Encrustation of crinoid holdfasts and pluricolumnals from the Pridoli (upper Silurian) of Saaremaa Island, Estonia

Abstract

The Pridoli (upper Silurian) of Saaremaa Island, Estonia contains crinoid holdfasts and pluricolumnals that are colonized by many epibionts (encrusters). They comprise Palaeoconchus tenuis, Cornulites sp., Anticalyptraea calyptrata, Favosites sp., as well as hederelloids, cystoporate and trepostome bryozoans, and stromatoporoids. The taxonomic composition of encrusters of crinoid pluricolumnals differs from that of crinoid holdfasts. The encrusters on pluricolumnals were numerically dominated by trepostome bryozoans and microconchids; the other taxa formed just a minor part of the sclerobiont association. The trepostomes dominate by the area of encrustation on pluricolumnals. The encrusters on holdfasts were numerically dominated by microconchids, followed by cystoporate and trepostome bryozoans; the other taxa formed a minor part of the sclerobiont association. The bryozoans dominated by the encrustation area on holdfasts. There is likely a negative correlation between potential substrate mobility and abundance of microconchids. The cystoporates colonized only relatively stable substrates such as crinoid holdfasts whilst trepostomes colonized also mobile substrates.