How can one tell if a prehistoric mammal was a marsupial?

Question

For example, how can paleontologists tell if the Marsupial Lion had a pouch? What kind of evidence is there to tell from a Placental from a Marsupial from millions of years ago?

Answer 1

As Howard H said there are skull differences. But I think you need to be convinced specifically how a scientist knows the animal is placental or marsupial...so here it is....

Placental mammals bear live young. These young must pass through an opening in the pelvis. Thus they have a few features that marsupials do not have. Without these features they could not bear young the size placental mammals do. One of those features is the opening in the pelvis is larger. Another is that the joint between the coxa (the two pelvis bones) in front is made of fibro cartilage which allows the joint to flex so that the young can fit out the opening. Thus a marsupial 'lion' would have a different pelvis than an African lion.

In fact a scientist can tell the pelvis of a female placental mammal from the pelvis of the male of the same species.

Answer 2

Without going into technical detail, there are features of the skull bones that distinguish a marsupial from a placental. There is also the tooth pattern. Marsupials have more premolars than molars in each jaw; placentals have more molars than premolars.