Evidence that malformed plankton may indicate alternative drivers of mass extinction has been presented this week in Nature Communications. Although glacial episodes have previously been linked to the oldest major mass extinctions, the authors suggest that the cause is more likely to be spreading oxygen depletion and harmful metals in the oceans.
Ordovician-Silurian extinction events are thought to have occurred in pulses more than 420 million years ago, when the vast majority of life was flourishing in the sea and very little survived on land. Previous studies have suggested that these events were the result of cooling climate and reductions in habitat, but those models do not account for all palaeontological and geochemical observations.
Thijs Vandenbroucke and colleagues show that malformations in plankton during the initiation of a Late Silurian extinction event occurred in a very similar manner to the response seen in modern organisms to high levels of metallic toxins today. The authors suggest that metal poisoning may also have caused the ancient malformations, as the misshapen plankton coincide with dramatic increases in the metal content of the rocks they are preserved in. The high levels of metals suggest changes in the ocean chemistry at that time and that spreading anoxia may have been a contributing kill-mechanism during these early extinction events.
More recent work implicates anoxia in the oceans as a driving force behind mass extinctions, and the current study supports this and suggests that malformed fossil plankton may be a new forensic tool to identify the onset of these periods of low oxygen.