A geometric morphometric analysis of cranial and mandibular shape variation of didelphid marsupials

Abstract

The New World marsupial family Didelphidae is one of the oldest among mammals and is usually regarded as a morphologically conservative group. We analyzed cranial shape vari- ation among six species of the six largest living genera of the family using two-dimensional land- mark data. We captured and digitized video images of the skull and mandible for the following species: Caluromys philander (n = 65), Chironectes minimus (n = 30), Didelphis aurita (n = 70), Lntreolinu cwssicmdata (n = 37), Metachirus nudicuudatus (n = 77) and Philander frenuta (n = 62). Fourteen landmarks were defined for the lateral, 25 for the ventral, 23 for the dorsal views of the skull, and nine on the mandibular lateral view. Sex, species, and interaction effects were ana- lyzed with a two-way MANOVA on the matrices of coordinates aligned by general least squares. All four views had significant interactions. Canonical Variates Analysis was performed on sexes and species, and shape was regressed on the canonical variate scores for each species. Culuromys philander was clearly the most distinct species, with paedomorphic features that can be related to its arboreal habits. A conspicuous shortening of the rostrum distinguishes the highly carnivorous Lutreolinu r.rassic,audatn. Didelphis aurita and Philundel- fi-enata overlapped somewhat, reflecting shape similarities associated with their phylogenetic affinities, while the few differences observed are probably allometric consequences of size differences. P hilundu fienufa and Chironcctes min- inzus showed similar cranial shapes, while MPtuc,hirus nrrdicaudatlrs was distinctive with a broad and elongated rostrum. In spite of an overall similar shape, the geometric morphometric approach revealed several marked differences among species that can be related to their phylogenetic origin and their adaptive zone.