That's not implying the avian-like genes are actually avian. It's possible those genes lay dormant in early vertebrates (ancestors of both mammals and dinosaurs/birds) until selected upon when reproduction became more complicated. Covergent evolution and the like.
As for the general evolution, platypuses show characteristics of both mammals (protolactation, fur, endothemry), and reptiles (soft-shelled eggs, venomous spurs), so it is likely to have evolved around the time that mammal-like reptiles (http://en.wikipedia.org/wiki/Synapsid) were replacing dinosaurs, then filled a niche so well it didn't need to continue down the mammal path. Same with the echidna. Then of course Australia broke off Gondwana and marsupials evolved, where (almost) everywhere else eutherians evolved.
Here's an interesting but simplistic video: http://www.youtube.com/watch?v=4hG4dcTj408
As for the general evolution, platypuses show characteristics of both mammals (protolactation, fur, endothemry), and reptiles (soft-shelled eggs, venomous spurs), so it is likely to have evolved around the time that mammal-like reptiles (http://en.wikipedia.org/wiki/Synapsid) were replacing dinosaurs, then filled a niche so well it didn't need to continue down the mammal path. Same with the echidna. Then of course Australia broke off Gondwana and marsupials evolved, where (almost) everywhere else eutherians evolved.
This is also very good reading: http://en.wikipedia.org/wiki/Evolution_of_mammals - don't just read the article, click and read the references at the bottom too.
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