The preceding provided examples of the most common ways in which plants become fossilized, but there are other forms as well, or combinations of the preservational types just discussed. For example, some stem casts may contain a faint outline of the conducting system in the center of the cast. In this case, the cast is not simply a three-dimensional replica of the original plant part, but may contain some components of the original plant. Certain types of algae are more common as fossils because they precipitate or bind calcium carbonate around or in their cells. This calcium carbonate skeleton can build up to a considerable thickness and provides excellent preservational potential. When the alga dies, the calcium carbonate skeleton persists, often for millions of years. Sectioning of these calcium carbonate residues allows one to reconstruct the three-dimensional appearance of the alga by following the configurations of the hollows within the calcium carbonate sheath. These limestone-precipitating algae play a very important part in the build up of some so-called coral reefs; in these cases, the bulk of the reef is produced by the accumulation of CaCO3 precipitated by algae, rather than by the corals living on the reef (see Chapter 4).
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