The evolution of the irreducibly complex mammalian ear bone.
First, the mammalian middle ear has three bones (incus, stapes, and malleus) where the removal of any one of these bones results in the middle ear no longer functioning.
Reptiles have a single middle ear bone and three lower jaw bones. Mammals have three middle ear bones and a single lower jaw bone. Two of those jaw bones evolved to become middle ear bones, and we have the step by step process preserved in the fossil record.
The lower jaw bones evolved to take on a new function as part of an irreducibly complex system, and we can see the steps in the fossil record.