A recent study has found that the Arctic Ocean was very brackish and had reduced salinity back then, indicating that Sharks thrived some 50 million years ago.
The study, carried out by University of Colorado Boulder and the University of Chicago, indicated that the Eocene Arctic sand tiger shark was thriving in the brackish water, while the modern sand tiger sharks living today are very intolerant of low salinity.
The modern sharks require three times the saltiness of the Eocene sharks in order to survive.
The findings have implications that today's sharks survive in a warmer Arctic region, which is heating up at about twice the rate of the rest of the planet due to increase in greenhouse gases. The consequences of this warming in the Arctic include changes in freshwater runoff and atmospheric water vapor and decrease in salinity that can affect marine biology.
The study was published online in the journal Geology.