathering and erosion.
In the early twenty-first century, some scientists suggested that a burst of gamma ray radiation from a distant exploding star may have impacted Earth, stripping away the planet’s protective ozone layer. Such an occurrence would not only have caused a mass extinction, it could also have contributed to the rapid planetary cooling. Although the theory is plausible, no evidence of such a gamma ray burst has ever been found. In 2015, researchers in Europe proposed that a depletion of oxygen levels in the ocean caused the seawater to absorb toxic metals from the ocean floor. The suddenly toxic oceans would have killed off many lower organisms on the marine food chain, causing many higher organisms to starve.
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