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It is a trace fossil, a trace of the animal having moved over the landscape and not part of its body.
If not specified as trace fossil of Ammonites, it should be a body fossil. Ammonites is the name of the creature.
This is called a trace fossil. or a mold.
a trace fossil is like cast, mold, or trace fossil
A trace fossil was an imprint left behind by an animal that hardens and turns to stone