The Pattern of Extinction

Each of the two pulses of extinction affected different parts of the biota. The initial pulse of extinction at the Rawtheyan-Hirnantian boundary primarily affected deeper water ben-thic organisms. Those taxa that were hardest hit include the trilobites (45 to 75 percent generic loss), brachiopods (25 percent generic loss), cystoids, a group of primitive echino-derms with 70 percent generic loss, and grap-tolites. In addition to those benthic groups, the pelagic acritarchs and conodonts became rare in temperate regions.

Following the extinction of many of the previously dominant Ordovician biota, a residual fauna, the Hirnantia fauna, became well established in temperate regions. The Hir-nantia fauna is considered a cool-water fauna that ranged from circumpolar to subtropical latitudes. The Hirnantia fauna was composed of brachiopod-dominated communities of low diversity that had a global distribution. Some additional extinction of shallow-water bra-chiopod and coral species may have occurred during the interval between extinction pulses.

The Hirnantia fauna became extinct during the second pulse of extinction in the mid-Hirnantian stage. During this phase of extinction, shallow marine benthic organisms suffered heavy extinction, including 40 percent of brachiopod genera, 70 percent of the coral genera, and several trilobite and cystoid families, as well as many ostracode genera. Pelagic groups such as the conodonts, acritarchs, and chitinozoans also experienced mass extinction within the tropical latitudes at this time.

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