Can their arms be a possessive pronoun?

The Word their is a pronoun called a possessive adjective, a Word that is placed before a noun to describe that noun as belonging to two or more people or things.

The possessive pronoun form is theirs, a Word that takes the place of a noun that belongs to two or more people or things.

Examples:

The models all extended their arms to show the jewelry. (possessive adjective)

When we captured a bunker, we found all the arms that were theirs. (possessive pronoun)