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Why were similar shelly invertebrates found on different Silurian continents?

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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.

The presence of similar shelly invertebrate species across different Silurian continents is primarily attributed to the unique paleogeographic conditions of the period, which facilitated widespread faunal distribution.1

During the Silurian, global sea levels were significantly higher than they are today, leading to the flooding of extensive continental regions by shallow seas. This resulted in muted seafloor topography across large areas of these flooded continental platforms. As a result, the strong faunal endemism that characterized the preceding Ordovician Period was replaced by a more uniform environment where many species were distributed globally.12

Because these shallow, interconnected marine environments were remarkably consistent across different regions, the faunas of shelly invertebrates—such as brachiopods—became widely distributed. This consistency allows geologists and stratigraphers to correlate layers of Silurian rock found on different present-day continents.13