Missing molluscs as evidence of large-scale, early skeletal aragonite dissolution in a Silurian sea

In the Silurian carbonate platform succession of Gotland, Sweden, interreef limestone and marl sequences have shallow-marine shelly assemblages dominated by brachiopods, typical of many Paleozoic benthic faunas. Locally, from similar facies, where early skeletal silicification occurred associated with bentonites, there are rich and diverse assemblages of originally aragonitic molluscs. Quantitative faunal analysis suggests that massive depletion of skeletal aragonite, chiefly molluscs, took place, and the entire molluscan fauna may generally be underrepresented by at least 100-fold. Previous workers on the Gotland succession have already shown that abundant diagenetic limestones were sourced by early aragonitic remobilization. Our findings indicate molluscs as the probable source of this diagenetic calcite, not aragonite mud as was previously thought. The taphonomic window represented by the silicified faunas illustrates the importance of aragonitic shells among the original Silurian shelf benthos, and raises the possibility that the paucity of such shells among many fossil faunas may be largely an early taphonomic effect.