You can not get an Armor Fossil in HeartGold.
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You cannot get a Skull Fossil (Cranidos) in Pokemon HeartGold.
by reviving it from a fossil
You get it by getting a life...and not playing Pokemon...
diamond- skull fossil- cranidos-rampardos pearl-armor fossil- sheldon-bastiodon
after you beat the elite four, you can find they fossil in Cliff Cave. good luck!