Rare symbiotic rugose corals in tabulates occur in the lower Rhuddanian strata of Estonia. Thirteen out of 1115 coralla of early Rhuddanian tabulate corals of Estonia and two out of 353 coralla of Sheinwoodian tabulate corals of Gotland and Estonia contain rugose coral endobionts. Endobiotic rugose corals occur in Paleofavosites balticus (Linnaeus 1767), Paleofavosites sp., Mcleodea sp., Heliolites interstinctus (Linnaeus 1767), and Catenipora gotlandica (Yabe 1915). Endobiotic rugose corals include Tryplasma sp., Helicelasma sp., Streptelasma estonica Dybowski 1873, Streptelasma sp., Kodonophyllum cf. tubaeformis Kaljo 1957, and Palaeophyllum? sp. The latter rugose corals commonly occur independently from tabulates in the early Silurian of Estonia. It is most likely that symbiosis between rugose corals and tabulates was not obligate, but facultative. Symbiotic rugose corals presumably benefitted from growth within a stable (positionally) substrate. Rugose corals seem to have no strong negative effect on the growth of tabulates.

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