The first of the “Big Five” mass extinctions, the Late Ordovician event, occurred approximately 445 million years ago, marking the boundary between the Ordovician and Silurian periods. At this point in Earth’s history, complex life was almost entirely confined to the oceans. The land was largely barren, with only primitive plants beginning to colonize it. This extinction profoundly reshaped the marine world and set a new course for the evolution of life.
The World Before the Catastrophe
The Ordovician period was characterized by a warm, greenhouse climate with exceptionally high sea levels. These conditions submerged large portions of the continents, creating vast, shallow inland seas ideal for life to flourish. Most of the world’s landmass was collected into a single supercontinent, Gondwana, in the Southern Hemisphere. These tropical waters became the cradle for the Great Ordovician Biodiversification Event, which saw a tripling of marine diversity.
The Ordovician seas were populated by a host of organisms, including trilobites, brachiopods, and the first true coral reefs. Communities of tiered suspension feeders, like crinoids and bryozoans, created complex vertical ecosystems, while cephalopods established themselves as formidable predators.
The Two-Pulse Extinction Event
The Late Ordovician extinction was not a single event but a drawn-out crisis that occurred in two distinct pulses over half a million to two million years. These phases were separated by several hundred thousand years, each driven by different environmental pressures. The first pulse began at the start of the Hirnantian stage, primarily affecting organisms in warm, shallow seas. Planktonic and free-swimming creatures like graptolites, along with many types of trilobites and brachiopods adapted to tropical conditions, were hit particularly hard.
Following this phase, surviving fauna adapted to the new, colder global conditions. A specialized, cold-adapted group of brachiopods known as the Hirnantia fauna spread across the globe. This stability was short-lived, as a second pulse of extinction occurred later in the Hirnantian, wiping out many survivors of the first wave, including the widespread Hirnantia fauna. This second event targeted species that had adapted to the cooler environments, leaving behind an altered marine ecosystem.
Proposed Causes of the Extinction
The leading scientific theory links both pulses of the extinction to a brief ice age. This glaciation, centered on the supercontinent Gondwana as it drifted over the South Pole, triggered a cascade of environmental changes. The rapid shift from a greenhouse to an icehouse climate was a shock to a biosphere adapted to stable, warm conditions.
The first pulse of extinction corresponds with the onset of this glaciation. As massive ice sheets formed on Gondwana, large quantities of water were locked away as ice, causing global sea levels to fall by as much as 100 meters. This drained the vast, shallow epicontinental seas, destroying the habitats of many marine species and contracting the habitable zone for warm-water life.
The second pulse was triggered by the abrupt end of the ice age. As the climate warmed and glaciers melted, a swift rise in sea levels flooded the exposed continental shelves. This flooding is thought to have created widespread anoxia, or low-oxygen conditions, across the seafloor as ocean circulation patterns changed. The influx of toxic, sulfide-rich waters onto the shelves was fatal to many remaining species, particularly the cold-adapted fauna. While glaciation is the most widely accepted cause, some research has explored other possibilities, such as intense volcanic activity or a nearby gamma-ray burst.
Life’s Toll and Aftermath
The Late Ordovician extinction eliminated an estimated 85% of all marine species. Around one-third of all brachiopod and bryozoan families disappeared, along with numerous groups of trilobites, conodonts, and corals. The complex, tiered ecosystems of the Ordovician were dismantled, leaving behind a world with significantly lower biodiversity.
The survivors were often generalist species or those adapted to colder, deeper waters that were less affected by the initial sea-level drop. The aftermath was a period of ecological instability, but it also created an opportunity. With many ecological niches cleared, the stage was set for a new wave of diversification in the following Silurian period. The recovery was relatively rapid, with diversity returning to pre-extinction levels within about 5 million years.
This recovery saw the rise of new forms of life, as surviving groups of brachiopods and corals radiated rapidly. The Silurian seas also witnessed the diversification of the first jawed fish, our distant ancestors, and the first creatures to permanently colonize land appeared.