What major sea-level event occurred at the Mississippian-Pennsylvanian boundary?

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

At the boundary between the Mississippian and Pennsylvanian subperiods, a major drop in sea level occurred, resulting in a worldwide lowstand. This significant regression of the seas from continental interiors created an unconformity—an interruption in the sedimentary rock record—and was associated with a global extinction event that particularly affected crinoids and ammonoid cephalopods.12

This sea-level event was driven by the expansion and contraction of widespread continental ice sheets in Gondwana, which began to influence global sea levels during the later part of the Carboniferous Period.2