The reasoning might go like this. How do you know it’s an 8.7 million year old fossil? Well it was in an 8.7 million year old rock. But how do you know it was an 8.7 million year old rock? Because there was an 8.7 million year old fossil in it. That’s like asking why is my friend not alive? Because he is dead.
Isn’t radiocarbon-14 dating a fairly reliable technique to give the approximate age of organic material?
But how do you know it was an 8.7 million year old rock?
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You know that because you know the periodic table and how fast elements decay into an other element found in your rock. Elemental, Watson.