The Ordovician-Silurian extinction, occurring approximately 445 million years ago, was the first of the “Big Five” mass extinctions in Earth’s history. This event unfolded over several million years, ultimately wiping out an estimated 85% of all marine species. At the time, life was almost entirely confined to the oceans, making this a devastating blow to the planet’s biosphere. The scale of this extinction reshaped the trajectory of evolution and set a new course for the development of life.
The Geological and Climatic Context
During the Late Ordovician, the world was vastly different from today. Most of the planet’s landmass was consolidated into a single supercontinent known as Gondwana, which was situated over the South Pole. The rest of the globe was a vast ocean, with smaller continents scattered in the warmer, tropical latitudes. This geological arrangement had a profound influence on the planet’s climate and marine environments.
The era had a warm, greenhouse climate, with atmospheric carbon dioxide levels significantly higher than today. This warmth prevented the formation of significant polar ice caps, leading to high sea levels. These waters flooded large portions of the continents, creating extensive, shallow epicontinental seas. These warm, sunlit inland oceans were stable environments that fostered an explosion of biodiversity over millions of years.
A Two-Phase Extinction Event
The primary trigger for the extinction was a brief but severe ice age. As the supercontinent Gondwana drifted over the South Pole, conditions became favorable for the formation of massive ice sheets. The growth of these glaciers locked up enormous volumes of seawater, causing a rapid and drastic fall in global sea levels. This drop drained the vast, shallow epicontinental seas, destroying the stable habitats where a majority of marine species lived.
The glacial period was relatively short-lived, and as the climate began to warm again, the massive ice sheets on Gondwana melted. This released immense quantities of freshwater back into the oceans, causing a swift rise in sea levels. The rising waters flooded the newly established coastal environments, creating another wave of habitat disruption for the surviving species.
This second phase of the extinction was compounded by changes in ocean chemistry. The influx of cold, fresh meltwater is thought to have disrupted oceanic circulation patterns. Evidence suggests this led to widespread anoxia, or periods of low oxygen levels, in the water. The combination of rising seas and oxygen-depleted waters delivered a final blow to many species that had survived the initial cooling.
Life Forms Most Affected
The Ordovician-Silurian extinction was an almost exclusively marine event, as complex life had not yet established a significant foothold on land. The devastation was widespread, affecting nearly all major groups of marine organisms. Among the hardest-hit were brachiopods, shelled animals resembling modern clams that were diverse in the Ordovician seas. Many of their families, particularly those endemic to specific regions, were wiped out.
Trilobites, armored arthropods that were once ubiquitous across the ocean floor, also suffered immense losses. While they had been declining prior to the event, the extinction pulses severely reduced their numbers and diversity. Other heavily impacted groups included:
- Graptolites, feathery, filter-feeding organisms that floated in the water column and faced near-total extinction.
- Reef systems, built by various corals and sponge-like organisms, which collapsed as their shallow-water habitats disappeared.
- Conodonts, which were early, eel-like vertebrates.
- Various types of echinoderms, bivalves, and bryozoans.
The extinction disproportionately affected species that were highly specialized or confined to narrow geographic regions. Cosmopolitan species, those that were more widely distributed, tended to fare better and were often the ones to repopulate the seas after the event.
The Aftermath and Silurian Recovery
Following the loss of life, the subsequent Silurian Period was a time of ecological recovery and evolutionary innovation. The extinction vacated numerous ecological niches, creating opportunities for surviving lineages to diversify into new roles. The biosphere began a slow process of rebuilding that took several million years to gain full momentum.
Reef ecosystems, which had been devastated, began to recover with new types of corals and sponges taking over as the primary builders. The faunas that emerged in the Silurian were initially composed of widespread, “disaster taxa”—hardy species that survived the crisis. Over time, brachiopod and trilobite communities recovered, though they were composed of different, more globally distributed genera than their Ordovician predecessors.
The most significant long-term consequence of the extinction was the evolutionary trajectory of vertebrates. The environmental shifts and the decimation of many dominant invertebrate groups appear to have paved the way for the rise of fish. The Silurian saw the first major radiation of jawed fish, known as gnathostomes. This group would go on to dominate marine ecosystems and eventually give rise to all jawed vertebrates, including amphibians, reptiles, birds, and mammals.