TAPHONOMY, PHYTOPLANKTON EVOLUTION, AND THE FOSSIL RECORD OF MARINE BIODIVERSITY

The rise of the Modern marine fauna is well-documented. Less appreciated is the diversification of modern phytoplankton taxa like coccolithophorids beginning in the Mesozoic. Moreover, we suggest that the two are linked. Modern phytoplankton taxa are carbon-poor and macronutrient (phosphorus)-rich, whereas their Paleozoic counterparts (“acritarchs”) were carbon-rich and phosphorus-poor. Nutrient-poor food at the base of food webs may have delayed the appearance of more advanced carnivores until the later Paleozoic. A shift to more nutrient-rich conditions began by the Permo-Carboniferous, as hinted by strontium isotope ratios, biomarkers, and petroleum source rocks; widespread orogeny, glaciation, sea level fall, and the spread of terrestrial forests increased rates of weathering, erosion, and macronutrient runoff to the seas. Continued nutrient input from land during the Meso-Cenozoic enhanced phosphorus availability, thereby increasing the inorganic nutrient content of phytoplankton and their population densities. Consequently, unlike their Paleozoic counterparts, Mesozoic marine metazoans had to expend less energy to “burn off” (respire) excess carbon to obtain inorganic nutrients necessary for growth, leaving more energy available for reproduction and, potentially, micro- and macroevolution of the Modern Fauna. Calcareous phytoplankton of some sort may have existed during the Paleozoic but only in numbers insufficient to be preserved in the fossil record because of an extremely shallow CCD (resulting from widespread limestone deposition on the cratons) and accelerated diagenesis (due to elevated levels of atmospheric carbon dioxide). Along with the erosion of shallow-shelf limestones during the Permo-Carboniferous sea-level fall, rising nutrient levels provided positive feedback on the expansion of calcareous plankton populations and deepening of the CCD, setting the stage for more widespread deposition and preservation of deep-sea calcareous oozes in the Mesozoic.