Let's start with the following statement:
Everything that bears live young, nurses them, and everything that lays eggs, gathers food for its young, except for the bat, which, even though it lays eggs, nurses its young. (Bechoros 7b)
You might ask a similar question about the continuation of the Gemara, which states that the only living things that copulate face-to-face are people, snakes and fish. As far as I am concerned, this is a valid statement. Yet the more zoologically knowledgeable of you might be wondering: What about the bonobo and stitchbird?
But as far as I'm concerned, platypus and echidnas and bonobos and stitchbirds do not present a problem. They are obscure animals from remote regions. Chazal never in the first place meant to be giving an absolute statement covering all species in the universe. True, Chazal did not know about the platypus or echidna or bonobo or stitchbird, but if you were to go back in a time-machine and tell them, they would justifiably shrug them off as irrelevant.
This is something that I explained at great length in my book The Camel, The Hare And The Hyrax. It is based on the Gemara's own principles that one does not need to concern oneself with rare cases (miyuta d’miyuta), and the principle that ain lemedin min haklalos, “we do not take general rules as being absolute,” and they can have exceptions. On numerous occasions, the Rishonim themselves observed that there were exceptions to the Talmud’s seemingly absolute statements about factual reality. They pointed out that such minor exceptions do not undermine these rules because they were not intended to be absolute in the first place. And R. Yonasan Eybeschitz and R. Yaakov Tzvi Mecklenberg make precisely this point about the Talmud’s seemingly absolute rule that all fish with scales have fins.
I applied this principle to further cases. The Gemara states that there are only four animals with one kosher sign; I argued that there are further types, but these do not undermine the Gemara's statement. Likewise, I argued that the principle of psik raisha velo yamus would not be undermined by discovering a headless chicken that survives.
But there are those who are vehemently opposed to my approach. They insist that if the Gemara states a principle, it is absolute and can have no exceptions. According to them, when Chazal said that there are four animals with one kosher sign, there cannot be any others; when Chazal said that every fish with scales has fins, this is an absolute principle that demonstrates confidence in supernatural wisdom. For these people, then, the platypus, echidna, bonobo and stitchbird contradict Chazal's principles. How ironic!