"It may be guessed that, wherever they originated, monotremes radiated in Australia before marsupials did, and that the old Australian monotreme fauna may have been at least as diverse as the ...

Clockwise from lower left: Opalios splendens, a newly described species dubbed an ‘echidnapus’; Stirtodon elizabethae, the largest monotreme of the time; Kollikodon ritchiei, with hot-cross ...

Even in monotreme mammals, like platypuses and echidnas, XY chromosomes are ordinary. Within the last 166 million years, the ...