Some of the world's oldest preserved art is the cave art of Europe,
most of it in Spain and France.
is the same as: Some of the world's oldest preserved art, most of it
in Spain and France, is the cave art of Europe.
For me, the phrase: most of it in Spain and France is an appositive phrase. The reason is that it adds extra information about the first part.
However, it is not easily identifiable coming at the end of the sentence and appositive phrases are supposed to be next to what they describe. Appositives are usually next to the noun they explain.
But, the comma allowed me to change its position in the sentence.
Also, some and most are in contrast to each other. Another reason that supports moving it for purposes of explanation and structural analysis.
"An appositive is a noun or noun phrase (appositive phrase) that gives another name to the noun right next to it. It adds descriptive words about a specific thing (the noun), which helps make a sentence more detailed; or, it adds essential information to make the sentence’s meaning clear."