WHERE HAVE ALL THE ENCRUSTERS GONE? ENCRUSTING ORGANISMS ON DEVONIAN VERSUS MISSISSIPPIAN BRACHIOPODS

Brachiopods are common substrates for Paleozoic encrusters. However, across the Frasnian-Famennian boundary, brachiopod assemblages were greatly affected, including the loss of atrypide and stropheodont brachiopods. Common encrusting organisms (bryozoans, corals, Spirorbis, and brachiopods) in Devonian communities of the Midwestern U.S. occur mainly on stropheodonts, atrypides, and spiriferides. Mississippian communities retain spiriferide brachiopods, and common encrusters (bryozoans, cornulitids, and brachiopods) occur mainly on Composita and spiriferides. Devonian brachiopod assemblages (Cedar Valley Formation, Iowa, and Silica Shale, Ohio, Givetian; Lime Creek Shale, Iowa, Frasnian) and Mississippian assemblages (Edwardsville Formation, Indiana, Osagean; Beech Creek Limestone, Illinois, Chesterian) were compared for encruster preferences across the Frasnian-Famennian extinction. Although every sample is distinct in presence, abundance, and distribution of both brachiopod substrates and encruster taxa, notable differences in pre- and post-extinction brachiopod-encruster relationships occur. With the loss of atrypides and stropheodonts across the F-F boundary, abundance of costate and plicate brachiopods in the studied communities decrease, while smooth-shelled and spinose brachiopods increase. Encrusters shifted from high rates of encrustation on Devonian costate brachiopods to Mississippian smooth or plicate brachiopods.

In the samples studied across the Frasnian-Famennian crisis, host abundance has the greatest affect on encrustation rates of smooth and costate brachiopods, but patterns in plicate and spinose brachiopods do not follow host abundance patterns. Although other environmental and biotic factors likely influenced each community, and other substrates were available for encrusting organisms across the Frasnian-Fammenian boundary, with the loss of atrypides, encrusters faced a reorganization of brachiopod assemblages and a subsequent change in available substrate types in late Devonian-early Mississippian benthic communities.