A mass extinction is a rapid, widespread decrease in Earth’s biodiversity, resulting in the loss of a large percentage of species within a short period. This crisis is distinct from background extinction rates, representing a catastrophic collapse of ecosystems. The first such global catastrophe was the End-Ordovician Extinction Event, which unfolded approximately 443 million years ago. It is recognized as the first of the “Big Five” mass extinctions that have punctuated the history of life on Earth. This crisis was caused by the complex interplay of climate shifts and ocean chemistry, fundamentally reshaping life in the early Paleozoic Era.
Defining the End-Ordovician Event
The End-Ordovician Extinction, also known as the Late Ordovician Mass Extinction (LOME), occurred during the final stage of the Ordovician Period, called the Hirnantian. This event ranks as the second-largest in Earth’s history based on the percentage of marine species lost. The extinction eliminated an estimated 85% of all marine species and about half of all marine genera globally.
Since life was almost exclusively confined to the oceans at this time, the extinction primarily struck marine invertebrates. Organisms like trilobites, a dominant group of arthropods, suffered massive losses, with many families vanishing entirely. Brachiopods (shelled organisms resembling clams) and graptolites (colonial organisms) also saw significant reductions in diversity.
These groups were vulnerable because they inhabited the vast, warm, shallow continental shelf seas that covered the continents. The destruction of these epicontinental seaways, the primary ecological niche for the Ordovician marine biota, was a major factor in the crisis’s severity.
The extinction unfolded in two distinct pulses, separated by a brief period of recovery.
The Initial Trigger: Global Glaciation
The hypothesis for the first, or cold-phase, extinction pulse points to a dramatic shift in global climate driven by continental drift. As the supercontinent Gondwana drifted across the South Pole during the Late Ordovician, it initiated a massive, short-lived glaciation. This transformed the planet from a greenhouse state to an icehouse state.
The formation of extensive continental ice sheets over Gondwana drew down immense amounts of water from the ocean. This led to a significant eustatic (worldwide) sea level drop, draining the shallow seas where most marine life thrived. The loss of these stable, warm habitats was devastating, causing the first wave of extinctions among warm-adapted species.
The glaciation also required a substantial drawdown of atmospheric carbon dioxide, which reduced the greenhouse effect and caused abrupt global cooling. This rapid temperature change severely impacted species adapted to the previously warm, tropical conditions. The combined shock of plummeting temperatures and habitat loss defined this first extinction pulse, which occurred at the start of the Hirnantian stage.
The Second Pulse: Warming and Anoxia
The glacial period was relatively brief, and the planet began to warm rapidly as the ice sheets melted. This deglaciation triggered the second, and often more devastating, extinction pulse near the end of the Hirnantian. The rapid melting caused a sudden rise in global sea levels, which created new environmental stresses.
The influx of cold meltwater and subsequent changes in ocean circulation led to the widespread development of oceanic anoxia. Anoxia, a severe lack of dissolved oxygen, is lethal to most complex marine life. In many areas, conditions progressed to euxinia, where the water became rich in toxic hydrogen sulfide gas, creating a poisonous environment.
This deoxygenated and toxic water mass, proven by geochemical proxies and the presence of black shales, expanded from the deep ocean onto the continental shelves. The second pulse primarily affected taxa that had survived the initial cold phase or adapted to the cooler glacial climate. This rapid shift from a cold, low sea-level crisis to a warm, anoxic, high sea-level crisis delivered a two-part shock that caused the End-Ordovician Mass Extinction.