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The empty spaces within an organism (spaces filled with liquid or gas during life) become filled with mineral-rich groundwater.

Minerals precipitate from the groundwater, occupying the empty spaces.

Specimens are usually considered to be fossils if they are over 10,000 years old.

The observation in the 19th century that certain fossils were associated with certain rock strata led to the recognition of a geological timescale and the relative ages of different fossils.

Some fossils consist only of skeletal remains or teeth; other fossils contain traces of skin, feathers or even soft tissues. In some cases the original remains of the organism completely dissolve or are otherwise destroyed.

The remaining organism-shaped hole in the rock is called an external mold.

If this hole is later filled with other minerals, it is a cast.

An endocast or internal mold is formed when sediments or minerals fill the internal cavity of an organism, such as the inside of a bivalve or snail or the hollow of a skull. If the chemistry is right, the organism (or fragment of organism) can act as a nucleus for the precipitation of minerals such as siderite, resulting in a nodule forming around it.

Often, however, the phytoleim is lost and all that remains is an impression of the organism in the rock—an impression fossil.In 2014, Mary Schweitzer and her colleagues reported the presence of iron particles (goethite-a Fe O(OH)) associated with soft tissues recovered from dinosaur fossils.Based on various experiments that studied the interaction of iron in haemoglobin with blood vessel tissue they proposed that solution hypoxia coupled with iron chelation enhances the stability and preservation of soft tissue and provides the basis for an explanation for the unforeseen preservation of fossil soft tissues.For this reason, one term covers the two modes of preservation: adpression.Because of their antiquity, an unexpected exception to the alteration of an organism's tissues by chemical reduction of the complex organic molecules during fossilization has been the discovery of soft tissue in dinosaur fossils, including blood vessels, and the isolation of proteins and evidence for DNA fragments.

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