Few Konservat-Lagerstätten are known from the Ordovician, and most preserve atypical marginal marine communities. Thus, we have little idea of how animals with a low preservation potential evolved during the Great Ordovician Biodiversification Event. Here we report the newly discovered Llanfawr Mudstones Lagerstätte from the basal Sandbian (Late Ordovician) of central Wales, UK. This biota, which has been studied through X-ray radiography and microtomography, is dominated by sponges. It also includes cnidarians (the oldest known solitary hydroid), arthropods, priapulids, various worm-like forms, entoproct-like organisms, and a variety of enigmatic fossils. The fauna includes taxa that are rarely preserved even in exceptional fossil biotas, and offers the potential for a new perspective on Ordovician ecosystems. The dominantly filter-feeding assemblage resembles modern abyssal sponge-dominated communities, although it was formed in much shallower water. The unusual Llanfawr Mudstones fauna shows that Ordovician ecological development was considerably more advanced in offshore environments than the mineralized fossil record implies.