There are three groups of modern mammal placental (which carry their young inside themselves for the full pregnancy, like ourselves), marsupials (which give birth to under-developed young which are then carried in the pouch, like the kangaroo) and monotremes (the egg laying mammals like the duck-billed platypus and the echidnas). These groups seem to have diverged from a common ancestor in the Jurassic or early Cretaceous.
Placental mammals, like ourselves, carry their young inside their body until they are almost fully formed. There are a huge number of placental mammals alive today, but recent DNA analysis has given us a great insight into how they are related to each other. It seems that they split from the marsupials in the Late Jurassic or Early Cretaceous. During the Cretaceous, around 100 million years ago, the placentals themselves split into several groups, of which four survive today. As the southern continents began to split apart, the 'Xenarthra' were restricted to South America, and the 'Afrotheria' to Africa. In the northern hemisphere the ancestors of the rodents and primates, and those of the 'Laurasiatheria' also diverged, giving rise to the four major groups that exist today.
The DNA analysis doesn't help us understand how many of the extinct animals we find in the fossil record. Many of these are so similar in some ways to modern animals and their ancestors that we can confidently trace their origins. Others, though, are quite unusual and it will not be until we find more fossils that we can understand their relationships with other groups of mammals.
http://www.abc.net.au/beasts/familytree/
