Tabulate corals and stromatoporoids.
Some of the largest know Hirnantian reefs are developed along Mirny Creek (NE Russia), part of the Omulev uplft: these are coral-stromatoporoid bioherms up to 20m thick, which end somewhere below the O/S boundary (Oradovskaya & Sobolevskaya, 1983): there are no reefs in the Llandovery strata immediately above.
Older reef growth with mud mound in underlying Padun Fm.