Large marine megafauna, including megalodon, the largest shark to have ever lived, disappeared during a global extinction event that had previously not been recognised as such.
At the end of the Pliocene, between two and three million years ago, the planet entered a phase of great climate change, marked by important fluctuations in the level of the sea.
Scientists know that these climatic shifts often go hand in hand with a loss of habitat and that they can cause major threats to a whole range of species.
Previous studies had documented individual examples of marine creatures becoming extinct around that time, including megalodon, and a number of ancient penguins and sea turtles.
However, it remained unclear whether these were simply isolated background extinctions, or if they formed part of a global marine extinction event resulting from environmental and climatic changes.
"There had been a lot of studies showing individual extinctions around that time, but no one had recognised these as a global extinction event of marine megafauna as a whole, until now," Dr. Catalina Pimiento, from the Paleontological Institute and Museum of the University of Zurich, told IBTimes UK.
In a study now published in the journal Nature Ecology and Evolution, Pimiento and colleague investigate severity of the extinction of marine megafauna during the Pliocene, and examine the potential causes and consequences of this event on the planet's biodiversity.
They argue that a large, previously unknown extinction event occurred in the oceans worldwide around the end of the Pliocene.
Study of ancient fossils
The team studied fossils of marine megafauna from the Pliocene and the Pleistocene epochs (dating back to between 5.3 million to around 9,700 years BCE).
They showed that at that time, marine megafauna disappeared at rates three times greater than those seen at any other time throughout the period - a third of the marine megafauna is thought to have disappeared between two and three million years ago.
Mammals were the most affected, with a loss of 55% of their diversity. Furthermore, the scientists estimate that as many as 43% of sea turtle species were lost at that time, along with 35% of sea birds and 9% of sharks.
The scientists say that climatic shifts and sea level changes might have played an important role in triggering these extinctions around the world. Indeed, combined with other factors such as altered sea currents, these factors might have led to sudden losses of the productive coastal habitats that many animals relied on - particularly warm-blooded animals like megalodon.
"This is not like the mass extinction event that caused the end of dinosaurs, we can't pinpoint a single major factor to explain its cause. The extinction of many marine species happened over time, and we propose that an important factor was the fluctuation in sea levels which caused a loss of coastal habitats, where charismatic species like megalodon got their food," Pimiento said.
The findings suggest that marine megafauna were far more vulnerable to global environmental and climatic changes than had previously been believed.
That being said, the scientists note that new types of marine species developed and thrived right after the global extinction event, during the Pleistocene epoch. These included the polar bear Ursus, the storm petrel Oceanodroma or the penguin Megadyptes, which had not existed during the Pliocene.