Before the body was sent to the crematorium, shilling and Crump filled the casket with animal bones, meat, and a mannequin.
Object of the preposition: genius. The subject of the sentence is called a noun clause. It is a clause because it has its own subject and verb. It is also a dependent clause which cannot stand on its own because of the word that (a relative pronoun) at the beginning of the clause. A noun clause functions as a regular noun in a sentence.
Prepositions are not in the habit of standing alone. A word group with a preposition at the head followed by an object (or complement) is called a prepositional phrase. The object of a preposition is typically a noun or pronoun: Gus put the horse before the cart. Prepositional phrases add meaning to the nouns and verbs in sentences.