Radiocarbon dating laboratory australia
Replacement occurs when the shell, bone or other tissue is replaced with another mineral.
In some cases mineral replacement of the original shell occurs so gradually and at such fine scales that microstructural features are preserved despite the total loss of original material.
Often what remains is a carbonaceous film known as a phytoleim, in which case the fossil is known as a compression.
Often, however, the phytoleim is lost and all that remains is an impression of the organism in the rock—an impression fossil.
A shell is said to be recrystallized when the original skeletal compounds are still present but in a different crystal form, as from aragonite to calcite.
These types of fossil are called trace fossils or ichnofossils, as opposed to body fossils.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.The development of radiometric dating techniques in the early 20th century allowed scientists to quantitatively measure the absolute ages of rocks and the fossils they host.
There are many processes that lead to fossilization, including permineralization, casts and molds, authigenic mineralization, replacement and recrystalization, adpression, carbonization, and bioimmuration.Some fossils are biochemical and are called chemofossils or biosignatures.