Mammaliaformes

Mammaliaformes ("mammal-shaped") is a clade that contains the mammals and their closest extinct relatives. Phylogenetically it is defined as a clade including the most recent common ancestor of Sinoconodon, morganuconodonts, docodonts, Monotremata, Marsupialia, Placentalia, extinct members of this clade, and all of its descendents. 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 extinct lineage of pre-mammals; Docodonta, including close relatives such as Morganucodonta; and Symmetrodonta, the most basal of modern mammals. Mammaliaformes radiated from Cynodontia. The Probainognathians of the Eucynodont clade probably evolved into the early mammaliaformes, but the branch Allotheria was so different that they may have come from an entirely different group of cynodonts.

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 arrangement of teeth is nearly unique. Instead of having many teeth that are frequently replaced, mammals have one set of baby teeth and later one set of adult teeth which fit together precisely. This is thought to aid in the grinding of food to make it quicker to digest. Being warm-blooded requires more calories than "cold-blooded" animals, so quickening the pace of digestion is a necessity. Early mammaliaformes were probably nocturnal.

Mammaliforms have several common structures. 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 a unique exception.

Some non-mammal mammaliformes still retain reptile-like traits. Some mammaliformes had reptile-like locomotion. Furthermore, these mammaliformes still had some bones on their lower jaw seen in reptiles.

Phylogeny
Cladogram from an online synthesis by Mikko Haaramo.