Multiple Lines of Chemical Evidence for Pelagic Neoproterozoic Acritarchs  

For over 1300 million years, acritarch diversity (organic-walled microfossils of unknown origin) was dominated by non-ornamented leiospherid forms.  In the late Neoproterozoic, a morphological radiation produced large spiny forms, the acanthomorphs.  As both Proterozoic leiospheres and acanthomorphs were orders of magnitude larger than their Paleozoic counterparts, it has been suggested that these forms represent benthic, not pelagic, algae.  The Early Cambrian radiation of small rapidly diversifying acanthomorphic acritarchs is thought in turn to represent the advent of true plankton in the ocean.  Some have even speculated that this ecological shift from the benthic to the pelagic helped spur the Cambrian Explosion. 

However, with the exception of acritarchs found attached directly to sediment surfaces, determining the habitat of these enigmatic fossils is difficult.  Today, both pelagic and benthic algae eventually settle on the sea floor, and while size can be an indicator of buoyancy, algae are known to precipitate CaSO^­₄ or BaSO₄ in their vesicles to make themselves more or less buoyant.  While morphology is not sufficient to determine the palaeoecology of acritarchs, their chemical makeup can provide clues to their habitat.  Here we infer their palaeoecology based on multiple lines of chemical evidence, including the carbon isotopic fractionation preserved within individual acritarchs; their biopolymer composition; and their bioinorganic minerals.  While these techniques all examine different chemical features, the resulting data all indicate that leiospherid acritarchs were likely to have been benthic organisms, but the acanthomorphic acritarchs that radiated in the late Neoproterozoic were likely to have been pelagic, despite their large size.