Phys.org has an interesting article about the structure of mammal jaws.  Vertebrate skeletons and thus mammal skeletons are very conservative.  We all have pretty much the same bones, however they are generally reshaped, reoriented, and possibly repurposed.  The general vertebrate lower jaw consists of multiple bones, for mammals some of the smaller bones around the joint have been repurposed into inner ear bones, leaving just one bone to serve as the mandible.  A new paper in the journal Philosophical Transactions of the Royal Society B: Biological Sciences looks into this evolutionary development from the viewpoint of jaws rather than the ear, which has been the traditional approach.

So why did mammals lose the extra bones in their lower jaw? Well, they didn’t. Instead, the extra bones that vertebrates had in the lower jaw, which were clustered around the hinge between the lower and upper jaw, evolved into the mammalian inner ear, perhaps giving mammals better hearing than their vertebrate cousins.

“A solid, stiff jaw in mammals is thought to be a side effect of establishing a uniquely mammalian hearing system,” Tseng said.

Co-opting these jaw bones into the ear left mammals with only one lower jawbone per side, making for a rigid jaw that gave mammals some advantage in terms of stiffness—enough to crack bones, for example—but limited their descendants to variations on a single bone, even when a stiff lower jaw was not needed to eat soft food. Anteaters, for example, evolved a down-curving jaw that serves as a slot for their long tongue to slide through.

To date, this major evolutionary transition in mammals—to a complex inner ear, but simple jaw—has been studied primarily as it relates to the ear.