Late Ordovician mass extinction

A mass extinction around 444 million years ago that sharply reduced marine biodiversity during the transition from the Ordovician to the Silurian.

Hirnantian glaciation: A major glaciation during the Hirnantian Age, near the end of the Ordovician Period. Its growth lowered sea levels and cooled oceans during the extinction’s first phase.
Brachiopoda: A phylum of marine invertebrates with two shells, common in Paleozoic seafloors. Many brachiopod lineages disappeared as shallow marine communities collapsed.
Ordovician Period: A geological period spanning roughly 485 to 444 million years ago, marked by diverse marine life. The extinction ended this period and its exceptionally rich marine ecosystems.
Silurian Period: A geological period following the Ordovician, spanning roughly 444 to 419 million years ago. Its ecosystems developed after the extinction’s losses and environmental disruption.
Ordovician–Silurian boundary: The geological boundary separating the Ordovician and Silurian periods, about 444 million years ago. The extinction’s two pulses cluster around this boundary.
Trilobite: An extinct group of marine arthropods whose segmented exoskeletons are abundant in Paleozoic fossils. Trilobites suffered major losses, with extinction patterns varying among groups.
Hirnantian: The final age of the Ordovician Period, lasting roughly from 445 to 444 million years ago. The extinction’s major pulses occurred during this brief interval.
Biodiversity: The variety of living organisms across genes, species, and ecosystems. The extinction caused a major decline in marine biodiversity.
Anoxia: A condition in which water or sediment contains little or no dissolved oxygen. Oxygen-poor marine waters contributed to losses during the extinction’s later phase.
Graptolite: An extinct group of colonial marine animals known chiefly from distinctive fossilized structures. Graptolite fossils document pronounced turnover in open-ocean communities.

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