A paleoecological study of stromatoporoid endobionts was carried out to discern the relationships between symbiotic rugosans and their stromatoporoid hosts. The earliest endobiotic rugosan symbiont Palaeophyllum sp. in Baltica has only been found in the stromatoporoid Ecclimadictyon astrolaxum from Saaremaa, Estonia. The rugosans are vertically oriented inside the stromatoporoid skeleton. Numerous rugosans have their corallites open at the upper, external surface of stromatoporoids, but many are completely embedded within the stromatoporoids. Stromatoporoid hosts were presumably beneficial for rugosans as elevated substrates on a sea floor that offered a higher tier for feeding. Relative substrate stability in the hydrodynamically active shallow waters may have also been beneficial for the rugosans.