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How did some reptile jaw bones become mammalian middle-ear bones?

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polar bear and cubs — mammal Mother polar bear nursing her cubs (*Ursus maritimus*).

The evolutionary transition from reptilian jaw bones to mammalian middle-ear bones is a classic example of mosaic evolution, where skeletal elements were repurposed to enhance auditory sensitivity.12

In ancestral reptiles, the lower jaw was composed of several bones, with the articular bone forming the jaw joint by articulating with the quadrate bone of the skull. Over a long evolutionary sequence, these posterior jaw bones underwent a gradual reduction in size and shifted their function.31

As the dentary bone expanded to become the sole bone of the mammalian lower jaw, the following reptilian jaw bones were incorporated into the middle ear as auditory ossicles:31

Articular: This bone evolved into the malleus (hammer), which rests against the tympanic membrane.31
Quadrate: This bone evolved into the incus (anvil), which articulates with the stapes.31
Angular: This bone evolved into the tympanic bone, which supports the eardrum.1

This transition increased the mechanical efficiency of the middle ear, allowing mammals to detect higher-frequency sounds compared to reptiles. This shift is a defining diagnostic feature used to distinguish mammals from their reptilian ancestors.312