What is the Evolutionary History of Mammals?
Mammals represent the latest evolutionary stage of one of the two major groups of amniotes (non-amphibian tetrapods), the synapsids. Non-mammalian synapsids, often called therapsids (although mammals are technically therapsids too) branched off from the other main group, the sauropsids (reptiles), in the Carboniferous period. The first known synapsid was Archaeothyris, a small lizard-like creature that lived 320 million years ago. Like the reptiles, synapsids evolved from Carboniferous amphibians. The synapsids would go on to develop into the pelycosaurs, which would be the dominant terrestrial vertebrates for about 40 million years, until the middle of the Permian period, about 275 million years ago. Pelycosaurs are famous for being the first large (1 meter up to 3 meters or more) terrestrial amniotes. They had large tail sails, made from extended vertebral spikes, which are thought to have been used to regulate body temperature and for mating displays. Pelycosaurs, despite being syna
