Coelibites and spatial refuges in a Lower Cretaceous cobble-dwelling hardground-fauna

A diverse hardground fauna of encrusters, nestlers and borers has been found on calcareous cobbles in the Lower Cretaceous (Upper Aptian) Faringdon Sponge Gravel of south-central England. The bulk of the fauna consists of coelobites that inhabited the vacated borings of bivalves. These coelobites often clustered near the cavity openings, apparently to escape epifaunal predators and physical abrasion, and to avoid sedimentary infilling of the cavity interiors. The few encrusting species common on the outside surfaces are robust and apparently adapted to abrasion resistance. Some species, notably the serpulid Glomerula gordialis, had a growth strategy that exploited the advantages of cavity-dwelling as juveniles and the resources available on the outside cobble surfaces as adults. The fauna thus shows adaptations to life in cavities and to existence on a mobile hardground in a high energy environment. The increase in hardground boring in the Upper Palaeozoic and Mesozoic may have caused a general increase in hardground faunal diversity by providing more niche space for coelobites.