Synapsid

A member of the amniote lineage characterized ancestrally by one temporal opening on each side of the skull. Synapsids include mammals and their extinct relatives.

Amniote: A tetrapod whose embryo develops within an amnion or related set of extraembryonic membranes. Synapsids are one of the two major living amniote lineages, alongside sauropsids.
Hylonomus: An extinct Carboniferous amniote known from fossils in Nova Scotia, among the earliest well-documented members of the group. Its age places early amniotes near the beginning of synapsid history.
Mammalian jaw: A jaw in which the dentary bone forms the lower jaw and meets the squamosal bone at the jaw joint. The shift to this joint from an older arrangement marks a major synapsid transition.
Edaphosaurus: An extinct herbivorous Permian synapsid genus with a prominent back sail bearing transverse projections. It demonstrates that early synapsids included large herbivores, not only predators.
Permian–Triassic extinction event: The largest known mass extinction, about 252 million years ago, which eliminated many marine and terrestrial species. Its aftermath opened ecological space for surviving synapsid groups, including therapsids.
Temporal fenestra: An opening in the skull behind the eye socket that accommodates jaw muscles and related structures. The ancestral synapsid skull has one temporal opening on each side.
Ophiacodontidae: An extinct family of early Permian synapsids, including large-bodied predators such as Ophiacodon. Ophiacodontids represent an early major diversification of synapsids.
Dentary: The main tooth-bearing bone of the lower jaw in mammals and many extinct vertebrates. The dentary enlarged through mammal evolution as other lower-jaw bones changed roles.
Lystrosaurus: An extinct dicynodont therapsid genus that survived the Permian–Triassic extinction and became widespread afterward. Its fossils show how one synapsid lineage persisted through a major mass extinction.
Mesozoic mammal: A mammal or mammaliaform that lived during the Mesozoic Era, alongside dinosaurs and other vertebrates. Mesozoic fossils show how synapsids diversified while dinosaurs dominated many large-animal niches.

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