Researchers found about 6,000 shark teeth at a coastal site on Banks Island, N.W.T. Teeth determined to be from the early Eocene epoch were compared to those of modern-day sharks. The Eocene was a warm period of about 50 million years ago (when ice was absent from the poles and there were no glaciers) that’s often studied to understand how today’s wildlife might cope with global warming.

“Very little is known about the Arctic Ocean at the time during the Eocene, and we were hoping to study the sharks to get a better sense about the environment,” says Jaelyn Eberle, an associate professor of geological sciences at the University of Colorado at Boulder and co-author of the study.

Analysis of oxygen isotopes in the teeth showed the water the ancient sharks lived in was not at all similar to that of today’s Arctic. “It wasn’t very saline at all. In fact, it was closer to fresh water than it was to salt water,” says Eberle.

The finding suggests that today’s sharks may be able to cope with a warming ocean and subsequent decrease of water salinity. Sand tiger sharks live in ocean waters that are three times as saline as the Eocene Arctic Ocean. “To me this suggests something positive if global warming was to occur,” Eberle says, “because in the past they did OK.”