Cranial characters associated with the proboscis postnatal-development in Tapirus (Perissodactyla: Tapiridae) and comparisons with other extant and fossil hoofed mammals

Abstract

Among extant hoofed mammals, only elephants and tapirs exhibit a true proboscis. We investigated the development of cranial characters potentially associated with the presence of a short proboscis in tapir species, following a comparative framework with a selection of extant and fossil hoofed mammals that exhibit modified narial structures. The core characters present in all taxa (with or without true short proboscis) develop prenatally in tapirs, and other characters that complement the osteological “proboscis syndrome” develop postnatally and are absent or irregularly distributed in other taxa. The taxa examined here showed varying degrees of cranial modification. On the basis of our comparisons, some fossil taxa are indicated as candidates to have possessed a true short proboscis. A terminal addition sequence of character evolution is suggested to explain the development and convergent evolution of the proboscis in hoofed mammals.