dolmen vs megalithic structure

dolmen and megalithic structure both are nouns.

Word	Noun	Adjective	Verb	Adverb
dolmen	Yes	No	No	No
megalithic structure	Yes	No	No	No

As nouns, megalithic structure is a hypernym of dolmen; that is, megalithic structure is a word with a broader meaning than dolmen:

dolmen: a prehistoric megalithic tomb typically having two large upright stones and a capstone
megalithic structure: memorial consisting of a very large stone forming part of a prehistoric structure (especially in western Europe)

Other hypernyms of dolmen include megalith.

dolmen (noun)	megalithic structure (noun)
a prehistoric megalithic tomb typically having two large upright stones and a capstone	memorial consisting of a very large stone forming part of a prehistoric structure (especially in western Europe)