Identifying a Metatherial Skull (1)

The first step in identifying which order and family a skull in Subclass Theria belongs to is to identify its infraclass. There are two characteristics that almost all metatherians share that set them apart from eutherians: they have distinctive openings on the underside of the top part of their skulls called palatal vacuities, and the angular process (a projection at the back on the base of the mandible) is reflected, which means that it is angled inwards. In eutherians the angular process is in line with the other features of the back of the mandible (the coronoid process and the mandibular condolyte) and there are no palatal vacuities. If these features are present, the next step is to look at the teeth and identify what type of dentition is present. There are two possibilities: polyprotodont dentition and diprotodont dentition. In polyprotodont dentition, the mandible is not shortened and the lower incisors are small and unspecialized, whereas in diprotodont dentition the mandible is shortened and the first pair of lower incisors are enlarged and jut forward to meet the upper incisors. If the dentition if diprotodont the skull belongs to Order Diprotodontia, but if it is polyprotodont it could belong to Order Dasyuromorphia, Order Didelphimorphia, or Order Peramelidae.