Here's why:
* Predicate Adjective: A predicate adjective describes the subject of the sentence. In this case, the subject is "paleontologist." The adjective "excited" is describing the paleontologist's state of being.
* Predicate Noun: A predicate noun renames the subject. The sentence doesn't have a word that renames "paleontologist."
Breakdown:
* Subject: The paleontologist
* Verb: Seems
* Predicate Adjective: Excited
* Object of the Preposition: By stone
The phrase "by stone" functions as a prepositional phrase modifying the adjective "excited" and tells us what the paleontologist is excited *by*.