Fossils and How They Exist

Fossils are some of the only remains of the deep past that can tell us what prehistoric life was like long ago. The fossils we have are extremely valuable to paleontologists, with the possession of them variable, yet numerous in different clarity and quantity. You might find fossils owned in museums, miscellaneous institutions, universities, or in the houses of wealthy people displayed as commodities (which actively works against scientific research majority of the time since they're privately owned). Nevertheless, the fossil record is only 1% of what the past of our planet was truly like within the grand scheme of life and its evolution; it is what allows us as humans to catch a glimpse into prehistory, the shared ancient history, of the menagerie of organisms known and unknown on Planet Earth.

Furthermore, fossils preserve in various ways and thus can appear in many different forms. Some are common finds that can be easy to come across, and others are rare cases which happen only under specific circumstances. At their scientific core, fossils are extinct organisms that are usually encased in sedimentary rock dubbed “matrix”. Sedimentary rocks are one of three rock types (the others being igneous and metamorphic) that are made up of deposited material from prehistory, a time which is defined approximately from 11,700 years ago or older, stretching back millions, even billions of years ago in the form of a fossil/rock record. All organic material must die first to be permanently buried and preserved in the rock record as fossils. This condition defines protection from decay, destruction, or any processes of erosion on the surface, and thus the organic material can then become a fossil by using the right tools of geology and specifically taphonomy (the study of how organisms’ decay and become preserved in the fossil record). The sediments under pressure & time form into the prior mentioned sedimentary rock, and the fossils then survive geological processes whilst under layers of preserved sediments as rock layers, or “strata”. A fossil must then finally become exposed on the surface in sedimentary rock, to then be discovered by paleontologists and fossil hunters alike.

Most fossils that are commonly encountered will usually be body fossils. These types of fossils are usually mineralized, which means that the organic material that was originally part of the organism, has become chemically replaced by minerals within the sedimentary rock to be permanently preserved. This is known as permineralization, and examples would include shark teeth, dinosaur bones, or clam shells. Permineralization is a common way of fossilization, however in other more uncommon cases, wood and plant tissue can also become mineralized by silica, which is the same material that can be found in quartz (a common mineral in the earth’s surface).?

This process is known as recrystallization, and a good observable example of this is in the state of Arizona, USA, within a place called Petrified Forest National Park (pictured above). This area has trees from millions of years ago, which have been preserved by this silica within the process of recrystallization. However, some specimens such as petrified wood which form via silicification (a method of recrystallization using the mineral silica/quartz) can become distorted much easier when compared to the common process of permineralization. In both processes of fossilization, details in the fossil can become omitted, both possible inside and outside of the fossil, messing with the preservation of anatomy/hard parts. Hard parts/body fossils can also be preserved by other elements and geological material outside of the two processes mentioned, such as phosphorus, pyrite, and even coal/carbon-based material (pictured below are carbon-based fossils of fern plants, found in St. Clair, Pennsylvania, USA).

The first of these more specific mineral-based methods of fossilization, is known as phosphatization. This process allows a given fossil to come in different grades of coloration, namely in Otodus megalodon teeth. This is based on the mineral replacement process of phosphatization, which deals with minerals like apatite, a major component of sediments which possesses phosphorus. The second of these various methods of fossilization, is known as pyritization, and this happens in association with the mineral pyrite (sometimes called “fool’s gold” by the layman), requiring a key reaction in the sediments with iron and sulfur, the elements associated with pyrite. Moreover, the final specific method of fossilization based on specific mineral composition would be carbonization, where organic material becomes preserved through burial. Like mentioned earlier, this wards off other elements such as hydrogen, nitrogen, and oxygen. Then, either hard or soft parts can become preserved in the fossil record via a film of carbon-based material such as coal. Carbonization can preserve plants and small invertebrates from the deep past (like what is seen above).

Hard parts/body fossils aren’t the only kind which can be preserved through a variety of ways thanks to Earth’s geological diversity. Other types of fossils would be molds/casts. These don’t directly preserve those prior mentioned hard parts or body fossils but will preserve impressions of them with variable detail as expected. Moreover, another kind of fossil which can be encountered is as a trace fossil. As the name suggests, this kind of preserved material represents a trace of the day-to-day lives concerning an extinct animal. Footprints and trackways are good examples of showing the activities of an animal from the deep past, along with burrows, eggs, nests, and even fossilized poop known as coprolites! These are all trace fossils which can be found, and a good real-life example would be Dinosaur State Park located in the state of Connecticut, which preserves numerous dinosaur footprints. The study of trace fossils is a branch in paleontology, which is known as ichnology.

A third kind of fossil that can be unearthed are “chemical fossils”. These are the remains of organic molecules which have become trapped in sediments and turned into sedimentary rock during mineralization. When this process happens, the molecules are preserved in the sediments, and thus at the end of the fossilization process you get the “chemical fossils”. An example of this kind of fossil would be coal (which few people realize!) and other rarer examples of chemical fossils include preserved proteins or blood cells, which have been uncovered before in Tyrannosaurus rex (meat-eating dinosaur) bones dated to millions of years ago.

As mentioned earlier, there are various ways organic material can go under the process of becoming a fossil. However, there are also rare cases where organic material is preserved in more unique ways unrelated to geological processes. Firstly, organic material can be frozen in permafrost, protecting it from elements that can threaten the preservation process. This is mainly seen in places such as Russia within the Siberian wilderness, where frozen mammoths, wooly rhinos, cave lions, and even possibly early domesticated dogs have been uncovered! Secondly, organic material can also be trapped in amber, which is fossilized tree sap. A variety of things from bugs to dinosaur tails and lizards can be found within this material when in the right place! Below is a piece of amber with a feathered dinosaur tail trapped inside showcasing this.

Finally, to leave off on this article about fossilization, there is a process which can occur called mummification. This is when organs such as skin or tendons that are usually not fossilized become preserved when dried out. After drying out, they need to go under the process of mineralization to become preserved, in each specific depositional environment. Not many of these kinds of fossils have been found, making this kind of fossilization extremely rare. Examples such as “dinosaur mummies”, which are sometimes represented by plant-eaters known as hadrosaurs have been uncovered from the process of mummification. One such mummy is on display at the American Museum of Natural History, located in New York City.