Tubeworms form an important part of the modern marine fauna. They were also common in the geological past. We discovered a new genus and species of tubeworms from the latest Ordovician of Estonia. These tubeworms grew on the lithified sea floor during the time of the end-Ordovician mass extinction. Our discovery helps better understand and reconstruct the marine life during this extraordinary time interval.

A new cornulitid genus and species, Porkuniconchus fragilis new genus and species, is here described from the Ärina Formation (Hirnantian, Porkuni Regional Stage) of northern Estonia. This new taxon differs from most cornulitids by having a fusiform ornamentation pattern that is somewhat similar to that of Kolihaia. All studied specimens are attached to a carbonate hardground. The hardground fauna is by abundance and encrustation area dominated by cornulitids. Other encrusters are represented only by a single sheet-like cystoporate bryozoan. The cornulitid specimens represent different growth stages, which suggest that the hardground was continuously colonized by cornulitid larvae. The high encrustation density indicates that the studied hardground may have represented a high-productivity site in the Hirnantian of the Baltic Basin.