It lived around 125 million years ago, based on the dating of radioactive elements in the sediments that encased the fossil.
研究者通过对包裹化石的沉积层进行放射性元素年代测定，推断出中华龙鸟生活在1.25亿年前。
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Most famously, uranium-238 decays into lead-206 with a half life of 4.5 billion years, though radioactive rubidium and samarium are also useful for dating things billions of years old.