It has been shown to occur naturally only in birds and placental mammals.
Within placental mammals, the size and complexity of the neocortex increased over time.
Since human settlement many placental mammals have been introduced to Australia and are now feral.
All placental mammals have a navel, and it is quite conspicuous in humans.
Most adult placental mammals have no remaining trace of the cloaca.
There is a big difference between placental and marsupial mammals.
Xenarthrans share several characteristics that are not present in other placental mammals.
"This may give us the first really good anatomy of placental mammals that far back."
The tooth is from a placental mammal, and it is 55 million years old.
