The first question we might clarify is what is a microfossil? It is a fossil between 0.001mm and 1mm in size, requiring a light or electron microscope to study it visually. There are siliceous, calcareous, phosphatic and organic microfossils.
Palynology is the study of organic microfossils, which are composed of entirely unmineralized proteinaceous materials and include pollen, spores, chitinozoans, scolecodonts, acritarchs, some bacteria, dinoflagellate cysts and fungal remains. The word palynology comes from the Greek word “paluno” meaning “strew” or “dust”. It is literally the study of dust.
Note that organic walled does not mean soft walled, but only that there are no minerals in the wall.
Pollen and spore cell walls contain sporopollenin, while dinoflagellates contain dinosporin. Both organic compounds are extremely resistant and show chemical and structural similarities but evolved independently. 3
This outer sheath allows them to resist the rigors of fossilization which destroy weaker organisms.
Chitinozoans are composed of a pseudo-chitinous molecule, meaning that it resembles chitin, which makes up the exoskeleton of crustaceans and insects, therefore a tough layer protecting the cell. It has been found that this layer is actually made of a network of kerogen with aromatic compounds.
Acritarchs, meaning “uncertain origin” in Greek, are also most commonly composed of kerogen, a carbon compound which is thermally altered, insoluble in acid, therefore confers a resistance to fossilization. Acritarchs are the remains of eukaryotic cells.
Scolecodonts are the jaws of polychaete annelids, a common type of fossil-producing segmented worm. Most annelids, such as earthworms and leeches, lack hard parts, so only sea-dwelling polychaetes are commonly fossilized due to their chitinous teeth and their dwelling tubes made of durable calcium carbonate, hardened mucus and/or chitinous cement.
Fossils of bacteria are very rare because their membrane is fragile and usually destroyed. An exception is blue-green algae, or cyanobacteria, the bacteria which are capable of photosynthesis, which are sometimes found in stromatolites which were infiltrated with a mineral rich solution, thus preserving the bacteria itself and not just its tubal constructions.
Other types of bacterial fossils are much more rare, but sometimes occur under certain chemical conditions where the bacterial cell is replaced with minerals, such as pyrite or siderite (iron carbonate), forming what are called pseudo-morphs, replicas of the once living cells. 667