Synapsida

Synapsida is an amniote clade that includes mammals and their extinct relatives. Its members ancestrally had one temporal opening behind each eye.

Amniote: A tetrapod whose embryo develops within an amnion, including reptiles, birds, and mammals. Synapsida is one major amniote lineage, alongside the sauropsid lineage.
Mammalian jaw: A jaw in which the dentary bone forms the lower jaw and articulates directly with the squamosal bone. This distinctive jaw joint emerged late in synapsid evolution.
Carboniferous: A geological period lasting from about 359 to 299 million years ago, marked by extensive coal-forming forests. The earliest known synapsids appeared during this period.
Sauropsida: The amniote lineage that includes reptiles and birds, distinct from the synapsid lineage. Sauropsids form the other major living amniote lineage besides mammals.
Mammaliaform: A mammal and its closest extinct relatives, grouped by shared derived features of the mammalian lineage. Mammaliaforms record the transition from broader synapsids to mammals.
Temporal fenestra: An opening in the skull behind the orbit that provides space for jaw muscles and associated structures. The ancestral synapsid skull is defined by one such opening on each side.
Dentary: The bone that forms the principal portion of the lower jaw in mammals and their evolutionary relatives. The dentary expanded across synapsid history and became the mammalian lower jaw.
Ophiacodon: An extinct genus of early synapsids that lived in the Permian and included large, long-snouted predators. Its fossils document an early branch of synapsid diversity.
Diapsid: An amniote whose ancestral skull has two temporal openings behind each eye. The diapsid skull pattern contrasts with the single ancestral opening in synapsids.
Morganucodon: An extinct early mammaliaform known from the Late Triassic and Early Jurassic. Its fossils show a transitional stage in mammalian jaw and ear evolution.

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