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Animals belong to these clades are found in Australia and New Guinea, with some marsupials additionally being found in south and North America. Both clades are classified as mammals and therefore have the following mammalian characteristics:
- Hair - all mammals process this characteristic,
although in some cases it is not as obvious. However, if you look closely at some of these apparently naked mammals you will notice regions where hair is visible, for example in whales and dolphins you will find hairs around their eyes and snouts. The evolution of hair allowed mammals to regulate their body temperature and invade cooler climates that other animals, such as dinosaurs, were unable to. Hair in mammals can also act as a sensory structure e.g. allowing cats to judge how wide an opening is before attempting to squeeze through, or as a form of protection e.g. the modification of hairs into sharp, stiff spines in hedgehogs.
- Mammary glands - all female mammals process these structures, which are used to secrete milk. Milk is a fluid rich in fat, sugar and protein, making it a highly calorific food and critical for the high-energy needs of a rapidly growing newborn mammal
- A single bone in the lower jaw
- Three bones in the middle ear (the incus, stapes and malleus)
- They are endothermic - produce their body heat internally rather than in reptiles where the body's temperature is regulated by the external temperature.
Marsupials and monotremes have distinct characteristics.
Images used, with permission, from Wikipedia
Author: Catrin Roberts
Last updated: 23.11.83
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