According to the author of the fascinating book, THE GREAT DYING, whose name I cannot recall off hand, the egg casings of ammonites were primarily calcium carbonate, while the egg casings of nautiloids were primarily magnesium carbonate. Thus, as a result of acid rain, the oceans became sufficiently acid as to dissolve egg casings of calcium carbonate (those of ammonites), while egg casings comprised of magnesium carbonate (those of nautiloids) endured sufficiently to allow the nautiloids to reproduce right on through the periods of unusually acid oceans.