synapsids

Synapsids are a clade of amniotes that includes the mammals and their extinct relatives. A defining feature is a single temporal opening behind each eye socket, or temporal fenestra, which allows space for larger jaw muscles and helps distinguish them from other amniote groups. The skull opening is a key synapomorphy that has influenced the evolutionary trajectory of this lineage.

The earliest synapsids appear in the late Carboniferous to early Permian, around 320 million years ago. Early forms, often grouped informally as pelycosaurs, gave way to more derived therapsids, including cynodonts, from which the mammal lineage eventually arose. Over time, cynodonts developed several mammalian traits, such as increased tooth differentiation, a more advanced jaw mechanism, and hints of a secondary palate. The transition from reptile-like jaw joints (articular-quadrate) to a mammal-like jaw (dentary-squamosal) became more pronounced in the cynodont lineage.

Mammals eventually emerged from among the cynodonts in the Late Triassic and diversified to occupy a wide range of ecological roles. Early mammaliforms were small and often nocturnal, and true mammals with more specialized dentition and other mammalian features continued to evolve after the Permian–Triassic extinction event. Today, the synapsid lineage is represented solely by the mammals, while all non-mammalian synapsids are extinct.

Synapsids have had a profound impact on our understanding of vertebrate evolution, illustrating the gradual accumulation of mammalian traits and the deep evolutionary roots of living mammals.