A 514-MILLION-YEAR RECORD OF SKELETAL CONTENT IN MARINE LIMESTONES

Compilations of the stratigraphic ranges of marine families and genera reveal long-term increases in diversity across the Phanerozoic punctuated by mass extinction events. Secular changes in the abundance of skeleton-forming organisms in the marine realm are also biologically important, with implications for understanding patterns in Phanerozoic marine diversity, primary productivity, resource utilisation, faunal abundance, and overall energetics of marine ecosystems, but are challenging to quantify. Indirect measures, such as increases in animal body size, depth of bioturbation, predation intensity, and the thicknesses of shell beds are suggestive of increasing abundance, but more direct measures have yet to be compiled at Phanerozoic scale. In this study, we use compositional data from marine limestones to develop a more direct record of Phanerozoic variation in the abundance of skeletal forming organisms. The dataset comprises point counts of new samples collected by the research team as well as data compiled from the published literature. It contains data from 2231 carbonate thin sections, totaling ~667,000 counted points. The dataset spans ~514 million years, from Cambrian Series 2 to the Quaternary . In the dataset, the abundance of skeletal grains in marine limestones has increased across the Phanerozoic. A two-way ANOVA indicates that sample age (p<0.01, η²=0.30) explains more of the variation in skeletal grain content than does the depositional architecture (carbonate platforms versus ramps) from which the sample derives (p<0.01, η²=0.005). We interpret this overall increase in the skeletal content of marine limestone to result from an increase in skeletal grain production, most likely reflecting increased resource availability and utilisation in the benthic marine realm across the Phanerozoic.