User:Nwbeeson/Characteristics of Mammals

The Characteristics of Mammals are those features of anatomy shared by all members of the clade Mammalia. The living Mammals are a monophyletic clade of animals, descended from one ancestral species. Using these characters, biologists have for centuries, correctly assigned living animal spieces to the Mammalia, and excluded others. There are two lists of characters: a list of hard parts, accessible to paleontologists in fossils; and a list of soft parts tissues, inaccessible in fossils, but accessible to mammologists from current specimens.

Hard Parts

Character	Description
Inner ear bones	At least five bones found in the jaws of other amniotes have moved to skull, and become part of the inner ear.
Absence of squamosal bone.	The squamosal bone fuses with the periotic bone and the auditory bulla to form the temporal bone.
Absence of quadratojugal bone.
Coronoid bone, small or fused.	In many mammals, it is fused to the upper surface of the jaw bone just behind the teeth.
Small stapes bone.	All amnionts have one inner ear bone, homologous to the stapes. It is much larger in relative to the skull.
Absence of Meckelian groove.	The embryonic cartilage from which the jaw bone develops, is completely covered, or absorbed, in mammals, and not in the other animals with jaws.
Pterqoid flange vestigial.	The pterygoid bones are a pair of bones found in the roof of the mothe of amniots, but has fused with the sphenoid bone in mammals.
Absence of proatlas bone.	The proatlas bone has fused with the occipital bone to from the Foramen magnum, the hole through which the spinal cord enters and exits the skull vault.
Atlas bone fused.	Example
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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 (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 the 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 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.