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Fossils are a record of organisms that lived in the past. are two main categories of fossils: body fossils and ichnofossils. Body fossils are the result of preservation of parts or the entire body of a plant, animal, or microorganism. These are the fossils that people are most familiar with, consisting of skeletons, teeth, shells, carapaces, organisms in amber, petrified wood, plant material, pollen, etc. Ichnofossils (also called trace fossils) are evidence of an organism’s activity. Common ichnofossils are animal footprints and trackways, burrows, traces of plant roots, coprolites (fossil feces), and borings in rocks, bones, wood, shells, or other substrates. The study of fossils provides information about ancient biological communities, and the physiology, behavior, and ecology of organisms.
Fossils are mostly found in sedimentary rocks, which are formed by deposition of sand and mud, or precipitation of minerals like calcite and silica. Most sedimentary rocks and the fossils therein contain evidence of aquatic deposition. The study of the fossils and the associated rocks in which they are preserved gives us information about ancient conditions in which organisms lived, called paleoenvironments, and the pathways leading to their fossilization.
Fossilization is a physical-chemical process that typically requires three conditions; 1) possession of hard parts, 2) escape from immediate destruction, and 3) the right geochemical conditions in the sediment. First, organisms with hard parts like wood, bones, teeth, shells, or other mineralized parts, are much more likely to leave a fossil of some kind than those that only have soft parts; like jellyfish, worms and slugs. Soft parts, like hair, feathers, skin, internal organs, flowers, etc., are extremely rare in the fossil record of the majority of animal and plant groups. The reason is that destruction of soft tissues by microbial decay or scavenging occurs rapidly after death, and only hard, mineralized structures survive long enough to be buried and preserved. That is the main reason why the fossil record is incomplete because, in comparison with the abundance of soft-bodied organisms present in modern environments, few organisms that consisted of only soft parts are represented in the fossil record. The fossil record is thus biased towards the mineralized parts of those organisms that produce them. Soft tissues do not last as long as hard parts, but even with hard parts, some materials endure longer than others. For instance, teeth last much longer than bones, while shells and cellulose endure longer than chitin (the protein that forms the arthropod’s exoskeleton), although the latter is more labile and fragile than lignin. However, some remarkable examples of preserved soft parts exist, including entire mammoths preserved in Siberian permafrost; salamanders, flowers, insects, arachnids, and other invertebrates preserved in amber; and blood vessels and cells preserved in dinosaur bones, bacteria in mineral salt, and bacteria in Cretaceous dinosaur skull bones. An exceptional case is the fossilization of muscles, which is very rare and has been documented in extraordinary specimens as the fossil fish of the Santana Formation, in Brazil. These rare fossils are cases of exceptional preservation (Fig, 1). Despite this bias toward the hard parts of organisms, the fossil record is still considered adequate to study the history of life on earth.
A second important factor is that fossilization requires escape from immediate destruction after death. Modern observations tell us that only a very small proportion of the organisms living in a given environment will eventually become fossilized. As indicated above, soft-bodied animals are subject to destruction by predation, scavenging, or decay, and normally no remains are left after a short period of time. Plants are also destroyed by herbivores or decomposed by bacterial and fungal activity. Wood can last for relatively long periods of time making it more likely to be buried and fossilized. Animals with hard skeletons are also subject to destruction, but their hard parts may remain and become fossilized.
Most organisms do not become fossils. Why is that? The reason is that both organic and mineral matter are destroyed by bacterial decay and physical damage. Soon after the organism dies, bacteria initiate the process of decomposition by breaking down molecules and tissues. Moreover, physical processes, like water currents, trampling or scavenging, can contribute to the destruction of the remains.
But bacteria may also play an important role in the opposite process—preservation, the mineralization of remains in certain environmental settings. Bacterial activity plays an important part in the chemistry of calcite (CaCO3), which is the main component of calcareous rocks and many fossils. Dolomite (CaMgCO3), siderite (FeCO3), and phosphates are also precipitated by bacterial activity. Bacteria’s extraordinary capacity for multiplication favors the early onset of certain biomineralization processes that prevent destruction of organic remains. Thus, fossilization depends on two chemical parameters: decay, which destroys the remains of organisms, and mineralization, which preserves a record of their existence. Interestingly, decay is carried out by bacterial activity, which also is the cause of many instances of mineralization.
