what i meant, and i hope im explaining it right, was that given a particular atmospheric ratio of c14/c12 at a given temperature, a given type of creature will take in a steady ratio of c14/c12, closely matching that found in the environment (not exactly, but thats another story belonging to a discussion on paleoceanography) and since it is continually processed while alive, it remains the same during the creature's lifetime.
your comment seemed to be saying (perhaps i misunderstood) that despite differences in c14/c12 ratio in the atmoshpere in different eras or due to nuclear testing, creatures had some kind of equilibrium that was independant of environmental variations. that was what i was commenting on. as near as i understand, the ratios are entirely dependant on the environmental ratio and temperature and this ratio does fluctuate, and does cause radiocarbon dates to be wrong, that is, until they are calibrated.
now im getting a headache again...
mox