This previously unknown extinction event not only had a considerable impact on the earth’s historical biodiversity but also on the functioning of ecosystems. This has been demonstrated a team of researchers, including co-author Dr John Griffin of Swansea University’s Department of Bioscience. 

The disappearance of a large part of the terrestrial megafauna such as saber-toothed cat and the mammoth during the ice age is well known. Now, the team of researchers led by the University of Zurich, and the Natukunde Museum in Berlin have shown that a similar extinction event had taken place earlier, in the oceans.

New extinction event discovered

The international team investigated fossils of marine megafauna from the Pliocene and the Pleistocene epochs (5.3 million to around 9,700 years BC). “We were able to show that around a third of marine megafauna disappeared about three to two million years ago. Therefore, the marine megafaunal communities that humans inherited were already altered and functioning at a diminished diversity”, explains lead author Dr Catalina Pimiento, who conducted the study at the Paleontological Institute and Museum of the University of Zurich.

‌The tooth of a Megalodon, the largest shark to have ever lived, which became extinct over two million years ago.

Contributing author Dr John Griffin of Swansea University, added: “It’s astonishing that an extinction event like this, among the biggest animals in the oceans, could go undetected until now. It overturns the assumption that the oceans’ biodiversity was resistant to environmental change in Earth’s recent history”.

Above all, the newly discovered extinction event affected marine mammals, which lost 55 per cent of their diversity. As many as 43 per cent of sea turtle species were lost, along with 35 per cent of sea birds and 9 per cent of sharks. On the other hand, the following new forms of life were to develop during the subsequent Pleistocene epoch: Around a quarter of animal species, including the polar bear Ursus, the storm petrel Oceanodroma or the penguin Megadyptes, had not existed during the Pliocene. Overall, however, earlier levels of diversity could not be reached again. 

‌Effects on functional diversity

In order to determine the consequences of this extinction, the research team concentrated on shallow coastal shelf zones, investigating the effects that the loss of entire functional entities had on coastal ecosystems. Functional entities are groups of animals not necessarily related, but that share similar characteristics in terms of the ecological roles they play in ecosystems. The finding: Seven functional entities were lost in coastal waters during the Pliocene.

Even though the loss of seven functional entities and one third of the species is relatively modest, this led to an important erosion of functional diversity: 17 per cent of the total diversity of ecological functions in the ecosystem disappeared. Previously common predators vanished, while new competitors emerged and marine animals were forced to adjust.

“Our analyses provide a new perspective for modern conservation” explains Dr Griffin. “Large marine animals show little functional redundancy, meaning there are few if any species ‘waiting in the wings’ to take over the ecological role of a lost species. Losing a single species of marine megafauna may well entail the loss of an entire ecological function”. In addition, the researchers found that at the time of the extinction, coastal habitats were significantly reduced due to violent sea levels fluctuations.

Large warm-blooded marine animals are more vulnerable to global environmental changes

The researchers propose that the sudden loss of the productive coastal habitats, together with oceanographic factors such as altered sea currents, greatly contributed to these extinctions. “Our models have demonstrated that warm-blooded animals in particular were more likely to become extinct. For example, species of sea cows and baleen whales, as well as the giant shark Carcharocles megalodon disappeared”, explains Dr Pimiento. “This study shows that marine megafauna were far more vulnerable to global environmental changes in the recent geological past than had previously been assumed”. The researcher also points to a present-day parallel: Nowadays, large marine species such as whales or seals are also highly vulnerable to human influences. 

Read the research on Nature.com