Present participle of fossilify; the process of turning into a fossil through the -ify verb form.
From fossilify (fossil + -ify) + -ing. This represents the gerund/present participle form of the archaic verb 'fossilify'.
This word is so rare that most spell-checkers won't recognize it—but it perfectly demonstrates how English verb formation works: fossil → fossilify → fossilifying, even though scientists just use fossilize/fossilizing instead!
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