Synapsids are reptiles that look like mammals. In fact, mammals, animals which give milk to their young, arose from reptiles. Synapsids have a single opening on the side of the skull, behind the eye socket. Primitive synapsids are called pelicosaurs. More advanced mammal like ones are known as therapsids. Several sub groups of pelicosaurs exist. Some ate fish. Dimetrodon was a pelicosaur that preyed on other animals. It reached a length of about three metres. Edaphosaurus was a ‘vegetarian’ pelicosaur. It had strange long spines on its back. This could have supported a large ‘sail’ rich in blood vessels. The edaphosaurus turned the sail towards the sun to warm up in the cold, and away from the sun to keep cool.
Pelicosaurs were replaced by therapsids 299 to 251 million years ago in the Permian period. Some therapsids ate plants, while others preyed on animals. Therapsids in turn, gave rise to the mammals, 250 to 200 million years ago in the Triassic Age. Like mammals, some therapsids had different types of teeth, and limbs pulled beneath the body, lifting it off the ground. However, we are yet to find out exactly when the reptiles became mammals.