Why are marsupials primitive mammals?

Answer 1

A misnomer posed by some earlier poster is that it is like a reptile. The correct answer is, it is not considered the most primitive mammal. This was incorrectly presumed by 19th century scientists. It is actually uniquely evolved.

If you're attempting to find out the most primitive "modern" mammals, they could be argued to be the Soricidae, the shrews and the Talpidae, the moles, but any modern creature is simply not primitive. More basic, more complex, more or less evolutionary divergent from their ancestral norms, but not more primitive.

Answer 2

I know of three ways in which marsupials are less advanced than placental mammals. First, they do not have a complex placenta. They do not need one, as the young are delivered prematurely anyway. Second, marsupials lack a corpus callosum, which connects the two hemispheres of the brain. Third, marsupials urinate and defecate through the same opening (a cloaca), while placental mammals have different openings. There are other differences between marsupials and placental mammals, but only these three portray marsupials as "primitive" as compared to placental mammals.

Answer 3

Monotremes are no longer regard as the primitive mammals they were once thought to be. Scientific thought now recognises their unique adaptations and specialised characteristics as highly advanced.

However, they do have some primitive features, in particular, the fact that they lay eggs. Certain skeletal features are regarded as primitive, for example, the shoulder girdle and skull characteristics that placental mammals and marsupials do not have.