Coal seams are fossilized accumulations of plants which lived and died in swamps that were so devoid of oxygen that few microbes or other critters could survive to feed on their remains.
(plants; lived; ) (plants; died; L:in swamps) (swamps; were; so devoid of oxygen that few microbes or other critters could survive) (few microbes or other critters; could survive; to feed on their remains)
plants plants swamps few microbes or other critters
lived died were could survive
L:in swamps
so devoid of oxygen that few microbes or other critters could survive
to feed on their remains