once means one time & if an organism is dead then that organism was living at one time ->  if an organism is dead then that organism is once-living  if an organism is dead then that organism is once-living & fossil fuels / fossils are formed from dead organisms over time ->  fossil fuels are formed from once-living organisms over time  to produce something can mean to form something & a source of something emits / produces / generates / provides that something ->  a source of something forms that something  fossil fuels are formed from once-living organisms over time & a source of something forms that something -> once-living organisms are a source of fossil fuels