Wonderfully symmetrical and large stromatoporoid (a kind of fossil sponge) preserved in the Undva Cliff, northwestern Saaremaa coast. The surrounding blue-gray sediment is a calcarous clay with other smaller fossils such as brachiopods and bryozoans. The stromatoporoid itself is a community with endosymbiotic corals and worms. (Mustjala Member, Jaani Formation, Wenlock).