Diverse early endobiotic coral symbiont assemblage from the Katian (Late Ordovician) of Baltica

A diverse early endobiotic coral symbiont assemblage has been detected within heliolitid tabulate corals of Katian age from northwestern Estonia. This assemblage indicates that the earliest endobiotic coral symbiont communities were not restricted to North America. The symbiotic endobiont assemblage comprises abundant Cornulites aff. celatus and rare Conchicolites hosholmensis and Chaetosalpinx sp. The cornulitids are the earliest known symbiotic endobionts of this group. The symbiotic endobionts presumably occurred in certain hosts only and preferred Protoheliolites dubius over Propora speciosa. This record indicates that early endobiotic cornulitids were more diverse than previously thought and appeared in large numbers in the Katian. It is possible that the appearance of abundant skeletal symbiotic coral endobionts may coincide with the Global Ordovician Biodiversification Event, supporting the hypothesis of escape from increased predation.

► Heliolitids were hosts to various endobiotic symbionts already in the Katian.
► Early endobionts assemblages were more diverse than previously known.
► Early endobionts had wider geographic distribution than previously known.
► There were presumably differences in host coral tolerance in the Katian.
► The earliest endobiotic cornulitids had appeared already by the Katian.