<P> 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 . In 2014, Mary Schweitzer and her colleagues reported the presence of iron particles (goethite - aFeO (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 . However, a slightly older study based on eight taxa ranging in time from the Devonian to the Jurassic found that reasonably well - preserved fibrils that probably represent collagen were preserved in all these fossils, and that the quality of preservation depended mostly on the arrangement of the collagen fibers, with tight packing favoring good preservation . There seemed to be no correlation between geological age and quality of preservation, within that timeframe . </P> <P> Carbonaceous films are thin coatings which consist predominantly of the chemical element carbon . The soft tissues of organisms are made largely of organic carbon compounds and during diagenesis under reducing conditions only a thin film of carbon residue is left which forms a silhouette of the original organism . </P> <P> Bioimmuration occurs when a skeletal organism overgrows or otherwise subsumes another organism, preserving the latter, or an impression of it, within the skeleton . Usually it is a sessile skeletal organism, such as a bryozoan or an oyster, which grows along a substrate, covering other sessile sclerobionts . Sometimes the bioimmured organism is soft - bodied and is then preserved in negative relief as a kind of external mold . There are also cases where an organism settles on top of a living skeletal organism that grows upwards, preserving the settler in its skeleton . Bioimmuration is known in the fossil record from the Ordovician to the Recent . </P> <P> Paleontology seeks to map out how life evolved across geologic time . A substantial hurdle is the difficulty of working out fossil ages . Beds that preserve fossils typically lack the radioactive elements needed for radiometric dating . This technique is our only means of giving rocks greater than about 50 million years old an absolute age, and can be accurate to within 0.5% or better . Although radiometric dating requires careful laboratory work, its basic principle is simple: the rates at which various radioactive elements decay are known, and so the ratio of the radioactive element to its decay products shows how long ago the radioactive element was incorporated into the rock . Radioactive elements are common only in rocks with a volcanic origin, and so the only fossil - bearing rocks that can be dated radiometrically are volcanic ash layers, which may provide termini for the intervening sediments . </P>

What are the 6 categories of fossil types