A mammal of the group Prototheria, which comprises the monotremes and their extinct relatives.
- ‘Extant mammals, including prototherians (egg-laying mammals) and therians (marsupial and placental mammals), probably last shared a common ancestor about 160 million years ago.’
Relating to or denoting prototherians.
- ‘In his classification of prototherian mammals Hopson queried the allocation of Haramiyidae to the Multituberculata.’
- ‘They had a common ancestor more recently than either had with the monotremes or, probably, any other prototherian group.’