Mesophotic vs. shallow water reefs: ecosystem connectivity in the Silurian of Gotland

The Lower Silurian Visby Beds (ca. 430 Ma) exposed on the island of Gotland (Sweden) yield the oldest so far described MCE’s (mesophotic coral ecosystems). Several basic questions concerning these ecosystems, including their extent, positioning, and particularly their spatial relation and affinity with the shallow water reefs, remain unanswered. We describe a deeply incised erosional channel infilled by redeposited detrital and biodetrital material including corals and stromatoporoids, which represents a unique example of a fossil depositional structure connecting shallow- and deep-water reef environments of that time. The mesophotic environment is embodied by limestones and marls, in which the channel is incised, while the shallow water habitats are represented by the redeposited material infilling the channel. The erosional channel contains material derived from hypothetical adjacent shallow-water reefal settings, which indicates that it served as an interconnecting zone between shallow- and deep-water benthic communities. The vertical connectivity of the shallow-water reefs and the adjacent mesophotic ecosystems results in a significant taxonomic overlap between these communities. This is especially important when taking into account that the Visby Beds are the oldest example of a fossil mesophotic ecosystem described so far. This indicates that the development of the earliest mesophotic ecosystems should not be attributed solely to gradual expansion of corals and stromatoporoids into greater depths, but probably took place also by means of redistribution of shallow water taxa during high-energy sedimentary events.