Mammals today can be divided into three groups
Monotremes (Platypus and the Echidna)
Marsupials (Kangaroos, Opossums, Koalas)
The Placentals (Humans, etc)
I was wondering in the past, if there have been any other type of mammals that don’t fit into one of these three groups?
There were Reptomammals, long ago.
During the Mesozoic, there were a variety of mammalian groups whose relationship to the modern three groups is uncertain.
The best known of these groups is the multituberculates, rodent-like animals that originated in the Jurassic and survived to the Oligocene.
Russell Brand.