Microconchid-dominated hardground association from the late Přidoli (Silurian) of Saaremaa, Estonia

A hardground fauna of moderate diversity from normal marine (shoal to open shelf) of the late Pridoli of Saaremaa (Estonia) contains: microconchids (Palaeoconchus cf. tenuis, and P. sp.), Anticalyptraea calyptrata, sheet-like trepostome bryozoans, discoid crinoid holdfasts, encrusting graptolites, Aulopora sp., Cornulites sp. and Conchicolites sp. Microconchids are most abundant in the association and form the second most prominent group by encrustation area in the association after trepostome bryozoans. There is a taxonomic polarity between sclerobionts on the upper surface of the hardground and the cryptic fauna beneath. This hardground community shows that the high abundance of microconchids characteristic of Devonian hard substrate communities had been achieved by at least the Pridoli. Microconchids preferred hardground upper surfaces and were able to symbiotically grow within bryozoans when overgrown.