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Fossilized Shark Teeth on the Internet
Fossilization
Processes:
There are 6 different ways that parts of ancient creatures become fossils.
The one that we are most concerned with is called MINERALIZATION. This process produces a Pseudomorph of the original object. The enamel of the megalodon tooth is original; the underlying dentin and root have been replaced by other materials.
Mineralization Subprocesses:
PETRIFICATION (or PETRIFACTION) (petros means stone) occurs when the organic matter is
completely replaced by minerals and the fossil is "turned to stone". This method reproduces the original material in every detail. Fossil wood is produced in this manner.
PERMINERALIZATION happens when the minerals have only filled in the spaces of the hard parts
of the object and the original, inorganic material remains.
RECRYSTALLIZATION: The original material is replaced by minerals which in growing, may destroy the shape of the structure it replaced. Example - aragonite changing to calcite.
REPLACEMENT: This process causes some of the original structures to dissolve and be replaced by precipitant substances. The shape of the original material is retained.
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Replacement Materials:
The most common replacement materials are:
The calcium carbonates - [CaCO3] like calcite & aragonite.
The silicates - [SiO2-NH2O] like quartz, opal & chalcedony.
Less common replacement materials are:
Iron disulfides - [FeS2] like pyrite & marcasite.
Limonite - [FeHO].
The sulphates like gypsum.
The phosphates like calcium phosphate, vivianite.
Glauconite. - (K,Na)(Fe+3,Al,Mg)2(Si,Al)4O10(OH)2
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In the United States, eastern coastal, ancient embayment fossil teeth were subjected to a combination of these mineralization subprocesses and materials, but in the opinion of most experts, were formed by Replacement Mineralization using the less common phosphates. Most of these fossils have the characteristic black root and black dentin.
In sharp contrast to these teeth, we have in our collection about 60 teeth from other parts of the world where the "replaced" root and underlying dentin is clearly silicate (quartzite).
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Shark teeth are (were) composed of four kinds of material; enamel, cementum, dentin and pulp. Pulp is the mostly organic, spongy material in the center of the tooth. It is surrounded by a largely inorganic substance called dentin (or dentine). You can see the dentin when you look at the root of a modern tooth. The dentin in the blade of a tooth is covered by a thin coating of enamel. The enamel is inorganic, extremely hard and crystalline in nature. It is only a few thousands of an inch thick and if you could slip it off the tooth would look like a triangular shaped "pita pocket". The cementum is the material that binds the enamel to the dentin. Much of the tooth is made up of calcium phosphate, a mineral with the
technical name HYDROXY-APATITE: chemical formula, Ca5[PO4]3OH.
There are three distinct areas on the surface of a tooth: (1) the root, (2) the blade and (3) the bourrelet. The bourrelet is the chevron-shaped area between the root and the blade enamel. It has its own type of enamel covering which is thinner and more delicate than the blade enamel. This enamel is usually the first thing "to go" as the tooth gets weathered and damaged by it's environment.
There are about 13 different processes grouped in two phases (Taphonomic and Diagenetic) that effect the tooth during fossilization, many of them destructive. Very few meg teeth go through the 20 thousand to 100 thousand or so years it takes to fossilize a tooth and the 20 million years of environmental changes without significant damage. One paleontologist estimates that perhaps only 10 to 15% actually make it.
After the megalodon lost a tooth, it settled to the bottom of the ocean and was covered by sediment. For the first few years the dentin absorbed water and became swollen. This swelling process cracked the enamel coating and began to separate it from the underlying dentin. The blade enamel is almost never cracked out to the edges because there is no dentin under the enamel in these areas and the enamel itself is too hard to absorb water.
During the next 20 - 100 thousand years, two things happened:
(1) Much of the pulp and dentin were dissolved by the water and replaced by precipitant substances, one molecule at a time. The "spaces" in between this material was also filled in by this same compound. (The same thing happens with "petrified wood" where the original material is replaced by minerals and only the shape of the original wood remains.) The tooth enamel, being much harder, is not affected except that it takes up the color of whatever impurities are in the sedimentary material the tooth falls in. This makes for the vast variety of colors of the fossil teeth. The original enamel, dentin and pulp were white, just like modern shark teeth.
(2) Over time, the tooth was buried deeper under tons of ocean floor and the material (matrix) around the tooth dried out and possibly heated up which caused the tooth to dry out. This caused the tooth to crack even more, generally in the root area (stress crack). You can see how this works as a physical cracking process in dried-up "mud flats".
For the above reasons, most teeth are found to have cracks in the enamel (hydration swelling) and root (dehydration stress), and some have much of the enamel and root cracked away. The enamel forms an attachment to the surrounding matrix that is stronger than it's weakened (cementum) attachment to the underlying (fossilized) dentin and comes away with the matrix when the tooth and matrix are separated.
The bourrelet enamel is particularly susceptible to being lost because it is thinner than the blade enamel and is over the area of maximum dentin swelling.
After many thousands of years, much of the original internal material is replaced by the predominant stable authigenic mineral in the surrounding environment, and the tooth is "fossilized". In the case of a marine, sedimentary environment; this material is FLUORAPATITE [calcium phosphate fluoride - Ca5F(PO4)3]
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