Mammaliforms

Mammaliforms, often referred to as Mammaliaformes, are a clade of synapsids that includes the earliest relatives of living mammals as well as the mammals themselves. They belong to the therapsid lineage and are defined as more closely related to mammals than to other synapsids. This group marks the transition from mammal-like reptiles to true mammals.

Key features show a shift in jaw mechanics and skull structure. The long-standing articular-quadrate jaw joint of earlier cynodonts gives way to a mammal-like dentary-squamosal joint. Teeth become increasingly differentiated, and the palatal, braincase, and auditory regions enlarge. In many mammaliforms, parts of the jaw bones are repurposed into the middle ear as the malleus and incus, with the stapes remaining as a hearing bone.

Fossils of mammaliforms date from the Late Paleozoic into the Mesozoic, with some of the earliest forms known from the Permian and Triassic. They are found on multiple continents and likely occupied small, insectivorous or omnivorous niches, coexisting with various synapsids and early dinosaurs. The lineage culminates in crown-group Mammalia, the therians and monotremes that survive to the present.

Taxonomy varies among authors. Some classifications treat Mammaliaformes as the formal clade containing all descendants of their most recent common ancestor, including crown Mammalia; others restrict Mammaliaformes to stem relatives. Regardless of naming, mammaliforms illuminate the origins of mammalian traits such as diverse dentition, jaw articulation, and the three-bone middle ear.