POST-PALEOZOIC INCREASE IN THE ABUNDANCE OF SKELETAL BENTHIC MARINE ORGANISMS

Increases in animal abundance and biomass have long been hypothesized to explain the increase in taxonomic diversity of marine communities during the post-Paleozoic (252 - 0 Mya). Compared to diversity, quantifying change in biomass is difficult, leaving this hypothesis poorly tested. For this study, the abundance of skeletal fossils (animals, algae, and foraminifera) in marine limestones was quantified (as a fraction of rock volume) from the Lower Triassic through the Neogene (252 - 0 Mya) in order to test the hypothesis that the overall abundance of animals increased across this time period. Skeletal abundance data were compiled from thirteen previously published papers reporting point count data for 1596 rock samples, reflecting the classification of approximately 400,000 points. These data on the volumetric contributions of skeletal grains to marine limestones were recorded along with the geologic age and depositional environment. Taxonomic identifications were standardized at the phylum and class levels. Overall, the skeletal abundance of animals increased significantly across the post-Paleozoic, with the exception of a short-term decline following the end-Triassic mass extinction (201 Mya). A similar trend occurs in data for algae and foraminifera as well as across all of the skeletal groups in aggregate. Looking at the aggregate data separated by depositional system types, carbonate platforms showed a significant increase as well as two of the three sub-environments of platforms. Carbonate ramps did not exhibit any significant trends, but data were sparser for these systems. Across the six sub-environments for ramps and platforms, only one had any negative trend, and this was not statistically significant. The fact that most of the environments studied show an increasing trend, in some cases significant, indicates that the overall trend in the data is better interpreted to result from increasing abundance of skeletal organisms rather than a shift toward the sampling of more skeleton-rich environments. These findings are consistent with the hypothesis that the abundance of marine biota has increased since the start of the Mesozoic, suggesting that increased trophic resources were at least one factor enabling the increases in taxonomic diversity and ecosystem complexity.