These 18 families are placed in 7 orders. This classification relies heavily on new information from studies of molecular biology, but it is increasingly supported by studies of morphology and the fossil record. This system is a substantial improvement over the old scheme, but we suspect that the last word about the higher classification of marsupials remains to be written. Early diversification of the metatheres is thought to have taken in North America, although by the middle Miocene the lineage became extinct on that continent, only reappearing around the time that North and South America regained contact in the Pliocene. A few fossil marsupials are known from Europe, Africa, and Asia, but clearly this group was never well established on those continents.
Marsupials differ from placentals in a number of important and obvious ways.
The braincase is small and narrow. It houses a relatively small and simple brain compared to that of similar-sized placentals. The jugal is large, extending posteriorally so that it actually contacts and forms part of the glenoid fossa . The lacrimal canal is slightly anterior to the orbit so that it opens on the surface of the face rather than inside the orbital space. The bullae are sometimes not ossified, and when they are, they are formed largely by extensions from the alisphenoid .