The half-life of uranium-235 is 710 million years. uranium-235 decays to lead-207. you are radiodating a fossilized organism, and find that 0.5 grams of uranium-235 and 0.5 grams of lead-207 is present in the fossil. that means the fossil is years old and lived during the eon. based on this age, you know this fossil is too old to be a(n) based on our current understanding of the fossil record.