THE PHANEROZIC RECORD OF ORGANIC-WALLED PHYTOPLANKTON

The fossil record of acritarchs and cysts of dinoflagellates is our best long-term direct sample of the global history of the phytoplankton. We have examined the Phanerozoic record of organic-walled phytoplankton taxon diversity by combining both the acritarch and dinocyst records. Our data comes from PALYNODATA, a compilation of palynological information developed by a consortium of oil companies and the Geological Survey of Canada over the last 4 decades. It contains records from about 20,000 publications. This picture of acritarch and dinocyst diversity has remained consistent since it was first studied by Helen Tappan some 30 years ago. Acritarchs have their acme in the Silurian, but they decline severely at the Devonian/Carboniferous boundary. Their diversity remains low until a gradual increase to the Recent begins in the Jurassic. The record of dinocysts begins in the Middle Triassic, peaks in the Cretaceous and declines in diversity after the Eocene. The composite organic-walled phytoplankton curve is decidedly different from benthic marine invertebrate diversity, which falls after the Permian and rises through the Tertiary. This is not surprising, given the heterogeneity of marine trophic resources and the lack of food specificity in modern benthic invertebrates. The Devonian acritarch extinction may have affected the vertebrate nekton indirectly (through a postulated demise of zooplankton) since both placoderms and ostracoderms suffered extinction at this time. Differences between the organic-walled phytoplankton curve and the classic marine invertebrate curve emphasize that most marine communities are not directly dependent upon phytoplankton as a food source, but rely upon a mix of sources including terrestrial particulate organic matter and zooplankton. The pattern of organic-walled phytoplankton diversity does appear to match recently postulated fluctuations in aragonite vs. calcite-dominated seas. This indicates that phytoplankton diversity over long time scales may be tracking changes in nutrient availability governed by ocean chemistry. Thus, phytoplankton history may be a more robust indicator of global physical/biotic interactions than the more well-known benthic marine invertebrate record.