Abstract

Reef-forming lithistid sponges in the Middle Jurassic (Bathonian) of Normandy, France, were lithified on the sea floor by a peloidal cement, probably of an original low-Mg calcite composition precipitated under bacterial influence. This early lithification permitted early dissolution of the siliceous skeleton without the collapse or distortion of the resulting moulds. These moulds were encrusted and bored. This pathway for the fossilization of sponges is shown by other examples of Jurassic lithistids and is likely to be widely applicable throughout the fossil record, particularly at times during which calcite was the primary marine precipitate.

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