Earliest symbiotic rugosans in cystoporate bryozoan Ceramopora intercellata Bassler, 1911 from Late Ordovician of Estonia (Baltica)

• Earliest symbiotic rugosans in bryozoans recorded in the Katian.
• Endobiotic rugosans benefitted from stable substrate.
• This symbiotic association was probably mutualistic.
• Symbiosis was common in Ordovician bryozoans in Baltica.

The earliest known endobiotic rugose corals are recorded in the Katian of Estonia. Multiple rugosans were partially embedded in colonies of the cystoporate bryozoan Ceramopora intercellata Bassler, 1911, leaving only their apertures free on the bryozoan growth surface. Bodophyllum sp. and Lambelasma sp. are rugosans that formed a symbiotic association with C. intercellata which may have been mutualistic. Rugosans presumably benefitted from growth within the stable substrate provided by the bryozoan, while bryozoans presumably benefitted by protection against some types of predators. Symbiosis between rugosans and the bryozoan Ceramopora intercellata was most likely facultative.