What are Monotremes? Monotremes are egg-laying mammals that maintain a relatively low body temperature and share some skeletal features with early mammals. These traits suggest that monotremes have been evolving on their own, apart from placentals and marsupials, for some time. There are only three living species, which are found only in New Guinea and Australia: the platypus and two species of echidna, the “spiny anteater.” There have been very few fossils of monotremes discovered to date, but the oldest fossils, some jaw fragments, have been found in Early Cretaceous rocks.
First known fossil occurrence: Cretaceous.
Last known fossil occurrence: Quaternary. This group has living relatives.
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