The oldest "brown mesophotic" coral-stromatoporoid ecosystem from the Silurian of Gotland was functionally similar to modern turbid reefs.

Coral reefs generally thrive under high light conditions. As light decreases with depth, corals may adapt their morphology to optimise light capture. However, these same changes in morphology (e.g., platy forms) may also occur in response to light attenuation caused by turbidity within shallow waters. In the fossil record the occurrence of turbid shallow-water ecosystems has been largely restricted to the Meso- and Cenozoic. Only a single example of an ecosystem functionally analogous to modern turbid reefs has been identified from the Palaeozoic (Devonian). Here we report a Silurian (~ 425 Ma) reef ecosystem from Gotland, Sweden, composed predominantly of platy tabulate corals. Sedimentological data suggest shallow depths, high sedimentation rates and an unconsolidated substrate. We interpret the Gotland reef to be functionally similar to modern turbid reefs, making it the oldest known turbid reef system, extending records by nearly 40 million years. This suggests that platy growth forms and colonies similar to modern funnel-shaped forms first emerged during the Silurian as a novel strategy that facilitated coral expansion in turbid, low-light environments. The recurrence of these forms across geological time suggests strong functional convergence driven by similar environmental pressures, evolving at least twice in both tabulate and scleractinian corals.