If this happens rapidly before significant decay to the organic tissue, very fine three-dimensional morphological detail can be preserved.Nodules from the Carboniferous Mazon Creek fossil beds of Illinois, USA, are among the best documented examples of such mineralization.
Minerals precipitate from the groundwater, occupying the empty spaces.Like extant organisms, fossils vary in size from microscopic, even single bacterial cells one micrometer in diameter, to gigantic, such as dinosaurs and trees many meters long and weighing many tons.A fossil normally preserves only a portion of the deceased organism, usually that portion that was partially mineralized during life, such as the bones and teeth of vertebrates, or the chitinous or calcareous exoskeletons of invertebrates.In this case the fossil consists of original material, albeit in a geochemically altered state.This chemical change is an expression of diagenesis.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.
The study of fossils across geological time, how they were formed, and the evolutionary relationships between taxa (phylogeny) are some of the most important functions of the science of paleontology.
Such a preserved specimen is called a "fossil" if it is older than some minimum age, most often the arbitrary date of 10,000 years.
Fossils may also consist of the marks left behind by the organism while it was alive, such as animal tracks or feces (coprolites).
These types of fossil are called trace fossils (or ichnofossils), as opposed to body fossils.
This repository contains files to produce the paper entitled "Bayesian phylogenetic estimation of fossil ages" by Alexei J.