Synapsid

Synapsids (Greek: "fused-arch") are a clade of animals that includes mammals. Non-mammalian synapsids are often called "mammal-like reptiles", though they were neither.

Synapsids were the largest land vertebrates in the Permian period. By the end of the Triassic, the group Probainognathia (which includes what would become modern mammals) were the only surviving synapsids. After the K-Pg extinction event 66 million years ago, synapsids again became the largest land animals.

Characteristics
Synapsid skulls contain a hole behind the eyes called the temporal fenestrae, which made their skulls lighter and less energy-intensive to grow and may have provided new attachment sites for jaw muscles. They also have differentiated teeth (canines, molars, and incisors) and evolved a different palate shape from their reptilian ancestors.

As soft tissue only preserves in rare cases (even less the further back in the record you go), it is unknown whether early synapsids had hair or mammary glands, though they are occasionally depicted with hair. Certain species of Dimetrodon have been shown to have tough scales on their undersides that extended to the tail, while Estemmenosuchus likely had no hair but had smooth skin with glandular depressions. Permian coprolites show that at least some synapsids back then already had hair.

The fossils of Lystrosaurus, as well as other extinct reptiles and land plants that lived from the Permian to the Cretaceous period, were used as evidence of continental drift due to fossils showing a clear correlation across the land, even though they were found across entire oceans.

Synapsids are significant in an evolutionary sense because of their perfect illustration of the process of transition, in this case from reptile-like amniotes to egg-laying mammals (Monotremes). They are no doubt one of the most frightening things to creationist nutters, who probably try to convince themselves they were eaten by velociraptors on the ark they lived peacefully with those loving, plant eating tyrannosaurs.

Classification
Synapsids are one clade of the amniotes (land vertebrates that lay eggs with shells). The other branch is Sauropsida, reptiles, which include birds (which are dinosaurs!). Synapsids are divided into two main clades. The extinct Caseasauria which resembled couch potato lizards and Eupelycosauria which included us and the sailed back herbivore Edaphosaurus. Going further down the line you have Sphenacodontia which includes the famous Dimetrodon and our ancestors.

Caseasaurs, edaphosaurs, and sphenocodonts were gradually replaced in the mid Permian by a new group of synapsids, the therapsids which experienced their own flowering. One group of therapsids, called dicyonodonts, were large pig-like browsers that evolved in the Permian and were very successful in the Triassic. They may have even survived until the early Cretaceous in Australia, proving that even back then Australia was strange.The other group were the cynodonts, dog-like animals that gave rise to true mammals.