Skeleton of Dimetrodon (image) Field Museum Caption This is the skeleton of Dimetrodon, an ancient relative of mammals. New research suggests that at least some species of Dimetrodon were active at night (nocturnal). Credit The Field Museum Usage Restrictions Must use credit 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.