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Monotremes (Monotremata, or egg-laying mammals) are a subclass of mammals distinguished from other extant mammals in that they lay eggs rather than giving birth to live young.
There are three living species:

The monotremes have separated from the therians (marsupials and eutherians) 150 200 million years (My) ago [1].

IMGT reference:
[1] Musser A.M., Comp. Biochem. Physiol. A Mol. Integr. Physiol., 136. 927-942 (2003) PMID: 14667856