Thrinaxodon (IMAGE) Field Museum Caption This is an illustration showing an early mammal relative, Thrinaxodon, which was part of the first group to have an extra fourth section of their backbones. Credit April Neander Usage Restrictions Please include credit to April Neander 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.