Mammaliaformes is a clade that contains the mammals and their closest extinct relatives. The precise phylogeny is disputed due to the scantness of evidence in the fossil record. However, it is thought that the Mammaliaformes were of three major groups: Allotheria, the longest exctinct lineage of pre-mammals; Docodonta, including close relatives such as Morganucodonta; and Symmetrodonta, the most basal of modern mammals. Mammaliaformes radiated from Cynodontia.
Early mammaliforms were generally rodent-like in appearance and size, and most of their distinguishing characteristics were internal. In particular, the structure of the mammaliform (and mammal) jaw and arrangment of teeth is nearly unique. Instead of having many teeth that are frequently replaced, mammals have one set of adult teeth which fit together precisely. Mammaliforms seem have been structured similarly. Most importantly, mammaliforms have highly specialized molars, with cusps and flat regions for grinding food. This system is also unique to mammals, although it seems to have evolved convergently in pre-mammals multiple times.
Lactation and fur, along with other characteristically mammalian features, are also thought to characterize the Mammaliaformes, but these traits are difficult to study in the fossil record. The fossilized remains of Castorocauda lutrasimilis are an unique exception.
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