Mammals have a specific skull type called a Synapsid. Synapsids have a single hole behind the eye (called the temporal fenestrae) which provides attachment for jaw muscles. All mammals have molar, canine, and incisor teeth. Mammals have had this skull type for almost 320 million years, starting with Pelycosaurs (early Therapsids) from the Permian (such as Dimetrodon and Edaphosaurus).
Originally, Pelycosaurs showed the same cold-blooded lifestyle as contemporary Diapsids and Anapsids. Pelycosaurs waddled on their spaced out legs, had reptilian skin and they laid eggs. Pelycosaurs shared almost complete similarity with reptiles, except for one thing. Pelycosaurs had managed to control their temperature. The earliest Pelycosaurs, Archaeothyris and Clepsydrops, were first classified as Synapsids due to the small hole behind the eye in their skull. Their jaws were more advanced than the other reptiles and amphibians with which they shared their homes. As they evolved, they grew into larger forms. They became the largest carnivores and herbivores on land.
Large Pelycosaurs were the first to control their temperature. In Texas, Dimetrodon and Edaphosaurus grew large sails on their back to control their heat. Strangely enough, these Permian Pelycosaurs would not be the first or the last to grow a sail. Platyhystrix was an amphibian from Texas, which also evolved a sail for temperature control. History continued to repeat itself in Texas. Like Dimetrodon, Edaphosaurus, and Platyhystrix, an early crocodile called Arizonasaurus evolved a sail to control the heat. Even larger forms of Pelycosaurs like Moschops (which weighed over a ton) evolved during the Permian. These were eventually wiped out by the Permian-Triassic Extinction.
The smaller Pelycosaurs, such as Robertia, became much more successful and lived into the Triassic. As the Pelycosaurs lived into the Triassic, the sails continued to disappear. Sphenacodon had about half as tall a sail as Dimetrodon. Pelycosaurs that survived into the Triassic no longer needed a sail and had proto-fur. The Pelycosaurs were extinct, but they had evolved into new forms: Cynodonts (like Procynosuchus), Dicynodonts (like Diictodon), and Therocephalians (like Theriognathus).
Therocephalians only survived into the early Triassic, while Dicynodonts and Cynodonts continued to thrive. Dicynodonts were originally small beaked mammals that grew into larger forms by the end of the Triassic (Placerias and Lystrosaurus), but they died out at the end of the Triassic. Cynodonts were herbivorous, insectivorous, and increasingly mammal-like throughout the Triassic. The largest Cynodonts during the Triassic were about dog-size (Cynognathus) and still walked like lizards. Cynodonts continued to flourish and evolved into a new group: the Eucynodonts (true Cynodonts). The first true mammals appeared 225 million years ago during the Triassic. These very early forms like Repenomamus were completely covered in fur and walked with their legs under them, instead of beside them. These were also the first animals to give birth to live young.