The Late Ordovician mass extinction, which occurred around 445 million years ago, wiped out 85% of marine species. The extinction event abruptly affected all major taxonomic groups and caused the disappearance of one third of all brachiopod and bryozoan families, as well as numerous groups of conodonts, trilobites, echinoderms, corals, bivalves, and graptolites. The extinction was confined to life in the seas since animals had not yet conquered land at this time. The extinction is traditionally considered to occur in two distinct pulses. The first pulse began at the boundary between the Katian and Hirnantian stages of the Late Ordovician epoch. This extinction pulse is typically attributed to the Late Ordovician glaciation, which abruptly expanded over Gondwana at the beginning of the Hirnantian and shifted the Earth from a greenhouse to icehouse climate. Two environmental changes associated with the glaciation were responsible for much of the Late Ordovician extinction. First, the cooling global climate was probably especially detrimental because the biota were adapted to an intense greenhouse, especially because most shallow sea habitats in the Ordovician were located in the tropics. The southward shift of the polar front severely contracted the available latitudinal range of warm-adapted organisms. Second, the glaciation caused a drop in sea level, which led to a reduction in the area of continental shelves and a decrease in the number of shallow-water habitats.

Recent research suggests that the extinction was associated with a cooling climate or changes in oxygenation of the ocean. US-led researchers have looked at the ocean environment before, during, and after the extinction event to find out what was brewing. In a study published in Nature Geoscience, the team took measurements of the iodine concentration of carbonate rocks from the Ordovician period, which served as an indicator for oxygen concentrations at various ocean depths. They then incorporated this geochemical data into numerical simulations and computer modeling to examine how oxygen levels changed in the ocean at the time. The study found that there’s no evidence that oxygen decreased during that time in the shallow marine.

In summary, the Late Ordovician mass extinction was likely caused by a combination of factors, including a cooling global climate, changes in oxygenation of the ocean, and a drop in sea level.