Why were shallow-sea organisms especially vulnerable during the end-Ordovician extinction?

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Ordovician paleogeography — Ordovician Period The distribution of landmasses, mountainous regions, shallow seas, and deep ocean basins during the Ordovician Period. Included in the paleogeographic reconstruction are the locations of the interval's subduction zones.

Shallow-sea organisms were particularly vulnerable during the end-Ordovician extinction primarily due to the onset of glaciation over the southern portions of the Gondwana supercontinent.1

This glaciation caused a significant drop in global sea level, which drained the vast epicontinental seas that covered much of the continents during the Ordovician Period. Because many species—such as brachiopods, trilobites, and bryozoans—were endemic to these specific, shallow-water environments, the reduction of available habitat left them with nowhere to retreat. Consequently, these regionally distributed groups suffered high rates of extinction compared to more cosmopolitan, globally distributed forms.12