Did I miss something? How did Jane acquire the skull?

I really liked the movie, but obviously they left something out -- or I missed something. How did Jane acquire the skull?

If Jane saw the killer/killers bury the body, then the mystery would have been solved early on, assuming she'd call the police, and there would be no need for a movie. If the skull became separated from the body, and she found it while walking then that would be the logical answer.

I don't think an explanation was ever given as to how the skull was discovered. Was there an explanation?

By the way, how old was the skull. It looked older than one buried recently.