Evolution of mammals

The evolutionary history of mammals, from their synapsid ancestors through the emergence of defining mammalian traits to the diversification of living groups.

Synapsid: An amniote lineage defined by a single opening behind each eye socket, including mammals and their extinct relatives. Mammals evolved within synapsids, a lineage extending far beyond crown mammals.
Amniote: A tetrapod vertebrate whose embryo develops within an amnion or its evolutionary equivalent. Mammals belong to the amniotes, alongside reptiles and birds.
Amniotic egg: An egg containing extraembryonic membranes that support development on land. The amniotic egg is part of the broader reproductive heritage mammals share with other amniotes.
Cretaceous–Paleogene extinction event: A mass extinction about 66 million years ago that eliminated non-avian dinosaurs and many other organisms. Its aftermath opened ecological opportunities for many surviving mammal lineages.
Molecular clock: A method that estimates evolutionary divergence times from genetic differences and mutation-rate models. Molecular estimates sometimes place mammal divergences earlier than their known fossils.
Mammalian jaw: A jaw in which the dentary bone forms the lower jaw and meets the squamosal bone at the joint. The shift to this joint accompanied the transfer of former jaw bones into the mammalian middle ear.
Pelycosaur: An informal name for several early synapsid groups that lived mainly during the Carboniferous and Permian periods. These early synapsids preceded the therapsid line leading toward mammals.
Cladistics: A method of classifying organisms by shared derived traits and common ancestry. Cladistic analysis organizes fossil mammals by evolutionary relationships rather than resemblance alone.
Adaptive radiation: The rapid diversification of a lineage into species adapted to different ecological roles. Mammals diversified into many body forms and niches after major ecological disruptions.
Mammal–bird divergence: The evolutionary split between the synapsid lineage leading to mammals and the sauropsid lineage leading to birds and reptiles. Its timing provides a deep boundary for reconstructing mammalian ancestry.

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