Mammal - Distinguishing Features

Distinguishing Features

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 (another small skull 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 the quadrate (a small bone at the back of the upper jaw).

• Middle ear - 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 therapsids.

• Tooth replacement - Teeth are replaced once or (as in toothed whales and murid rodents) not at all, rather than being replaced continually throughout life.

• Prismatic enamel - The enamel coating on the surface of a tooth consists of prisms, solid, rod-like structures extending from the dentin to the tooth's surface.

• Occipital condyles - Two knobs at the base of the skull fit into the topmost neck vertebra; most tetrapods, in contrast, have only one such knob.

For the most part, these characteristics were not present in the Triassic ancestors of the mammals.

For palaeontologists who define Mammalia phylogenetically, no limit can be set on the features used to distinguish the group. Any feature may be relevant to a fossil's phylogenetic position. Palaeontologists defining Mammalia in terms of traits, on the other hand, need only consider those features that appear in the definition. The dentary-squamosal jaw joint is generally included.