Looking at the base of the cladogram (think of it as a kind of family tree) of crown mammals, we have monotremes. Monotremes are the most basal mammals alive today, and they include just five animals: one species of platypus, and four species of echidna. Very few monotreme fossils have been found, and the earliest are all from the Cretaceous of Australia. The earliest, Teinolophus, was already a fully evolved platypus, suggesting monotremes evolved earlier, possibly in the Jurassic. Monotremes are considered to be basal mammals because of many features, including the fact that they still lay eggs, unlike other mammals.
|The very strange duck-billed platypus. Image by Maksim|
After a few minor extinct groups, we have the therians. This includes the two mammal groups we typically think of: the marsupials and placental mammals. Marsupials are the living representatives of Metatheria, which also include some fossil taxa. The first metatherian, Sinodelphys, lived during the Late Cretaceous of China, about 125 million years ago. Modern groups like opossums evolved during the Late Cretaceous, while diprotodonts (kangaroos, koalas, wallabies, etc.) first appeared in the fossil record during the Oligocene, about 25 million years ago. This is likely review for many people, but marsupials mainly differ from placental mammals in way of their reproduction. Marsupials do not have a placenta, and have very short gestational periods, causing them to "give birth" to very young, under developed offspring. The offspring are then kept in the mother's pouch, where they continue to develop for several months.
|Artists impression of Diprotodon, an extinct marsupial. Image by Nobumichi Tamura|
|Fossil of Icaronycteris, the earliest known fossil of a bat. Image by Andrew Savedra|
|Skeleton of a modern bat, which is not significantly different from the early fossil bat above. Image by Mnolf.|