The Late Ordovician Mass Extinction, often referred to as the Ordovician-Silurian extinction, represents the second-largest known extinction event in Earth’s history. Occurring approximately 445 million years ago, this global event significantly reshaped life on our planet, leading to a dramatic reduction in biodiversity and influencing the evolution of marine ecosystems.
The Ordovician World and the Extinction’s Timeline
During the Ordovician Period, which spanned from roughly 488.3 to 443.7 million years ago, Earth’s geography was different from today. Much of the world’s landmass was consolidated into the supercontinent Gondwana, which included parts of modern-day South America, Africa, Australia, Antarctica, and Southern Europe. Gondwana gradually drifted towards the South Pole throughout the period, eventually settling there.
Vast, shallow seas covered much of the continents, supporting a thriving marine ecosystem. The extinction event unfolded over about 1.4 million years, primarily within the Hirnantian stage of the Late Ordovician epoch.
The extinction occurred in two distinct pulses. The first pulse began at the boundary between the Katian and Hirnantian stages, coinciding with the abrupt expansion of a major glaciation over Gondwana. A second pulse followed later in the Hirnantian, as the glaciers receded and warmer conditions returned.
Leading Theories on the Causes
The primary scientific explanation for the Late Ordovician Mass Extinction centers on a global cooling event and its subsequent effects. As the supercontinent Gondwana moved over the South Pole, massive glaciers formed, leading to a substantial drop in global sea levels, potentially by as much as 100 meters. This sea-level fall drained vast epicontinental seaways, eliminating numerous ecological niches.
The expanding glaciation also caused a global cooling of ocean surface waters, detrimental to marine life adapted to warmer “greenhouse” conditions. This rapid shift from a greenhouse to an “icehouse” climate contracted the latitudinal range available for warm-adapted organisms. The initial pulse of extinction is linked to this cooling and sea-level decline, as well as changes in ocean circulation that brought nutrients from deeper waters to the surface.
The second extinction pulse is associated with the end of the glaciation and the subsequent warming and rising sea levels. This warming led to widespread anoxia, or a lack of oxygen, in the oceans. This oxygen depletion resulted from changes in ocean circulation patterns or nutrient cycling, which created stagnant conditions in the water column.
Some recent research, however, proposes an alternative or complementary view, suggesting that volcanism may have played a role. Volcanic eruptions could have released greenhouse gases, leading to warming and anoxia, which might have driven the first extinction pulse rather than cooling and glaciation.
Impact on Ancient Life
The Late Ordovician Mass Extinction significantly impacted marine life, which constituted nearly all complex multicellular organisms at the time. The event eliminated a large portion of marine biodiversity, with 49–60% of marine genera and approximately 85% of marine species perishing.
Marine invertebrates that dominated the Ordovician seas were particularly affected. These included groups such as trilobites, brachiopods, graptolites, conodonts, nautiloids, and crinoids. About one-third of all brachiopod and bryozoan families disappeared, along with many groups of trilobites, conodonts, and graptolites.
The extinction primarily impacted marine invertebrates, particularly those in shallow-water habitats. For example, widespread families of trilobites vanished, and graptolites faced near-total extinction.
Unraveling the Past: Scientific Evidence
Scientists reconstruct the Late Ordovician Mass Extinction by examining various lines of evidence in the geological record. The fossil record provides insights into changes in species diversity and abundance across different geological layers. Paleontologists identify extinction events and their severity by observing the disappearance of species and reduction in genera within rock strata.
Stratigraphic analysis, the study of rock layers, helps to understand past environmental changes. Geologists examine sedimentary rocks for indications of sea-level fluctuations and the presence of glacial deposits, which offer direct evidence of ancient ice sheets.
Geochemical proxies, which involve analyzing the chemical composition of rocks, offer further clues. Isotopic signatures in marine sediments, particularly carbon and oxygen isotopes, are indicators. Changes in carbon isotopes reflect disruptions in the global carbon cycle, while oxygen isotopes provide information about past ocean temperatures and glacial ice volume. Thallium and sulfur isotope ratios, for example, indicate past ocean oxygen levels.