mammaliaforms

Mammaliaformes is a clade within the Synapsida that includes the last common ancestor of living mammals and all extinct species more closely related to mammals than to other amniotes, together with all of its descendants. In cladistic terms, it is a broader group than the crown group Mammalia and is used to study the evolutionary transition from non-mammalian synapsids to true mammals.

Fossils attributed to mammaliaformes are known from the Late Triassic to the Early Jurassic, with a variety of forms representing different evolutionary experiments along the path to mammals. Well-known early mammaliaforms include Morganucodon and Megazostrodon, as well as members of Docodonta. The status of Haramiyida within this grouping is debated, and different analyses place some members inside or outside Mammaliaformes, reflecting ongoing questions about early mammalian relationships.

Morphology in mammaliaforms shows a mosaic of mammal-like traits alongside more primitive features. They typically possessed differentiated teeth, with incised canines and differentiated premolars and molars in many lineages, and a trend toward a dentary-dominated lower jaw. The jaw joint was transitioning from a multi-bone assembly toward the mammalian dentary-squamosal arrangement, while the articular and quadrate bones became the malleus and incus of modern mammals. Postcranial skeletons were generally small and light, suitable for an agile, insectivorous or omnivorous lifestyle.

Mammaliaforms occupy a key position in discussions of mammalian origins, illustrating important steps in the restructuring of feeding mechanics and the auditory system. Ongoing research and new fossil finds continue to refine their phylogenetic relationships and their exact role in the ancestry of living mammals.