ANATOMICAL DIFFERENCES BETWEEN MARSUPIAL AND PLACENTAL MAMMALS

The marsupial skull generally has a large face area, but only a small brain case. The rest of the skull is similar to a primitve mammal, with the eye socket and opening for the temporal muscles running together. Where the skulls of marsupials do differ from placentals is that typically the rear part of their jaw is turned inward (inflected jaw angle) rather than outward.
We also see a difference in the dentition of marsupials. Marsupials on the whole have more teeth than placental mammals. Marsupials may have 3 premolars and 4 molars on each side in both the upper and lower jaw (P3/3; M4/4). In placental mammals the general dental formula is 4 premolars and 3 molars on either side in both the upper and lower jaw (PM 4/4; M 3/3).
Marsupials which have 4 or more lower incisors are termed polyprotodont, for example the bandicoot which is insectivorous, has small even teeth of equal size, with sharp cusps for crushing insects. The bandicoot's dental formula is I4-5/3; C1/1; PM3/3; M4/4.
Whereas placental mammals typically have two sets of teeth, marsupials replace only some of their teeth.

A Bandicoot skull.

Marsupials are most easily distinguished from Eutherian mammals by the possession of a pouch. Although the typical image of a female marsupial is of an animal with a single offspring living in a pouch on the front of her abdomen, there are many types of pouches and some species do not have a pouch at all.

Most members of the families Didelphidae and Dasyuridae have large litters of 8 or more, with the pouch only being a flap of raised skin around the nipples. Even so, the young are able to remain firmly attached to the nipples until they are old enough to be left in the nest whilst the mother goes out to feed. The photo to the right shows the stages in development in the opossum. The young can be left in the nest when they are at the developmental stage seen at the bottom of the picture. In bandicoots and some of the smaller diprotodonts, the litter of 2-4 young are protected by a large fold of skin which more closely resembles the 'typical' pouch. The young are carried until they can be left on their own in the nest. This period typically corresponds to the time when the young are too heavy for the mother to carry. Hence the smaller the number of young born in each litter the longer the mother is able to carry them. The third type of pouch completely covers the young. These pouches tend to be home to one young only at a time. The front opening pouches of active climbers or jumpers, such as the kangaroo, are good examples. Rear opening pouches are more common in digging and burrowing marsupials such as the wombat, and help to protect the young from the dirt.

Anatomy of Reproduction

The reproductive anatomy of marsupials also distinguishes them from Eutherian mammals. The female reproductive system is very unusual in that it is almost entirely doubled. Females have a posterial urogenital sinus which recieves two lateral vaginae and the urethra. One of a pair of uteri and then a cervix leads from the top of each vagina. Fertilisation occurs via either of the vaginae. At the time of birth the two vaginae fuse to form a 'median vagina' or 'pseudo birth canal'. In some species, e.g. the brushtail possum, the septum which was breached during birth reforms; in others, e.g. the grey kangaroo, this median canal remains open permanently.
The reproductive system of the male is much more similar to Eutherian mammals than that of the female. The main differences in the male are external rather than internal and comprise a bifurcate penis which is posterior to the scrotum. When flaccid, the penis is held in an S-shaped curve withdrawn into the body.