For an explanation, see the main splats page
- Mammals make a natural group which subdivides into three, based on their methods of reproduction: the monotremes, the marsupials and the placentals.
- All mammals are warm-blooded, they all have at least a few modified hairs, and they generally care for their young. They are usually classified by their teeth.
- For each mammal, it is possible to write a dental formula, describing the numbers of each of the different types of teeth in each jaw.
- Teeth last long after an animal dies. While teeth are useful in identifying the groups that extinct mammals belong to, there are a few inconsistencies.
- According to the dental formula of a koala, it is some sort of fat possum. The molecular evidence, on the other hand, suggests that it is an arboreal wombat.
- All of the mammals other than monotremes are viviparous, bearing their young alive. Monotremes are the only furry and warm-blooded egg-layers.
- Monotremes have many reptilian features, including egg-laying, their gait as they walk, their egg-laying, and their body temperature, which is more variable.
- Marsupials have common features: most of the females (but not all) have pouches in which the young are carried, and the reproductive systems are distinctive.
- Placental mammals have their young develop within the uterus, getting nourishment from a placenta, which is fetal tissue in contact with the mother's tissues.
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