One of the traits that is unique to mammals is having a nose. I am not just talking about the nostrils, which lot’s of other vertebrates have as well. It’s the protruding and mobile (at least to some degree) structure of the mammalian face that includes the nostrils. Here a recent paper shows how this structure evolved from structures in the upper jaw.
The upper jaw consists of two major pairs of bones called the premaxilla and maxilla. The premaxiilla is the front-most portion of the upper jaw and at the back is the maxilla.The traditional view was that these bones developed in the same manner in both mammals and non-mammals. But it turns out that during the evolution of mammals, the premaxilla became increasingly smaller migrated upward and was eventually replaced by a bone in between these bones called the septomaxilla, which became the tip of the upper jaw. This caused the nose to seperate from the jaws, allowing mammals to sniff the air.
This is transition is seen in the fossil record, as well as the living monotremes (platypus and echidna).
And in embryological development.