Mammals’ clade once mislabeled mammal-like reptiles
Synapsida covers every mammal plus extinct cousins that share a single lower temporal fenestra behind each eye. They are amniotes’ other great branch beside Sauropsida. Early forms appeared late in the Carboniferous, about 315 to 307 million years ago, and ruled Permian land long before dinosaurs.
Classical textbooks called non-mammalian synapsids mammal-like reptiles or pelycosaurs because basal amniotes were once lumped as reptiles. Cladistics junked that story: synapsids are an independent amniote stem, and Reptilia sits entirely inside Sauropsida. The skull opening once housed enlarged jaw muscles; in advanced therapsids and mammals the sphenoid expands and leaves the zygomatic arch. Ancestral pineal foramina for a parietal eye closed along the cynodont line toward mammals, retiring that light-sensing organ.
Permian giants such as Dimetrodon and other eupelycosaurs were top terrestrial predators until the end-Permian crisis reshuffled ecosystems. Convenience still splits stem “pelycosaur-grade” sprawlers—possibly with horny scutes—from more erect therapsids, the clade that produced mammals, though phylogenetic usage folds mammals inside Therapsida. Heterodont teeth intensified from early sphenacodonts into therapsid incisors, canines, and postcanines; dicynodonts took an odd, often tusked path unlike most other synapsids.
Jaw evolution is the headline mechanism: multiple lower-jaw bones shrink in cynodonts until the articular and angular contribute to middle-ear ossicles, while a dentary–squamosal joint defines mammals against the ancestral articular–quadrate hinge. Palatal teeth vanished on the mammal line. Secondary palates grew from maxillary shelves into complete separators of airway and mouth by forms such as Thrinaxodon. Skin coverings later ranged from osteoderms and scutes to hair and even hair-derived scales in some later mammals, showing how far the clade wandered from its lizard-like beginnings.
Source: Synapsida
