Well, pollution is probably the main reason, but i think all of them are reasons why animals are dying off, if there isn't any choice for all of them, then your best bet would be to do a, pollution.
Matter into oil. (Another word for this kind of oil is petroleum.) Given enough pressure, organic matter can also become natural gas. Heat and pressure are the two main forces that transform organic matter into fossil fuels.
i think the answer to your question is the parietal bones
