Paleontological and Developmental Evidence for the Origin of Baleen and the Loss of Teeth in Baleen Whales

Postnatally, modern baleen whales do not have teeth. Instead, they have hundreds of baleen plates implanted in their upper jaw, and this system is used in procuring food. Fetal baleen whales do initiate the development of several dozens of teeth in upper and lower jaws. These tooth germs reach the bell stage and are sometimes mineralized, but toward the end of prenatal life, they are resorbed and no trace remains after birth. Around the time that the germs disappear, the baleen plates start to form in the upper jaw. Early cetaceans, so called archaeocetes, had two generations of teeth and were heterodont. The prenatal teeth of modern baleen whale fetuses are usually interpreted as an evolutionary left‐over. We investigated the development of teeth and baleen in bowhead whale fetuses using histological and immunohistochemical methods. We found that upper and lower dentition initially follow similar developmental pathways. As development proceeds, upper and lower tooth germs diverge developmentally. Lower tooth germs differ along the length of the jaw, reminiscent of a heterodont dentition of cetacean ancestors, and lingual processes of the dental lamina represent initiation of tooth bud formation of replacement teeth. Upper tooth germs remain homodont and there is no evidence of a secondary dentition. After these germs disappear, the oral epithelium thickens to form the baleen plates, and the protein FGF‐4 displays a signaling pattern that foreshadows the placement of baleen plates. In laboratory mammals, FGF‐4 is not involved in the formation of hair or palatal rugae, but it is involved in tooth development. This leads us to propose that the signaling cascade that forms teeth in most mammals has been exapted to be involved in baleen plate ontogeny in mysticetes.