You asked the OP, but you really don't see the hard part of the idea. The hard question hidden behind your OP is not when did one life form appear and was recorded in the strata, but is WHY do they keep becoming extinct and became fossils.
Most become extinct due to extinction events, of which there are 5 major and 20 minor ones. Keep in mind that they didn't happen over night, except maybe the last major one, the CretaceousPaleogene (KPg).
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