The Silurian of Saaremaa Island, Estonia: a window into ancient symbiosis and Palaeozoic ecosystems

The Silurian rocks of Saaremaa Island, Estonia, provide an exceptional record of ancient marine life, not just as individual fossils but as evidence of how organisms lived together. Around 430–420 million years ago, this region was a tropical sea where corals and stromatoporoid sponges formed thriving reef ecosystems. What makes Saaremaa unique is the frequent preservation of endobiotic relationships—organisms living inside the skeletons of others. Worms inhabited tabulate corals, cornulitid tubeworms embedded themselves in stromatoporoids and solitary rugose corals grew within coral colonies and sponges, using them as stable shelters and feeding platforms. These associations show that Silurian reefs and shelves were crowded, competitive, and highly interconnected environments where organisms coexisted through tolerance rather than destruction. Now exposed in coastal cliffs, these fossils offer a rare and detailed glimpse into early symbiosis and community structure in Palaeozoic seas.