Talking about mammals ...
Today, there are three types of mammals on Earth. One is marsupials, which carry their offspring in pouches. Kangaroos and opossums are marsupials.
Platypuses and echidnas are monotremes, which
lay eggs. Like birds, Monotremes only have one body opening, for excretion, urination and reproduction.
The third type of mammal – placental mammals – are the most common. Humans and animals such as prehistoric rats are placental - they feed their embryos via a placenta.