Abstract

Evolutionary changes in the ecology and diversity of organisms that produce and destroy calcareous skeletons suggest that bioclastic concentrations themselves might have changed in nature through the Phanerozoic. Empirical data from marine siliciclastic records of Ordovician-Silurian, Jurassic, and Neogene ages indicate a significant increase in the thickness of densely packed bioclastic concentrations over geologic time, from a primarily thin-bedded brachiopod-dominated record in the Ordovician-Silurian to a mollusk-dominated record with many more and thicker shell beds in the Neogene. Jurassic shell beds vary in thickness with the Paleozoic or modern affinities of the chief constituents, suggesting, along with other evidence, that the Phanerozoic increase was determined neither by diagenesis nor by a shift in taphonomic conditions on the sea floor but rather by the evolution of bioclast producers, namely, groups with (1) more durable low-organic skeletons, (2) greater ecological success in high-energy habitats, and (3) on the basis of indirect evidence, higher rates of carbonate production. These results suggest that (1) reproductive and metabolic output has increased in benthic communities over time and (2) the scale of time averaging in benthic assemblages has increased owing to greater hard-part durability of modern groups.

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