It's not yet clear why the isotopes should be different in bones and teeth, but DeSantis said it might have something to do with the enamel on the latter. "Enamel is highly inorganic, so it's less prone to alteration after it's been deposited [in a tar pit]," she said. That makes enamel the gold standard in terms of preserving isotopes.

My guess was exposure to solar radiation, which is why a radioactive difference was mentioned.

The ice age was ended by a massive solar outburst. It all went down in a matter of days, or possibly weeks.

The Earth's magnetosphere was overwhelmed and the megafauna died of radiation sickness. The enamel retained the evidence.