Mammal Anatomy – An Illustrated Guide is available to download free in pdf / epub format.
Living mammal species can be identified by the presence of sweat glands, including those that are specialized to produce milk to nourish their young. In classifying fossils, however, other features must be used, since soft tissue glands and many other features are not visible in fossils.
Many traits shared by all living mammals appeared among the earliest members of the group:
- Jaw joint – The dentary (the lower jaw bone, which carries the teeth) and the squamosal (a small cranial bone) meet to form the joint. In most gnathostomes, including early therapsids, the joint consists of the articular (a small bone at the back of the lower jaw) and quadrate (a small bone at the back of the upper jaw).
- Middle ear – In crown-group mammals, sound is carried from the eardrum by a chain of three bones, the malleus, the incus and the stapes. Ancestrally, the malleus and the incus are derived from the articular and the quadrate bones that constituted the jaw joint of early therapists.
- Tooth replacement – Teeth can be replaced once diphyodonty not at all monophyodonty.
- Elephants, manatees, and kangaroos continually grow new teeth throughout their life.