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c. 330 -346 million years ago

The Formation of Fossils

Fossils can be formed by a number of processes... some are better than others.

By Carin Stritch

This specimen consists of the fossil valve of a brachiopod from the Carboniferous, approximately 330 -346 million years ago, found in the Fergus Estuary area, Co. Clare.

In the Museum's accession register, the entry for this specimen reads: "NG:F31658; unidentified Brachiopoda; gift: Univ. of Southampton (Tattersall PhD Coll.); 6885; Fergus Estuary area, Co. Clare.; Carboniferous, Viséan; Beekitised.”

How are fossils formed?

Fossilisation refers to the preservation of organic remains or evidence of once living organisms, in the geological record.  Fossils may include: hair, teeth, bones, impressions left by skin or feathers, or trace fossils, such as, footprints.  There are a number of methods by which fossilisation may occur; the quality of the fossil resulting from each method of fossilisation will vary according to the method of preservation.

  1. Freezing and entrapment in amber will usually result in unaltered remains where both the hard and soft parts are preserved in fine detail, even allowing researchers to remove DNA for analysis.
  2. Compression results in a dark carbon impression which shows only the external details of the specimen being observable.
  3. Permineralisation (where the pores of the organism are filled with mineral rich waters, when the water evaporates the minerals replace the organic tissue).  In permineralisation the fine structural details of the organism are usually preserved allowing researchers to study the cellular structure of the fossil.  As permineralised fossils are preserved in three dimensions the tissues are not compacted and the relationship between organs and their sizes and locations can be studied.
  4. Replacement will usually result in much of the original morphological detail of the fossil being preserved, however, the level of preservation will be dependent on which mineral is deposited; commonly pyrite, calcite or silica.

Silicification as a method of fossilisation

The method of fossilisation which is the most variable is replacement.  This can result in fossils which show fine detail or, as in the fossil in Figure 1, it can result in a mere outline of the specimen being preserved.  The method which results in this is silicification, where silica is introduced into specimens as a replacement of the shell mineral calcite, particularly a form known as beekitisation.

Beekite is a form of chalcedony, a microcrystalline form of quartz, named after Henry Beeke, Dean of Bristol (1751-137).  It is usually recognisable as orbicular structures, concentrically layered spheroids.  Apart from the presence of orbicular structures of beekite there are a number of ways to identify if a fossil is silicified: Natural weathering will dissolve the limestone matrix in which the fossil is preserved leaving the fossil proud, steel will not scratch it and if acid is applied it does not fizz as limestone does.

There are a number of problems with silicified fossils:

  1. In order to free silicified fossils from the matrix in which they are preserved they are etched with acid, but as silicification of the fossil may not always be complete this may result in some of the specimen being lost. 
  2. All details of the structure may not be preserved in this method of preservation which may not allow an exact identification of the fossil. 
  3. Internal examination of the fossil composition is not possible.

Fossilisation is a rare occurrence; a plant or animal must die in the right place at the right time, not be predated or scavenged, but be buried under soil or mud or best of all on the seabed. Millions of years later they allow scientists to infer information about the life and evolution of the organism and to track environmental changes.

How did this specimen come to be in the Museum?

It is one of approximately 1,000 specimens collected as part of a PhD study in the 1960’s and was recently gifted to the National Museum of Ireland - Natural History by Southampton University.

Learn more

This particular specimen is part of the museum’s scientific collection, and is not on public display.  However, another specimen of beekite is on display it is part of the UCD mineral collection and it can be found in geology window case number 229 on the ground floor of National Museum of Ireland – Natural History in Merrion Street.


I am very grateful to Dr Matthew Parkes, Curator of Geology, for his assistance in preparing this article.


Baars, C. (2009) "Acid digestion of silicified shells", The Geological Curator, 9 (1), pp. 29-33.

McKenny Hughes, T.M. (1889) "On the manner of occurrence of Beekite and its bearing upon the origin of Siliceous Beds of Palaeozoic Age", The Mineralogical Magazine and Journal of the Mineralogical Society, 8, pp. 265–271.

Wright, A.D. (1963) "The Fauna of the Portrane Limestone. I". The Inarticulate Brachiopods, 8 (5),  London: British Museum.


The Formation of Fossils is located at:
In Storage

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The Dutchman’s Log Box

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The Signing of the Anglo-Irish Treaty, 1921

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