All these sentences use the word "so" correctly except:

A dinosaur egg is a fossil, and so is petrified wood.

A dinosaur egg is a fossil; so is petrified wood.

A dinosaur egg is a fossil. So is petrified wood.

A dinosaur egg is a fossil, so is petrified wood.

A dinosaur egg is a fossil. So is petrified wood.

'So' is a conjunction that depicts comparison between subjects in a sentence in this case. The use of 'so' in this sentence is incorrect because when comparing two subjects, which in this case are dinosaur egg and petrified wood, they should be in one sentence idea. When using the punctuation mark period '.' it ends the idea of the sentence in the first sentence given.