6.3: Early Mammals - Monotremes
- Page ID
- 62159
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Monotremes are mammals that lay eggs. Years ago, biologists often thought of the term 'egg-laying mammal' as synonymous with 'reptile-like mammal' or 'primitive mammal'. Now, with our greater understanding of monotreme biology, these emotive terms are disapproved of, since these animals have so many authentic mammalian features. For example, if echidnas didn't lay eggs, you might be forgiven for thinking of this animal as 'little more remarkable than a rather large and slightly chilly hedgehog'; the hedgehog is a 'true' (or placental) mammal, as you'll see if you study the next course. I've mentioned that the period of development within the egg is relatively brief, but many aspects of reproduction and maternal care in the monotremes are distinctly mammalian. And how long have monotremes existed? A platypus fossil is about 25 million years old, but the oldest monotreme fossil (a jaw bone) is over 100 million years old.