cynodont

Cynodonts are a clade of therapsids that includes the closest extinct relatives of living mammals, with all mammals descending from cynodont ancestors. The name means dog-toothed, reflecting early dental characteristics of some members. Within Therapsida, cynodonts are more closely related to mammals than to other therapsid groups, and mammals are nested within the cynodont lineage.

The cynodont lineage appears in the fossil record during the Middle to Late Permian and became ecologically diverse during the Triassic. They persisted through the Permian–Triassic transition and gave rise to the more mammal-like therapsids, ultimately leading to crown-group Mammalia. Fossil evidence shows a broad range of sizes and lifestyles, from small, insectivorous forms to larger carnivores and omnivores.

Morphologically, cynodonts display several features associated with mammalian evolution. They show differentiated teeth (incisors, canines, and postcanines), a more advanced jaw joint and jaw musculature, and a shift in the arrangement of jaw bones toward the mammalian condition. Over time, postdentary bones reduced and were repurposed as auditory ossicles in the middle ear, while the dentary-squamosal joint became the primary jaw joint in more derived members. These changes mark the transition from the reptile-like jaw to the mammalian jaw and hearing apparatus.

Notable cynodonts include genera such as Cynognathus, Diademodon, Thrinaxodon, and Galesaurus, which illustrate the group's diversity and transitional biology. Cynodonts are important for understanding the origin of mammals and the early evolution of mammalian characteristics.