Ontogenetic and phylogenetic transformations of the vomeronasal complex and nasal floor elements in marsupial mammals

Histological sections and three-dimensional reconstructions of section-series were used to document the anatomy of the vomeronasal complex and other aspects of the ethmoidal region in representatives of 13 families and six orders of marsupial mammals, including for the first time Microbiotheria. The changes during growth of several features were examined in ontogenetic series. Marsupials are very conservative in comparison with eutherians regarding the vomeronasal complex. All have a vomeronasal organ and a nasopalatine duct, have no nasopalatine duct cartilage, have no (or just an incipient) palatine cartilage, and the overall construction of the nasal floor is uniform across species. Most features examined show a high degree of homoplasy (e.g. presence of glandular ridges, isolated dorsal process of the paraseptal cartilage), and their systematic value is confined to low taxonomic levels. Significant ontogenetic changes occur in features usually discussed in the systematic/taxonomic literature. Amongst the didelphids examined, Caluromys philander shows several autapomorphies. It is hypothesized that the opening of the VNO into the upper end of the nasopalatine duct was present in the marsupial groundplan. Most marsupials have a large and horizontal anterior transverse lamina, the plesiomorphic condition, which becomes oblique in diprotodontians. Some features are autapomorphies of well-supported monophyletic groups of marsupials, e.g. the conspicuous internasal communication of perameliformes and the 'tube-like' or ring-shaped paraseptal cartilage of vombatiformes. An outer bar joining the middle (and not the dorsal-most portion) of the paraseptal cartilage characterizes Australasian marsupials and Dromiciops, with the exclusion of perameliformes, and evolved independently in Caluromys philander.