The End-Ordovician extinction was the first of the “Big Five” mass extinction events, occurring approximately 445 million years ago. This event was confined almost entirely to the oceans, as complex life had not yet moved onto land. It marked a dramatic turning point between two major geological periods and demonstrates how planetary systems can trigger catastrophic changes for life.
The Ordovician World Before the Extinction
Before the extinction, high sea levels flooded the continents, creating vast, shallow inland seas. This geography, coupled with a warm “greenhouse” climate, fostered an explosion of life in the oceans. The dominant continents were gathered into a supercontinent known as Gondwana, which was slowly drifting over the South Pole. Life in these warm seas was abundant, as marine invertebrates like trilobites, brachiopods, and bryozoans flourished, and early corals constructed the first large-scale reef systems.
A Two-Phase Catastrophe
The extinction occurred in two distinct pulses separated by roughly a million years. The first phase was initiated by a rapid ice age. As massive glaciers formed on Gondwana, they locked up enormous volumes of water, causing a drop in global sea levels of as much as 50 to 100 meters. This sea-level fall drained the shallow epicontinental seas that were the cradle of Ordovician life, and the loss of these habitats combined with the cooling climate led to the first wave of mass extinction.
The second pulse was driven by the opposite conditions. The ice age ended abruptly, and the glaciers melted quickly, causing sea levels to rise rapidly. This influx of freshwater and disrupted ocean circulation are thought to have created widespread anoxia, or a lack of oxygen. Survivors of the first pulse, now facing oxygen-depleted waters, were hit by a second wave of extinction.
The Prime Suspects and Triggers
The trigger for this ice age is believed to be geological. A major mountain-building event, the Taconic Orogeny, created the predecessors to the modern Appalachian Mountains. This process uplifted enormous quantities of fresh rock that, through chemical weathering, drew down immense amounts of carbon dioxide (CO2) from the atmosphere. This reduction in atmospheric CO2 is considered a primary driver of the global cooling that followed.
The planet’s geography played a supporting role. With the massive landmass of Gondwana situated over the South Pole, the Earth was predisposed to glaciation once a cooling trend began. This made the climate system highly sensitive to the drop in CO2 levels, pushing it into a full-blown ice age.
While the CO2 drawdown hypothesis is the most supported explanation, other theories have been proposed. One idea involves a gamma-ray burst from a nearby supernova stripping away Earth’s ozone layer. Less direct geological evidence exists for this scenario compared to the documented glaciation.
Life’s Devastating Toll
The biological consequences were immense, with an estimated 85% of all marine species eliminated, making it one of the most severe extinctions in history. The crisis did not affect all life equally, as certain groups were hit particularly hard due to their adaptations and habitat preferences. The greatest casualties included brachiopods, trilobites, graptolites, and conodonts. Many of these groups were overwhelmingly adapted to the warm, shallow inland seas that disappeared, and the collapse of early coral reef systems destroyed habitats that supported countless other organisms.
Aftermath and Recovery
In the wake of the devastation, the world entered the Silurian period. The species that survived the dual crises found themselves in a radically altered and largely empty world. These “survivor” species, often more adaptable, began to diversify into the ecological niches left vacant by the extinct fauna. The decline of previously dominant invertebrate groups, such as the trilobites, created an evolutionary vacuum. This opening allowed for the rise of other groups, most notably the early jawed fishes, which became prominent in the subsequent Devonian period.