Ancient Arctic Sharks Survived in Brackish Water 50 Million Years Ago

There's a reason that sharks have survived for so long as a species; they're hardy. Scientists have found that about 50 million years ago in an Arctic Ocean, sharks managed to tolerate water that contained the same percentage of freshwater as Louisiana's Lake Ponchatrain does today.

Sharks are known as animals that thrive in saltwater, yet millions of years ago it seems as if they could also tolerate brackish water. Known as the Eocene Arctic sand tiger shark, this species was a member of the lamniform group of sharks that includes today's great white, thresher and mako sharks.

In order to learn a bit more about the conditions in the Eocene, the researchers compared ratios of oxygen isotopes locked in ancient shark teeth found in sediments on Banks Island in the Arctic Circle and incorporating data into a salinity model. They then compared this information to prior studies of sediment cores extracted from an oceanic region in the central Artic Ocean.

"This study shows the Arctic Ocean was very brackish and had reduced salinity back then," said Sora Kim, first author of the new study, in a news release. "The ancient sand tiger sharks that lived in the Arctic during the Eocene were very different than sand tiger sharks living in the Atlantic Ocean today."

This actually has large implications for the sharks of today. The Arctic is currently warming, which means that more fresh water is entering the northern oceans. With freshwater runoff, salinity could be affected and, therefore, species that live in the area could also be impacted.

"As more freshwater flows into the Arctic Ocean due to global warming, I think we are going to see it become more brackish," said Jaelyn Eberle, a study co-author. "Maybe the fossil record can shed some light on how the groups of sharks that are with us today may fare in a warming world."

The findings are published in the journal Geology.