Mammalian face as an evolutionary novelty

The anatomical framework of the jaw has traditionally been thought to be highly conserved among vertebrates. However, here we show that the therian-unique face (muzzle) evolved via a drastic alteration of the common pattern of the tetrapod jaw. Through comparative morphological and developmental analyses, we demonstrated that the therian mammal's premaxilla (rostral-most upper jawbone) is derived from the maxillary prominence of the mandibular arch. The developmental primordium that produces the premaxilla in nonmammalian tetrapods rarely contributes to the upper jaw in therian mammals but rather forms a motile nose. We propose that these previously unrecognized rearrangements allowed key innovations such as the highly sensitive tactile perception and olfactory function in mammalian evolution.