Early cryptic microconchids and other invertebrates in empty nautiloid shells from the Silurian of Saaremaa Island, Estonia

The empty nautiloid shells from the Silurian of Estonia contain a cryptic fauna. The cryptic sclerobionts include microconchids, cornulitids, and trepostome bryozoans. Empty nautiloid shells could also have offered shelter to trilobites. The relatively deep-water cryptic fauna of the Jaani Formation differs from other Silurian cryptic communities by the numerically dominant tentaculitoid tubeworms. The cryptic fauna inhabiting the vacant body chambers of nautiloids within the Jaani Formation exhibited a low taxonomic diversity. One plausible explanation for this atypical community structure is the nature of the depositional environment, which is interpreted to represent relatively deeper marine conditions – likely corresponding to the lower part of the photic zone. The sparse encrustation observed in deep-water cryptic habitats during the Sheinwoodian of Estonia was likely driven by limited substrate exposure time, food limitation, and reduced energy availability, all of which hindered the establishment and growth of a more diverse and robust cryptic community.