Previously unknown extinction of large marine animals discovered

 Fossils from the Pliocene: A shark tooth from Carchahinus leucas on the left, and from Negaprion on the right. Image: University of Zurich.

Over two million years ago, a third of the largest marine animals like sharks, whales, sea birds and sea turtles disappeared. This previously unknown extinction event not only had a considerable impact on the earth’s historical biodiversity but also on the functioning of ecosystems. 
 

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

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.

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

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 function they play on ecosystems. The study found that seven functional entities were lost in coastal waters during the Pliocene.

Even though these losses are relatively modest, they led to an important erosion of functional diversity: 17%  of the total diversity of ecological functions in the ecosystem disappeared and 21% changed. Previously common predators vanished, while new competitors emerged and marine animals were forced to adjust. In addition, the researchers found that at the time of the extinction, coastal habitats were significantly reduced due to violent sea level fluctuations.

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”. Dr. Pimiento also points to a present-day parallel: Nowadays, large marine species such as whales or seals are highly vulnerable to human influences.

The study is published in Nature Ecology & Evolution. DOI: 10.1038/s41559-017-0223-6.