Mammaliaformes in lifeEarly mammaliaforms were generally shrew-like in appearance and size, and most of their distinguishing characteristics were internal. In particular, the structure of the mammaliaform (and mammal) Jaw#Mammals, jaw and the arrangement of teeth are 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 that fit together precisely. This is thought to aid in the grinding of food to make it quicker to digest. Warm-blooded animals require more calories than those that are ectothermy, cold-blooded, so speeding up the pace of digestion is a necessity. The drawback to the fixed dentition is that worn teeth cannot be replaced, as was possible for the reptilian ancestors of mammaliaforms. To compensate, mammals developed Tooth enamel#Structure, prismatic enamel, characterized by Grain boundary, crystallite discontinuities that helped spread out the force of the bite. Lactation, along with other characteristically mammalian features, is also thought to characterize the Mammaliaformes, but these traits are difficult to study in the fossil record. Evidence of lactation is present in morganucodontans, via tooth replacement patterns. Combined with the more basal tritylodontids that also display evidence of lactation, this seems to imply that milk is an ancestral characteristic in this group. However, the fairly derived ''Sinoconodon'' appears to have uniquely discarded milk altogether. Prior to hatching, the milk glands would provide moisture to the leathery eggs, a situation still found in monotremes. The early mammaliaforms did have a harderian gland. In modern mammals, this is used for cleaning the fur, indicating that they, contrary to their Cynodont ancestors, had a furry covering. An insulative covering is necessary to keep a homeothermy, homeothermic animal warm if it is very small, less than 5 cm (1.97 in) long; The 3.2 cm (1.35 in) '' Hadrocodium'' must have had fur, therefore, but the 10 cm (3.94 in) ''Morganucodon'' might not have needed it. The docodont ''Castorocauda'', further removed from crown group mammals than ''Hadrocodium'', had two layers of fur, guard hairs and underfur, as do mammals today. It is possible that early mammaliaforms had ''Whiskers#Evolutionary biology, vibrissae''; Tritheledontidae, a group of Cynodonts, probably had whiskers. A common ancestor of all therian mammals did so. Indeed, some humans even still develop vestigial vibrissal muscles in the upper lip. Thus, it is possible that the development of the whisker sensory system played an important role in mammalian development, more generally. Like monotremes today, the legs of early mammaliaforms were somewhat sprawling, giving a rather "reptilian" type of gait. However, there was a general tendency to have more erect forelimbs, forms like eutriconodonts even having a fundamentally modern forelimb anatomy while the hindlimbs remained "primitive"; this tendency is in some effect still seen in modern therian mammals, which often have more sprawling hindlimbs. In some forms, the hind feet likely bore a spur similar to those found in the platypus and echidnas. Such a spur would have been connected to a venom gland for protection or mating competition. '' Hadrocodium'' lacks the multiple bones in its lower jaw seen in reptiles. These are still retained, however, in earlier mammaliaforms. With the possible exception of ''Megazostrodon'' and ''Erythrotherium'' (as well as placental mammals), all mammaliforms possess epipubic bones, a possibly synapomorphy with tritylodontids, which also have them. These pelvic bones strengthen the torso and support abdominal and hindlimb musculature. They, however, prevent the expansion of the abdomen, and so force species that possess them to either give birth to larval young (as in modern marsupials), or produce minuscule eggs that hatch into larval young (as in modern monotremes). Most mammaliforms, therefore, probably had the same constraints, and some species could have borne pouches.
PhylogenyThe cladogram below follows the analysis of Luo and colleagues in 2015 in paleontology, 2015. ''Expanded from above'' Cladogram based on Rougier et al. (1996) with ''Tikitherium'' included following Luo and Martin (2007).
See also* Evolution of mammals * Permian–Triassic extinction event * Therapsid * Vertebrate paleontology