Mass Extinction: Ordovician-Silurian, Devonian, and Guadalupian

The geologic record documents at least six catastrophic events that brought much of the Earth’s existing life to relatively sudden and untimely ends.

Ordovician-Silurian Extinction

            The first mass extinction occurred 440-450 mya at the Ordovician-Silurian transition. At least two extinction events occurred and together are ranked by many scientists as the second or third largest of the five major extinctions in Earth’s history in terms of percentage of genera that disappeared from the geologic record. Twenty-seven percent of all families and 57 percent of all genera became extinct. The event eliminated many brachiopods and conodonts, which were marine animals with tiny, complex, specialized conical teeth-like structures that appeared long before land animals, fish, or animals with backbones that are now one of the most important biostratigraphic indices available in Paleozoic and Triassic strata. It severely reduced the number of trilobite species and was probably triggered by a major drop in temperature and CO₂ levels, ice sheet formation, and lowering of the ocean level, which combined to adversely affect marine and terrestrial lifeforms. Several scientists have theorized that the leading extinction cause may have been the gradual movement of Gondwana into the South Polar Region and consequent disruption of ocean currents.

Devonian Extinction

            The second extinction event, with effects similar to the first, took place during the late Devonian, about 364 mya in the transition from the late Devonian to the Carboniferous period, and also was probably due to similar causes: decreased temperatures and sea reliction. About 50 percent of all species disappeared. At this time no significant controversy has been generated among geoscientists concerning those events since so little is known of the geological or climatological processes that led to the extinctions.

            Contrary to other mass extinctions, the Devonian event was not sudden; evidence suggests that the extinctions took place in pulses over a period that ranged from about three million to fifteen million years, which is why many paleontologists do not include it as an “event,” since it consisted not only of many independent extinction pulses but perhaps also a multitude of causes including bolide impacts, global anoxia (widespread dissolved oxygen shortages), plate tectonics, sea level changes, global climatic change (cooling rather than warming), and the expansion of terrestrial plants (causing mass extinctions in the tropical oceans). Definitive reasons for the late Devonian extinctions have yet to be demonstrated.

Guadalupian Extinction

            Large-scale extinction event that occurred in the middle Permian around 260 mya that devastated marine life around the world, resulting in a 50 percent extinction in marine genus. Prior to the mid-1990s very little research had been done on this Permian extinction event because it closely preceded the great end-Permian mass extinction peak and for many decades was considered to be part of that larger event. One reason for the prior difficulty in separating these two extinctions was that the duration between them was very short, about eight million years, when compared with durations between the big five mass extinctions, between 41 to 145 million years. Only in 1994 did two research teams  independently identify a separate mass extinction late in the Guadalupian Stage of the Middle Permian followed by a phase of radiation and recovery prior to the end-Permian extinction event. The Guadalupian extinction is best known from evidence provided from shallow marine, equatorial carbonate environments and was particularly severe for brachiopods, corals, echinoderms — blastoids, crinoids, echinoids — foraminifera (especially fusulinids), and reef-forming sponges.
              Researchers led by Paul Wignall of the University of Leeds reported in the journal Science in May 2009 that the Guadalupian Mass Extinction was preceded by massive eruptions in the Emeishan geological province of southwestern China. Since the eruptions occurred in a shallow sea the researchers were able to study both the volcanic rock and the overlying layer of sedimentary carbonates containing fossilized marine life, making it possible to compare dates and directly monitor the relative timing of extinction and volcanism in the same locations. According to Wignall and his colleagues, between the layers of igneous rock are limestone deposits with fossilized evidence of widespread extinction. The injection of hot lava from the volcanoes into the sea would have produced massive cloud formations that likely spread around the world, cooling the planet and producing acid rain. For over half a million years the eruptions discharged about 120,000 cubic miles of lava, killing more than half of the life on Earth and leaving lava deposits 655 feet deep in some locations. Although the evidence discovered by Wignall is not proof of cause-and-effect, the researchers concluded that their study provided solid evidence potentially linking the mass extinction and eruption.