9.2: Types of Preservation

Turning a recently deceased organism into something that is likely to be preserved in the geologic record isn’t easy! Doing so typically requires one of the following:

Figure \(\PageIndex{1}\): In order to be preserved as a fossil, organisms must undergo one of the modes of preservation shown above (Page Quinton via Wikimedia Commons; CC BY-SA 4.0)

Hard and Soft Parts

Preservation without alteration can happen when particularly robust hard parts of organisms like bones, shells, teeth, etc. are buried and protected from decay and damage. Under very special conditions such as freezing, entrapment in amber, or mummification the soft parts of organisms like hair, skin, or muscle tissue can be preserved.

 

Figure \(\PageIndex{2}\): Hard parts and soft parts. A) The hard parts of this mammoth that are easily preserved include its bones and tusks (Zissoudistrucker via Wikimedia Commons; CC BY-SA 4.0). B) Under certain circumstances (like being preserved in permafrost), soft parts like the hide, hair, and flesh of this baby mammoth can be preserved (NOAA via Wikimedia Commons; public domain).

Preservation with Alteration

Preservation with alteration is the most common way of preserving body fossils and it happens when the original tissue is changed in some way from its original form. Recrystallization happens when the original material in the tissue is transformed into a more stable crystal form; the most common example of this is the transition from aragonite (iridescent nacre or mother of pearl) into chalky calcite. Carbonization happens when the organism is compressed, and water/gas are driven off so that only a thin carbon film remains. When minerals precipitate out of groundwater to fill in open spaces in wood, bone or other porous tissue the process of permineralization takes place (ex: “petrified” wood formed when microcrystalline quartz fills in woody tissue). And lastly, replacement takes place when one mineral is precipitated in place of another (ex: pyrite replacing calcite).

Figure \(\PageIndex{3}\): Preservation with alteration. A) Calcite replaced the aragonite that originally made up these fossil ammonites (Reinhold Möller via Wikimedia Commons; CC BY-SA 3.0). B) A carbonized leaf (Kevmin via Wikimedia Commons; CC BY-SA 3.0). C) Permineralized wood (Dhzanette via Wikimedia Commons; public domain). D) These shells were made of carbonate that was later replaced by pyrite (Luis Fernández García via Wikimedia Commons; CC BY-SA 4.0).

 

 

Casts, Molds, and Impressions

Sometimes even when the tissue itself isn’t preserved, a fossil may be still be there in the form of a cast, mold, or impression. A cast occurs when sediment or minerals fill in a void space left by a decayed or dissolved organism. A mold is the name we use for the actual void. An impression happens when organic material is pressed into sediment and the texture of that material is preserved.

Figure \(\PageIndex{4}\): Casts, molds, and impressions. A) The void inside of this shell could form a mold if infilled with sediment or minerals as in the next figure (Dr. Zachi Evenor via Wikimedia Commons; CC BY 2.0). B) This cast formed when sediment infilled the void inside of a gastropod shell like the one in the previous figure (Rowan-earth via Wikimedia Commons; CC BY-SA 4.0). C) Cast of a sleeping dog killed and entombed by the eruption of Mount Vesuvius in 79 AD. D) Dinosaur skin impression (replica) from the Jurassic Morrison Formation (James St. John via Wikimedia Commons; CC BY 2.0).