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.
Remember that the subject is never part of a prepositional phrase. The subject of a verb will never be part of a prepositional phrase. A prepositional phrase begins with a preposition [in, on, at, between, among, etc.] and ends with a noun, pronoun, or gerund.