The key to fossil formation is thus rapid burial in a medium capable of preventing or retarding complete decay. The remains of organisms must be buried before decay and scavenging completely destroy them (Fig, 2). The occurrence of fossil bones, shells, and wood indicates that not only were these remains buried before complete destruction occurred, but also that further decay ceased and chemical conditions in the sediment were appropriate for preservation. Therefore, the third condition for fossilization to occur is the existence of the right geochemical conditions in the sediment for remineralization to occur. The type of chemical conditions depends on the environment in which the organisms were buried. Different conditions and environments may yield different types of preservation, variation in abundance, or a complete lack of fossils. Marine animals living in shallow waters are the most likely to be preserved, especially if fine sediments like mud or sand cover them. Terrestrial organisms are not as likely to be preserved as those from marine habitats. The remains of terrestrial faunas and floras are normally encountered in lacustrine, swamp, and fluvial-alluvial deposits because water is almost absolutely necessary for fossilization.
Water flowing in the sediment surrounding buried organisms allows dissolved minerals to seep through bones, shells, wood or other hard parts and replace them with minerals. This process is known as permineralization. Calcium will precipitate into calcite, a form of calcium carbonate, and silicon will precipitate into silica; these are the two most abundant minerals or cements that produce mineralization of organic matter. Minerals containing copper, cobalt, or iron may add color to fossils.
Contrary to what many people believe, permineralization may not take a long time. Given the right geochemical conditions during burial, permineralization can occur rapidly: ranging from within a few hours to a few years, depending on the size and nature of the original material. Scientists have reported fossilized embryos of echinoderms (sea urchins), which are extremely delicate structures. Experiments carried out to replicate those fossilized embryos show that fossilization happened in a very short span of time. Experiments show that mineralization of soft tissue of shrimp with calcium phosphate mediated by bacterial decomposition may start in a few days and increase in 4 to 8 weeks after death, possibly leading to fossilization. This is an example of fossilization involving mineral precipitation that occurs during the decay process caused by bacteria. The British paleontologists David Martill studied in detail the preservation of fishes and other animals in rocks of the Lower Cretaceous of the Chapada do Araripe, north-east Brazil, and found that they have preserved the most delicate structures known in the fossil record. Gills, muscles, stomachs and even eggs with yolks have been found. These are cases of exceptional preservation by phosphatization—mineralization by calcium phosphate. Martill concludes that many of the fine details preserved in those fossils became mineralized within a span of time of 5 hours or less after death, and calls this instantaneous fossilization. I have studied fossils of whales in the Pisco Formation in Peru in which the baleen structure (the filtering organ in the mouth of the whales) has been partially mineralized and preserved in anatomical position, which is a case of exceptional preservation because baleen is not bony tissue and is not rooted in the maxillary. Baleen tends to detach from the whale and decay rather quickly after death, nevertheless it is preserved in life position in many of those fossil specimens. I have suggested that the whales must have been rapidly buried and the baleen rapidly mineralized in order to become preserved.
In conclusion, fossilization at least at the present time, is thought to be a very unlikely process and it is believed that only a very small fraction of organisms that lived in the past became fossils. The majority of these fossils were hard skeletal parts or wood. To become fossilized, organisms must be rapidly buried, preferably in a fine sediment with geochemical conditions that favor the exchange of minerals between the sediment and organic components of the organism, and that exchange of minerals is possible because of dissolved minerals in flowing water. If those conditions occur, fossilization must necessarily be a rapid process of a few hours to a few months if it is to occur before decay destroys any record of the organism. Fossilization does not take thousands or millions of years, but is most likely to occur in catastrophic conditions such as would have existed during the Genesis Flood.
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