Extinction During the Late Devonian
A Review of the Mass Extinction Event 376 Million Years Ago
Dec 3, 2008
Occurring approximately 376 million years ago, the late Devonian mass extinction is the only event that does not mark the boundary of a major time period change. Instead, the extinction takes place within the Devonian, between the Frasnian and Famennian ages. This is a relic of the determination of geologic boundaries, which were drawn in accordance to major changes in fossil content.
“Since mass extinctions caused marked changes in life, it is no surprise that period boundaries correspond to the extinction horizons,” says George McGhee, a paleontologist at Rutgers University. During the late Devonian, two horizons of biotic changes exist, says McGhee, and early geologists believed the second to be the most pronounced, thus choosing to draw the Devonian/Carboniferous boundary at that point.
“Many years later,” says McGhee, “we now know that the biotic change at the Frasnian/Famennian boundary was greater – in essence, the Devonian/Carboniferous boundary should have been drawn there.”
Pulses of Extinction
It is a point of contention amongst paleontologists whether the late Devonian event should be considered one prolonged extinction, two distinct events or a series of events. Thomas Algeo, a geologist at the University of Cincinnati, believes that there were at least ten episodes of higher extinction rates during the 25 million year duration of the event.
Regardless of the duration or number of individual extinction events, most scientists agree that at least 70 percent of marine species, including the elpistostegalian fish, a precursor to the modern amphibian, went extinct during this time. “Some estimates suggest that 82 percent of all species went extinct,” says McGhee, as land plants and insects were also affected.
The Devonian period was characterized by warm, tropical seas and a bustling marine realm. On land, plants and a few organisms including arthropods, arachnids and paleoinsects, were proliferating into new niche space. “Amphibians first evolved in the late Devonian,” says Algeo, “a few million years after the appearance of the first forests.”
Hypothetical Causes of the Late Devonian Event
Several theories have been suggested as to what caused the abrupt extinction of 70 percent of all marine species 376 million years ago. Anoxia, or oxygen depletion in marine waters, is a common hypothesis because of the large distribution of black shale in Devonian sediments. These rocks indicate a lack of oxygen, and if anoxic deep waters are brought to the surface, species intolerant to low-oxygen environments will die.
Recently, Algeo has published work identifying the evolution of trees as a contributing factor to the anoxic conditions of the oceans. He reports in his 2000 paper, “Effects of the Middle to Late Devonian Spread of Vascular Land Plants on Weathering Regimes, Marine Biota and Global Climate,” published in Plants Invade the Land from Columbia University Press, that deep-rooting plants lead to increased sedimentation. “After the plants die and decay,” Algeo says, “nutrients are released and ultimately washed through the freshwater system into the oceans.”
Algeo explains that “the spread of land plants resulted in ‘background conditions’ in which marine systems were generally stressed through changes in nutrient fluxes and dissolved oxygen levels,” Algeo says, “but the individual extinction episodes were triggered by some other external factor(s).”
However, McGhee disagrees with the “killer land plant” hypothesis. “It is true that the nutrient runoff from the land increased markedly due to the action of land plants, and it is true that this triggered eutrophication of many shallow seas and widespread oxygen depletion in marine waters,” he says, “but this happened many times during the late Devonian without triggering a planet-wide extinction.”
Instead, McGhee says that most of the evidence points to global cooling. For instance, the majority of affected marine species were those adapted to warm and shallow waters – deep-water, high-latitude and freshwater species were all relatively untouched. Global climate cooling would also account for the fact that terrestrial organisms were also affected. “The geochemical evidence shows that the Earth became colder at the extinction horizons,” McGhee says.
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