Mammals are derived from the synapsid lineage of reptiles, e.g. pelycosaurs and therapsids. Variety in teeth form was one indication of their relatedness to mammals. Mammalian skin has epidermal and dermal layers and the unique keratinized derivative of hair seated in epidermally-derived hair follicles. While hair is usually for insulation, it may also be for sense, e.g. the vibrissae or whiskers.
Horns have a bony core.
Antlers are all bone without a covering of keratin, but unlike horns, are shed every year. Mammalian teeth are highly variable or heterodont . A diastema is a gap between the anterior and posterior teeth, allowing for an elongated snout, e.g., in deer. Muscles attach at an origin and an insertion , and produce an action . 1,3 Digestive system .