Stromatoporoid-coral/tubeworm intergrowths in the lowermost Silurian Varbola Formation of Estonia: first evidence of competitive interaction

Tabulate corals, particularly auloporids and syringoporids, are commonly found as intergrowths in host stromatoporoids in Palaeozoic shallow marine limestones. These associations are, in almost all cases, interpreted to be favourable to the intergrown corals but neither favourable nor unfavourable to the host. Here, in order to further investigate their palaeoecological interactions, the growth behaviour of host stromatoporoid and intergrown corals is evaluated in the association of the stromatoporoid Clathrodictyon boreale Nestor 1964 and the tabulate Auloporoidea indet. from the lowermost Rhuddanian Varbola Formation (Juuru Regional Stage) at Reinu Quarry, central Estonia. Also, intergrown tubeworms (Cornulites and Helicosalpinx) occur in the stromatoporoid and are compared with the coral to address variation of growth interactions. Two types of syn-vivo interactions between the organisms are recognized near their physical contacts: (1) upward- or downward-bending laminae of the host stromatoporoid with high skeletal density; and (2) irregular shape of the outer wall of auloporid corals with uneven wall thickness. Auloporid corals and tubeworms probably benefitted from the stable growth substrate provided by C. boreale, but spatial competition with the host stromatoporoid, local to the tubes, occurred during their intergrowth, judging from their skeletal deformations. Distorted skeletal structures in the intergrown organisms and host stromatoporoids are not persistently found in Palaeozoic intergrowth examples, indicating variation of growth interactions between each intergrown coral/tubeworm and host stromatoporoid species. This finding implies that stromatoporoid–coral/tubeworm associations were more complex than the previous interpretations and emphasizes that evaluation of skeletal deformations is critical in understanding the nature of palaeoecological interactions.