Coryphodon (IMAGE) University of Colorado at Boulder Caption A hippo-like mammal known as Coryphodon was one of several ancient mammal groups that endured twilight winters in the high Arctic 53 million year ago, according to a new study led by the University of Colorado at Boulder. Credit Image copyright American Museum of Natural History/D. Finnin. Usage Restrictions None License Licensed content 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.