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.