A white dwarf is the answer
Answer:
Identifying the fossil is the first step. We have already identified this fossil as a clamshell. We know clam are from the sea, but if you didn't, you could conclude this from knowing where you found the fossil (likely near the beach). Discovering where it is can also lead you to a conclusion on it's arrival as a fossil, perhaps washed up on shore or buried by an animal.
Explanation:
Inert
That means it does not react with other atoms
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Bacteria, since most gas is due to the breakdown of fats or carbohydrates by intestinal flora (normally dwelling bacterial cultures)