Proboscidean jaw mechanics and the evolution of the proboscis and feeding apparatus in extant elephants

The proboscis, or trunk, in extant elephants is a rostral elongation of narial and upper lip musculature and has been suggested to have evolved with the lengthening of the mandibular symphysis in more basal proboscideans. The secondary shortening of mandibular symphyses in derived proboscideans is said to be associated with the evolution of a more vertical line of action of the temporalis muscle. It has been suggested that the caudally oriented temporalis in more basal proboscideans produced a caudodorsal power stroke whereas the derived vertical temporalis seen in extant elephants allows the masseter to produce a more proal, or rostrally oriented, power stroke. Given the absence of soft tissue preservation in extinct proboscideans, the temporalis line of action is used as a proxy to elucidate the supposed connection between jaw mechanics and proboscis evolution. In this study, lever arm analyses were performed, using lateral views of skulls, to quantify temporalis line of action and compare mechanical advantage of the temporalis across proboscidean taxa. Results corroborate previous hypotheses of temporalis lines of action and power stroke orientations. Lever arm analyses show no clear correlation between the orientation of temporalis line of action and relative bite forces across the tooth row. In many cases, taxa with a secondarily shortened mandibular symphysis show smaller relative bite forces across the tooth row. Given these mechanical consequences of shifting temporalis orientation, it can be inferred that masseter and pterygoid muscles became more pronounced in derived proboscideans, assuming they were to maintain stronger bite forces as a whole. This shift in jaw mechanics conveys a clear change in feeding behaviors throughout taxa, giving insight into the evolution of proboscis use in extinct proboscideans.