Of tusks and trunks: A review of craniofacial evolutionary anatomy in elephants and extinct Proboscidea.

While being the largest living terrestrial mammals, elephants are best known for their highly modified and uniquely elaborate craniofacial anatomy-most notably with respect to their often-massive tusks and intricately muscular, multifunctional proboscis (i.e., trunk). For over a century, studies of extinct proboscidean relatives of today's elephants have presented hypotheses regarding the evolutionary history of the crania and tusks of these animals and their bearing on the evolution of the proboscis. Herein, I explore major functional characteristics of the proboscidean head. I give a brief review of the anatomy of tusks and dentition, the feeding apparatus, and proboscis in extant elephants and explore their overall bearing in elephant feeding behavior as well as other aspects of their ecology. I also review the evolution of the proboscidean head, with a synthetic analysis of studies and further speculation exploring the interconnected evolutionary roles of tusk morphology and use, feeding anatomy and functional implications thereof, and proboscis anatomy and use in the ancestry of elephants. Notable emphasis is given to the evolutionary role of initial elongation of the mandibular symphysis in the development of the proboscis in many proboscideans. Subsequent secondary shortening of the symphysis and elevation of the temporal region and occiput allowed for a pendulous trunk and proal feeding in living elephants and other proboscidean groups with highly lophodont dentition.