Researchers studying malformed plankton in the fossil record have revealed that one of the oldest mass extinctions in the history of life on Earth may have been triggered by oxygen depletion and the increasing presence of harmful metals in the oceans. The findings are published in Nature Communications this week.

Life was thriving in the seas during the Ordovician Period (more than 440 million years ago), until the Ordovician-Silurian extinction events eliminated an estimated 85 percent of all Ordovician species. These events were thought to have occurred in pulses that correspond to glacial episodes of cooler climes and shrinking habitats. However, recent studies have begun to implicate the spread of anoxia (or low oxygen) in the oceans as an additional driving force of mass extinctions. 

To study the effects of paleo-pollution in marine environments, an international team led by Ghent University’s Thijs Vandenbroucke examined the geochemical signals of fossil plankton living in what’s now the Libyan desert during the initiation of a late Silurian (or mid-Pridoli) extinction event. They focused on chitinozoans: 100-500 micrometer microfossils that represent fossil egg cases of zooplankton that lived in shallow ocean layers. (The soft-bodied parents aren’t typically preserved in the fossil record.)

Aberrations in chitinozoans were found to coincide with a dramatic increase of metals – including iron, lead, manganese, and arsenic – in both the fossil specimens and the rocks they were preserved in. 

The abnormal growth of modern organisms living with high levels of metallic toxins are well documented. In fact, misshapen plankton, arthropods, and fish are routinely used as a forensic tool for detecting contamination in bodies of water. Metal poisoning, the team concludes, may very well have caused the ancient malformations. High levels of these harmful metals indicate changes in ocean chemistry. Together with the spread of anoxia, these factors may have been a contributing “kill mechanism,” making malformed plankton harbingers of mass extinction.