The evolution of mammalian auditory ossicles was an
evolution Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes that are passed on from parent to offspring during reproduction. Different character ...

ary event that resulted in the formation of the bones of the mammalian
middle ear The middle ear is the portion of the ear internal to the eardrum, and external to the oval window of the inner ear. The mammalian middle ear contains three ossicles, which transfer the vibrations of the eardrum into waves in the fluid and membran ...

middle ear
. These bones, or ossicles, are a defining characteristic of all
mammals Mammals (from Latin , 'breast') are a group of vertebrate animals constituting the class Mammalia (), and characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region ...
. The event is well-documented and important as a demonstration of transitional forms and
exaptation Exaptation and the related term co-option describe a shift in the function of a trait during evolution. For example, a trait can evolve because it served one particular function, but subsequently it may come to serve another. Exaptations are common ...
, the re-purposing of existing structures during evolution. The ossicles evolved from skull bones present in most tetrapods, including the reptilian lineage. The reptilian
quadrate Quadrate may refer to: * Quadrate bone * Quadrate (heraldry) * Quadrate lobe of liver * Quadrate tubercle of femur See also *Quadrat (disambiguation) {{disambig