Bats are mammals. How can we explain the Gemara that says that they lay eggs [and nurse]. Is it possible they were referring to a platypus, which is in fact the only animal to lay eggs and nurse its young (obviously Rashi didn't learn it this way, but could the Gemara have known about Australian wildlife?)
I don't think the Gemara can be referring to a platypus. The word used in the Gemara (Atalef) is used in the Torah as well as in many places in Shas, and it can clearly be identified with the nocturnal bat. As for the eggs, that's a very good question. Since bats are fairly common, it seems obvious that Chazal are referring here to a phenomenon that they themselves witnessed. There must be something that looks like an egg (or has the halachic status of an egg) which is produced by a bat.
I'm afraid that I don't have the background to answer your question from a scientific perspective, perhaps one of our readers has an insight on this matter.
L'hitra'ot!
Mordecai Kornfeld