Pollen grains are well preserved as fossils because of the presence of:

Pollen grains are well-preserved as fossils because of the presence of sporopollenin. The hard outer layer of pollen grain, the exine, is made up of sporopollenin which is one of the most resistant organic material known. It can withstand high temperatures and strong acids and alkali. No enzyme that degrades sporopollenin is so far known.