SPONGE-?HYDROID PALEOECOLOGIC RELATIONSHIPS IN SILURIAN MICROBIAL REEFS

Sphinctozoan (aphrosalpingid) sponges from Alaska and Russia inhabited high-energy microbial reefs that were built along the Uralian Seaway in the Late Silurian. Three species of sphinctozoans encrusted microbial laminae, cavity surfaces, and a variety of organic substrates, including the problematic hydroid Fistulella; possible stromatoporoids (recrystallized); crinoids; the possible cyanobacterium Ludlovia; corals; and unidentifiable shelly debris. This suggests that settlement of sponge larvae was largely accidental via random colonization of available hard substrates in the microbial-bound sediment. The sponges have well-defined attachment areas, such as enlarged "holdfasts," and exhibited a preference for erect growth off the sides of Fistulella. This suggests that some juveniles attached as epizoans to the surface of living, erect Fistulella. These taxa may have enjoyed a mutualistic relationship that afforded short-term feeding or anti-predatory advantages before the weight of the parasitic sponges caused their Fistulella hosts to topple over.

In addition to the ubiquitous microbial laminae, there were other biota, such as Ludlovia, Renalcis, and crinoids, that encrusted aphrosalpingids, Fistulella, or recrystallized stromatoporoids (?), creating an intricate stromatolite biofabric of intergrown micro- and macroorganisms. The intimate growth relationships shared by these and other biotas yields new insights into the fossil record of encrusting sponges, identifies a novel sponge-?hydroid association, and reveals the complex ecology that evolved in mid-Paleozoic microbial reefs.