In one sense, mammals are broken up into three types: monotremes, marsupials, and placentals. The monotremes are some of the oldest (by fossil record), probably developed before either of the other two, lay eggs, and produce milk but have no teats--they secrete the milk like sweat. These are the platypuses and echidnas. The marsupials are the pouched mammals, mostly associated with Australia, but opossums are quite successful on the American continents. Placentals are pretty much everything else, and get their name from the internal placenta that keeps the fetus safe from the mother's immune system, as by definition, a fetus acts like a parasite.
But really, I just wanted to list out the ten orders that make up about 90-95% of mammalian species. While the class Mammalia has about 25 to 30 orders, the top ten make up the vast majority of species.
If this kind of crap bores you, please skip this. I need to clean some natural science rust from the brain. I like this stuff.
The biggest order are the rodents(1), which make up almost 40% of all mammalian species. Next, surprising I thought, are the bats(2), which make up almost 20% of mammalian species. The next two are rather close in percentages, shrews(3) and primates(4). I guess it was a coup of sorts to wrest the shrews away from the rodents, but I guess that could have given rodents almost half of all mammal species. The next order, and only slightly smaller than shrews and primates are the carnivora(5), which have two subdivisions, feliforma and caniforma, basically your cats and dogs, in all forms, and which I plan on doing a special post about, just to clear some thoughts. Next are the even-toed ungulates(6), your pigs, goats, sheep, cattle, hippos, camels, but not horses. The fossil record seems to show that the odd-toed ungulates were by far more varied and successful, while the even-toed species filled special niches, until something happened, and now the tables are switched, with horses, zebras, and tapirs among the remaining species of odd-toed ungulates, and the variety has bloosomed in the even-toed. The next order in the list are Australian marsupials(7), and here the species are extremely varied, but doesn't include the marsupial predators. Some placental shrews were able to develop in Australia. Rounding out the list are rabbits/hares(8), opossums(9), the North and South American marsupial species, and the ceteceans(10), the whales, both baleen and toothed.
Cool guys like elephants, sloths, and tasmanian devils didn't make the list.
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