Sharks Teeth (image) University of Chicago Caption Sora Kim has analyzed oxygen 18 and oxygen 16 -- two isotopes of the oxygen atom that contain only slight subatomic variations -- of fossil and modern sharks teeth for insights into the potential adaptability of sand tigers to global warming. These teeth come from the Eocene epoch, which ended approximately 38 million years ago. Credit Robert Kozloff/University of Chicago Usage Restrictions News organizations may use this image in connection with reports about the research of Sora Kim and her colleagues. Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.