Silurian Chitons as Indicators of Rocky Shores and Lowstand on Gotland, Sweden

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Unusually abundant, diverse, and well-preserved assemblages of silicified chitons (Polyplacophora, Mollusca), dominated by Chelodes, from the Silurian of Gotland, Sweden, have a very limited stratigraphic and facies distribution. The Gotland succession (upper Llandovery-upper Ludlow) represents a segment of a low-latitude, low-angle carbonate ramp, with major reef systems, and is characterized by rich benthic faunas. Chitons are concentrated within two intervals in the late Wenlock (Gothograptus nassa-Pristiograptus ludensis biozones) and late Ludlow (Saetograptus leintwardinensis-Bohemograptus bohemicus tenuis-Neocucullograptus kozlowskii biozones), which correspond to periods of lowstand recognized as sequence stratigraphic boundaries. At both horizons, detailed carbonate lithofacies relationships indicate inshore, very shallow shelfzones with reduced sedimentation, and at the higher level there is karstification. Chitons are not preserved in situ but are derived very locally. Among high-energy calcarenites, assemblages are poorly preserved and of low diversity; the best preservation, of a high-diversity assemblage including small and delicate sclerites, occurs among pockets of soft-bottom, possibly lagoonal sediments associated with tidal, reef-shoal environments. Recent chitons are most common and diverse in high littoral to intertidal zones of rocky shores. Such environments have low preservation potential, and chitons have a sparse fossil record. The Gotland chitons are paleoloricates, but evidence from cyclic growth variations on sclerites suggests similar life histories to Recent, neoloricate chitons. Reconstructions based on sclerite morphology and degree of overlap illustrate the diverse assemblage; some are notably elongate by comparison with Recent chitons. The highly localized distribution of chitons on Gotland appears to reinforce broader carbonate lithofacies and biofacies evidence ofproximity to the shoreline, or to emergent tidal flats, of the Baltoscandian Basin.