Dimetrodon synapsid, illustration. This synapsid lived 300 million years ago during the Early Permian period in what is now the southern United States. Synapsids are a group of animals that include mammals and extinct amniotes more closely related to mammals. Dimetrodons measured up to 4.6 metres in length and weighed 250 kilograms. The carnivorous dimetrodon evolved ziphodont (serrated) teeth to tackle large prey.