You are referring to the Mishnah (126b) in which Rebbi Yehudah and the Rabbanan discuss a rodent which is formed "of its own" from a clod of earth. At one stage it is part earth and part rat. The existence of such a rodent certainly seems farfetched, in any case I don't think anyone will tell you that one who does not accept its existence is excluded from the Jewish religion. Depending on whom you consult, you may be shown one of many approaches on the subject, ranging from "Chazal accepted many of the scientific claims of the Greeks, which later were found to be mistaken," to "Who says today's scientists are more correct than Chazal -- they make new discoveries all the time!" Take whichever approach you like, there's no reason for any of them to shake one's religious foundations.

Interestingly enough, in the times of Chazal it was so broadly accepted that such a creature existed that Rav Ami used it's existence as irrefutable proof of Techiyas ha'Meisim, when confronted by an Apikores (SANHEDRIN 91a). Even more interesting, the RAMBAM writes in his Peirush ha'Mishnah (to our Mishnah) "The existence of such a rodent is well known; I can't number the amount of people who have told me that they saw such a creature -- even though it is utterly bizarre and has no physical justification whatsoever." TIFERES YISRAEL (Boaz to Mishnayos Chullin 9:6) continues the thread, saying that those who ridicule the Mishnah's discussion of a half-rat should be aware that the great naturalist Link, in his book "Urvelt," (Vol. I p. 327), mentions that such a rat lives in the area of Thebes (Egypt) and is known as Dipus Joculus (or Springmouse). (I heard from Dr. S.Z. Leiman, however, that the section of "Urvelt" which the Tiferes Yisrael is quoting is discussing Pliny the Elder's legendary creatures, it is not an actual reference to known animals.)

I would think that the Rambam's friends must have seen something, although it is not exactly clear what. If you'd like to know my own impressions on this matter, here goes: It is well known that certain species of small amphibians and mammals burrow in the mud and entomb themselves in a cocoon of solidified mud during the dry season. Entombed, the creatures bodily functions nearly cease and they remain entirely motionless until the first rain, at which point they shed their cocoon. If you were to see the picture of such a creature as it appears in Scientific American (March '94, p. 66) I think you will agree with me that when entombed it unquestionably appears as a clod of earth, and while leaving its cocoon (the South American toad in the picture, Lepidobatrachus laevis, pulls it over its head and eats it) it certainly looks as though it is half-earth half-creature. (The article informs us that various species in Africa also form such cocoons.)

Halachically, I'm not even sure that such a creature would not be granted the status of "newly born" as it emerges from its shrine. If it becomes dry and solid while buried, it may not be considered a living creature while entombed; when it emerges it returns to life. If so, there is no reason that the rat of Thebes should surprise us as naturalists nor as halachists. As for other instances where the belief in spontaneous generation may affect the Halachah, I think that each instance must be judged on its own -- but that, I'm afraid, will have to wait for another occasion.

Be well,

-Mordecai