The proboscis of tapirs (Mammalia : Perissodactyla): a case study in novelnarial anatomy

The trunk-like proboscis of tapirs provides a prime case study in the evolu tion of anatomical novelty. Morphological study of this unique structure wa s undertaken employing several specimens and a combination of analytical te chniques: gross anatomical dissection, radiographic imaging and histologica l sectioning. Evolution of the proboscis of tapirs entailed wholesale trans formation of the narial apparatus and facial architecture relative to peris sodactyl outgroups. This transformation involved retraction and reduction o f the bony and cartilaginous facial skeleton, such that several structures present in outgroups are completely absent in tapirs, including cartilages surrounding the nasal vestibule (e.g. alar and medial accessory cartilages, rostral portion of the nasal septum) and associated musculature (dilatator naris apicalis, lateralis nasi pars ventralis). At the same time, soft tis sues surrounding the upper lip and nose became elaborated to form a mobile, fleshy proboscis. Several key facial muscles (e.g. levator labii superiori s, levator nasolabialis, caninus, lateralis nasi) have been co-opted to fun ction in movement of the proboscis. The nasal vestibule is expanded and occ upies approximately 75% of the nasal cavity. Vestibular expansion has compr essed and simplified caudal components of the nasal cavity (e.g. reduction of dorsal and middle nasal conchae, loss of plica recta and plica basalis). The airway has become dorsally arched causing the ventral conchal complex to become inclined relative to the long axis of the skull. A few anatomical enigmas remain, such as the complicated maxilloturbinate that rostrally co ntacts the nasal septum and vomeronasal organ. Similarly, the meatal divert iculum, despite being both ancient and anatomically complex, has no obvious functional significance; it is clear that it is not homologous to the nasa l diverticulum of horses and other equids. The reduction of the osseocartil aginous portion of the proboscis, coupled with expansion of the muscular an d connective tissue components, has resulted in an organ that is best inter preted as a muscular hydrostat.