Sci-News "The study, published in the journal Geology, indicates that these Eocene sharks were thriving in the brackish water of the western Arctic Ocean."
'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.
'“This study shows the Arctic Ocean was very brackish and had reduced
salinity back then. 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,” said lead author Dr Sora Kim of the
University of Chicago.
'The new findings on Arctic Ocean salinity conditions in the Eocene
were calculated in part by comparing ratios of oxygen isotopes locked in
ancient shark teeth found in sediments on Banks Island in the Arctic
Circle and incorporating the data into a salinity model.'