How many major sea-level highstands occurred during Llandovery time?

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

During the Llandovery Epoch of the Silurian Period, there were at least four major global sea-level highstands, which are intervals where the sea level rises above the continental shelf edge.1

These highstands occurred at the following times during the Llandovery:

First highstand: Occurred during the transition between the Rhuddanian and Aeronian stages, likely corresponding to the melting of Late Ordovician glaciers.1
Second highstand: Mid-Aeronian in age.1
Third highstand: Early Telychian.1
Fourth highstand: Late Telychian.1

These fluctuations were reconstructed by analyzing biological community replacement patterns across stratigraphic sequences on multiple paleocontinents, with each cycle lasting approximately 2.5 million years.1