The earliest known rugosans attached syn vivo to vertical stems occur in the late Silurian of Saaremaa, Estonia. These rugosans display vertical to subvertical attachment scars and are more common in the Ludfordian than in the Pridoli. The unknown hosts provided a higher tier for the feeding, making the association beneficial for the rugosans. Several rugosans were themselves syn vivo encrusted by bryozoans and unknown endobiotic tubicolous organisms, possibly cornulitids. Estonian rugosans appear to have been host size selective and preferred substrates of certain size. Silurian symbiotic rugosans are more often endobionts in stromatoporoids than epibionts on the vertical stems.