Diversity and Evolution of the Marsupial Mandibular Angular Process
Marcelo R. Sanchez-Villagra1'2 and Kathleen K. Smith1
1 Department of Biological Anthropology and Anatomy, Duke University Medical Center, Box 3170, Durham, North Carolina 27710.
2 To whom correspondence should be addressed.
A medial inflection of the mandibular angular process is present in most marsupials. The few living marsupials that lack this trait either are very specialized forms (e.g.,Tarsipes) or show a medial inflection at some point in development that is lost in later ontogenetic stages (cf. Dactylopsila and Phascolarctos). A medially inflected angular process is not present in any known extant or extinct placental (including all Cretaceous taxa that preserve the back of the dentary bone). Some extant placentals with enlarged auditory bullae evolved a medial flange of the angular process as a strategy to increase gape, but this is not homologous to the marsupial condition. We conclude that the medially inflected angular process is a shared derived trait of extant and extinct marsupials. The significant diversity in the form of the medially inflected mandibular angular process in marsupials, documented here for 53 taxa, shows a general relation to dietary adaptations. Herbivores (with well-developed masseter and medial pterygoid muscles) tend to have a shelf-like angular process, while small, insectivorous marsupials generally have a rod-like angular process. A close connection between the angular process and the ectotympanic is maintained during early postnatal development in all marsupials examined, a relation not seen in the placentals examined. A previous hypothesis suggested that the angular process plays a role in hearing in pouch-young Monodelphis. Data on the maturation of the auditory system does not support this hypothesis. Currently there are no data on differences in muscular anatomy or mastication between marsupials and placentals that could serve as a causal explanation for the difference in adult form of the angular process between the two groups.