The Late Ordovician Mass Extinction
A Review of the Second-Largest Extinction Event in Earth’s History
Dec 12, 2008
Second only to the Permian-Triassic extinction, the late Ordovician event 440 million years ago, saw the demise of approximately 85 percent of marine species in two distinct pulses, according to Peter Sheehan, a paleontologist with the Milwaukee Public Museum.
In his 2001 paper, “The Late Ordovician Mass Extinction,” published in the Annual Review of Earth and Planetary Sciences, Sheehan describes a glacial interval with two episodes of extinction occurring at the beginning of the glacial phase, and one at the end.
“We have a good understanding of the basic changes during the extinction and recovery,” says Sheehan.
Faunal Overturn and Paleoecological Changes
Perhaps because of the general lack of diversity that early in Earth’s history, or the fact that the species existing at that time were in the marine realm, the resulting Silurian faunal composition looked much like the original Ordovician fauna.
According to Sheehan, after the first pulse of extinctions took place, in which graptolites and brachiopods were hard hit, the open niche space was filled with species adapted to survive the freezing temperatures of the ice age. Some species did not die off, but simply moved farther south, contracting on the equatorial region.
At the end of the ice age, as water returned to the shallow seas, another wave of extinctions took place as those cold-water species died off. The ecological habitats that were open niche space were then refilled by the families that were lost at the beginning of the glacial interval.
“In the Ordovician most major groups survived and the new fauna was actually quite similar to the extinct faunas,” Sheehan says. “The major carnivores were still nautiloid cephalopods. Brachiopods were still the most common organisms.”
Hypothetical Cause of the Ordovician Extinction
It is widely accepted in the scientific community that the late Ordovician extinction was caused by a period of glaciation. As water was frozen into glaciers and ice sheets, sea level dropped and shallow areas dried up. Sheehan reports that deep ocean currents could have brought toxic waters and excess nutrients to the surface waters as well.
“Extinction in the disappearing epicontinental seas was immense. Open ocean animals living in the water column also suffered greatly,” says Sheehan. “Prior to the glaciation, currents were very modest and the oceans were stratified.”
After approximately a half million years, Earth’s climate returned to its former ecological condition, causing another wave of extinctions. The cold-weather fauna that adapted to the glacial interval went extinct at the Ordovician/Silurian boundary.
Citing previous work, Sheehan and his team established that “even though the percentage loss of taxa is perhaps the second largest extinction, the ecological changes were probably the least among the five [Phanerozoic] extinction events.”
Triggers of the Ordovician Ice Age
Recent work from Adrian Melott of the University of Kansas and Brian Thomas from Washburn University has cited gamma rays from supernovae as a possible trigger to the glacial period of the late Ordovician.
They believe that a strong burst of gamma rays within 10 million light-years of the Earth could have substantially reduced the ozone layer, exposing the Earth to harmful UV radiation. Melott and Thomas argue that a burst this large and close could happen once in a billion years.
“There is a serious problem even with the theory because such an event is unlikely to have happened to the Earth,” says Sheehan who does not agree with this hypothetical trigger. “No suggestion has been made as to how a gamma ray burst could start a glaciation. Melott and Thomas merely took a known phenomenon and suggested it happened in the Ordovician.”
