SEASHELLS THROUGH TIME: A PHANEROZOIC RECORD OF THE ABUNDANCE OF SKELETAL ORGANISMS

The first shelled macroinvertebrates (Cloudinia and Namacalathus) evolved during the late Ediacaran. These organisms were small and lived in sheltered, shallow-marine habitats. Fast forward to modern times, the shell forming organisms have invaded almost all oceanic habitats and are found in a wide variety of body sizes. The exact patterns behind how small and rare shelled invertebrates in the Ediacaran became so ubiquitous in modern marine ecosystems is still a largely unanswered question in paleobiology. To develop a direct, quantitative record of skeletal abundance across geologic time, and thereby assess the potential controls on the abundance of skeletal organisms, we compiled compositional data from marine carbonate sediments deposited across the Phanerozoic. Compositional data from marine carbonates are in the form of point counts from petrographic thin sections of carbonate rocks. The point count data used in this study was compiled from published literature as well as new samples counted for the purpose of this study. We focused on carbonate-producing skeletal organisms because they form most of the fossil abundance and diversity of marine animals, algae, and foraminifera. We chose carbonate sediments because the precipitation of carbonate sediments in the marine realm has been a continuous phenomenon since the late Archean. The database consists of 7683 thin sections with each thin section having counts ranging from 200-500 points. This amounts to more than 1.4 million counted points for the entire dataset. We found a secular increase in the abundance of skeletonized taxa from the Cambrian to present. The abundance patterns capture key biotic transitions in the marine realm, including an increase in abundance across the Great Ordovician Biodiversification Event and decreases across the Late Devonian, end-Permian and end-Triassic mass extinctions. Abundance trends are consistent across platforms and ramps, shallower and deeper environments, and lower versus higher latitudes. Overall patterns closely resemble the taxonomic diversity of marine invertebrates across time, indicating a close coupling between abundance and diversity over geological time scales.