Mammaliaformes (Mammaliaformes) are a clade of probainognathian cynodonts that contains the crown-group Mammalia and their closest extinct branches. Cynodonts were decimated during the Triassic-Jurassic extinction and survivors underwent evolutionary radiation leading mainly to mammaliaforms. These group together the clade originating from the most recent common ancestor of Morganucodon and the crown-group of mammals; the latter [unclear] being the clade originating from the most recent common ancestors of monotremes, marsupials and placentals. In addition to Morganucodon and Mammalia, mammaliaformes also include Docodonta and basal genera such as Hadrocodium as well as Tikitherium, the oldest known member of the group.
Mammaliaformes taxon is a term of phylogenetic classification. In contrast, the assignment of organisms to Mammalia has traditionally been trait-based, and on this basis mammals are slightly more inclusive than mammaliaformes. In particular, trait-based taxonomy generally includes Adelobasileus and Sinoconodon in Mammalia, although they fall outside the definition of mammaliaformes. These genera are included in the larger mammaliamorph clade, phylogenetically defined as the clade originating from the last common ancestor of tritylodontids and the crown group of mammals.
The origin of the crown group of mammals dates back to the Jurassic, with significant finds in Late Jurassic outcrops of Portugal and China. Some fossilized specimens already had fur, indicating that the ancestors of mammals had already developed this major characteristic that defines them.
The oldest known mammaliaforms generally resemble shrews in appearance and size, and most of their distinguishing features were internal. In particular, the structure of the jaws of mammaliaformes (including mammals) and the arrangement of the teeth are almost unique. Instead of having many teeth which are replaced frequently, mammals have a set of milk teeth and later a set of adult teeth which fit together precisely. It is believed to help break down food to make it faster to digest. Warm-blooded animals need more calories than cold-blooded ones, so speeding up the pace of digestion is a necessity. The disadvantage of fixed dentition is that worn teeth cannot be replaced, as is possible for the reptiliomorphic ancestors of mammals. To compensate, mammals develop prismatic enamel, characterized by discontinuities of crystallites that help distribute the force of the bite.
Lactation, among other mammalian traits, is also thought to characterize mammaliaformes, but these elements are difficult to study in the fossil record. Evidence of lactation is present in morganucodonts, via tooth replacement patterns. Combined with the more basal tritylodontids which also display evidence of lactation, this seems to imply that milk is an ancestral feature in this group. However, the fairly derived Sinoconodon seems to have completely rejected milk. Prior to hatching, the mammary glands would provide moisture to the leathery eggs, a situation still found in monotremes.
Early mammaliaforms have Harder's glands. In modern mammals, this is used to clean the coat, indicating that unlike their cynodont ancestors, the latter would have had fur. An insulating covering is necessary to keep a warm-blooded animal warm if it is