Sharks were a tolerant bunch some 50 million years ago, cruising an Arctic Ocean that contained about the same percentage of freshwater as Louisiana's Lake Ponchatrain does today, says a new study. The study indicates the Eocene Arctic sand tiger shark, a member of the lamniform group of sharks that includes today's great white, thresher and mako sharks, was thriving in the brackish water of the western Arctic Ocean back then. In contrast, modern sand tiger sharks living today in the Atlantic Ocean are very intolerant of low salinity, requiring three times the saltiness of the Eocene sharks in order to survive....