Posted by: Rob Viens | September 25, 2012

A Fossil Primer II

On the 25th, Darwin was still having fun with his fossils:

“The Schooner has been taken to the Creek.— Mr Wickham & a party of men have erected tents on shore & are living there during the refit of the vessel.— I accompanied the little settlement & whilst they were rigging the tents I walked to Punta alta & again obtained several fossils.— I came quite close to an Ostrich on her nest; but did not see her till she rose up & with her long legs stretched across the country.” (Sept 25)

Most people are familiar with the general concept of fossils, but what many do not know is that there are a lot of different types of fossils out there. Today I thought I’d finish up our “fossil primer” with  brief description of some of the types of fossils you can find.  Darwin was most likely collecting fossils that were permineralized (bones) or recrystallized (shells).

Fossil Types

  • Unaltered Fossils (“Subfossils”):  This is organic material that has not significantly decomposed and is more-or-less preserved in its original state.  They are mostly found in relatively young deposits, and include such things as mammoths that have been frozen in ice, plants (or human remains) buried in oxygen-poor swamps, animals trapped in tar pits, or mummified (dried out) remains in arid regions.  Occasionally they can be older, such as insects trapped in amber.
    Unaltered remains – mammoth hair (from New York Times)
    mammoth hair
  • Recrystallized Fossils: In recrystallization the original chemical elements that make up the fossil material are reorganized into new crystal forms.  For example, most shells are actually made of the mineral aragonite.  During fossilization the chemical elements in the aragonite rearrange into a new crystalline structure – calcite. The fossil remains a closed system – no material is added or taken away. A similar process is…
  • Replacement: In this case the original materials are completely removed (decayed or dissolved away) and then replaced by a new material, such as pyrite, silica, gypsum, copper, phosphate minerals, etc.  Unlike recrystallization, this is a completely open system as elements come and go during the replacement process.
    Replacement – Ammonite replaced with pyrite (
  • Permineralized Fossils: Permineralization is a type of fossilization that is common in porous materials, such as bone or wood, where additional material is deposited in the pores making the final sample very hard and dense.  This is often the result of silica-laden water passing through the sample and gradual leaving the silica behind in the pore space, thereby “petrifying” the remains
    Permineralization – petrified wood (from Wikipedia Commons)
    petrified wood
  • Fossil Molds and Casts: In some cases the fossil material may completely dissolve or decay away leaving behind an impression called a mold.  These molds are often just made in the surrounding rock type. If the mold is later filled in by another material (mud, for example) it can make a cast of the original fossil. This is exactly the same concept as the molds and casts that are made in candle making or bronze casting.
    Mold and Cast – Trilobites (from R.Weller/Cochise College)
  • Carbonized Fossils: In carbonization, the original organism mostly decays away leaving behind only a film of carbon in the impression of the original organism.  These are most likely to occur with soft tissues and therefore, are common with plants (leaves, twigs, flowers, wood, etc.). Soft tissue of animals can also be preserved this way and some common carbonized fossils include fish and graptolites
    Carbonization – fossil fish (from R.Weller/Cochise College)
    fossil fish

In the modern sense of the word, fossils are the remains of once-living things.  But there are is other types of evidence left behind by living things, too.  These include:

  • Trace Fossils:These are “traces” of living things – indirect evidence for the presence of life. They include things left behind by organisms such as, tracks, trails, burrows, nests, egg shells, coprolites (poop), etc.
    Mammoth tracks from Alberta (from Quaternary Science Reviews 2005)
    mammoth tracks
  • Chemical Fossils: There are organic chemicals and isotopes that are unique to living things.  So even though an organism may have decomposed it may leave behind a chemical signature in the rocks that is unique to life.  (For example, the chemical compound chlorophyll has a unique chemical signature and the ratio of carbon isotopes (carbon-12 to carbon-13) in plants is different from that in nonorganic rocks.

After all the dust had settled (literally) what had Darwin collected?  Well, more on that next time! (RJV)


  1. […] PS – It has been about 10 years since it came out, but if you want to know more about trilobites, I would recommend Richard Fortney’s book, Trilobite: Eyewitness to Evolution. For more on Darwin and fossils, you can also see A Fossil Primer (Part I and Part II). […]

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