Fossilization and the Science of Taphonomy

This article in the Introduction to Fossils and Paleontology series introduces what fossils are, explains fossilization, and how paleontologists study fossilization.

The views expressed in this article reflect those of the author mentioned, and not necessarily those of New Creation.

What is a Fossil? 

The word fossil means “dug up.” Before paleontology (the study of fossils) took off, people called all kinds of things fossils, including crystals and interesting rocks. Today, the term refers to the remains of organisms or traces of their activity (e.g., footprints) preserved within sedimentary rocks.

How did fossils get inside sedimentary rocks? The answer might seem obvious now, but it wasn’t always obvious to observers of fossils. People in earlier times saw the similarities between fossils and living things, but struggled to account for that similarity. Did mysterious forces create structures inside rocks that looked like organisms or their parts? Seventeenth-century Danish scientist Nicholas Steno argued that fossils are organic remains that were covered by sediment. After covering the remains, the sediment hardened into solid rock.¹ Steno’s insight laid the groundwork for modern paleontology.

Fossilization

Once buried, a variety of processes help to preserve organic remains as fossils. We call the entire process whereby fossils form fossilization. Some common types of fossilization are permineralization, replacement, and recrystallization.^{2, 3} Permineralization occurs when minerals like calcite and quartz crystallize within empty spaces in organic remains, like pores in bone. In this case, the original material may still be present, only “frozen in stone.”

Replacement occurs when minerals replace the original materials of the bone, wood, shell, or soft tissue. Recrystallization is when the original minerals in the hard parts of the organism change structure into a different mineral. This commonly occurs in seashells made of aragonite, a form of calcium carbonate that frequently converts to calcite after burial. 

Other modes of fossil preservation include impressions, compressions, preservation as molds and casts, and whole preservation.⁴

Impressions are a common way that leaves preserve as fossils. These form when the sediment encasing the leaf turns into hard rock. All the original leaf material decays away, leaving only an impression of the leaf in the rock. 

Compressions are another common way that leaves, insects, and even fish preserve as fossils. These fossils preserve remnants of the original organic material. A related mode of preservation is carbonization. Carbonization occurs when the biochemical components of the original soft tissues convert to more stable carbon compounds. The result is a black or brown film that forms on the bedding plane. Carbonaceous fossils may preserve the outline of the entire body, details of internal anatomy, and even remnants of skin pigment!

Invertebrates like mollusks commonly occur as natural molds. After the rock hardens around the shell, the shell dissolves away leaving a void. But the rock retains the original shape of the shell as internal and external molds. If sediment or minerals fill the void, they form a replica of the original shell, which we call a cast. Trace fossils–fossilized evidence of animal behavior, like tracks and burrows–may also occur as molds and casts.⁵

Whole preservation in amber is a rare form of fossilization. This type of preservation occurs when sticky tree resin traps an organism or part of an organism and then transforms into hard, often transparent, amber. Amber sometimes contains entire insects and spiders, and more rarely, parts of frogs, lizards, and birds. Freezing in permafrost or desiccation (extreme drying) in caves can also preserve whole organisms.⁶

Why Fossilization Matters

As we’ve seen, there are lots of different modes of fossil preservation, which all require unique sets of conditions to occur. An organism can be preserved in a variety of different ways depending on the conditions present before, during, and after burial. Now, think about this: What if we could figure out what those conditions were? What could young-age creationists learn from this? Young-age creationists generally attribute much of the fossil record to the worldwide flood of Genesis 6-9. As such, we predict that fossilization, at least in its initial stages (i.e., burial), happened relatively quickly, and that the time between burial and discovery for all fossils is only thousands (not millions) of years.

Now, what if we could actually unravel the processes that formed specific fossils? Imagine how much deeper of an understanding we could get about what was going on before, during, and after the Flood, not just generally, but at specific times and places! We could point to particular fossil beds and (with much fear and trembling) describe how and when they likely formed! But figuring out the conditions of fossilization isn’t just a hypothetical goal or idea. It is the work of paleontologists who study taphonomy.

Taphonomy: Law of the Grave

Taphonomy is the study of how fossils form, from the moment of death to the moment of discovery. The word taphonomy comes from two Greek words that mean “grave” and “law,” which is fitting given that the taphonomists’ goal is to discover the principles (“laws”) by which organisms go from the living world to becoming part of the earth itself (the “grave”).

When an organism dies, the normal processes that uphold life, including all the trillions of chemical reactions carried out by enzymes within cells, no longer function for their intended, life-giving purpose. Instead, enzymes begin tearing down cells and tissues (autolysis). Microorganisms that normally help sustain multicellular organisms begin to consume them from the inside. Microorganisms and scavengers from the environment then begin to do their best work to destroy the remains.

When acting on organisms that are not biblically-defined as “living creatures,” like plants, this process is normal and would have functioned the same before the Fall. But for us and the other “living creatures” (see Genesis 1:20-21), this process is one of the ongoing effects of sin on creation (Genesis 3:19; Romans 8:19-22). Yet it is also a reminder of the promise that God made to preserve life under the Noahic covenant (Genesis 8:20-9:17). God’s creatures, great and small, are programmed to lay the foundations for new life.⁷

These same processes also make fossilization extremely unlikely under most conditions. Everyday experience and scientific research show us that remains of living things are normally quickly destroyed. Vultures and ravens quickly pick roadkills clean. The bones get disarticulated (separated at the joints), scattered, broken, and trampled into the earth by animals. Insects and moss make habitats out of fallen logs, while fungi and wood-eating insects go to work slowly breaking them down.

In the aquatic realm, things aren’t much better. Scans of lake-floor sediments rarely yield anything more than scales and scattered fish bones.⁸ Whale falls are rapidly picked clean, after which bone-boring organisms help destroy the skeleton.⁹ Marine invertebrates disarticulate, with their hard parts getting attacked by other invertebrates like snails that rasp holes into hard shells. Physical processes like dissolution, weathering, transportation, and abrasion also destroy hard parts.

And despite all this, billions of fossils remain for us to study. How did this happen? How did so many organisms manage to bypass these processes? Taphonomists use two complementary approaches to answer these questions.

Approaches to Taphonomy 

The first approach is making observations and conducting experiments on what occurs when organisms die in the present day. It is reasonable to think that the processes that affect the remains of living things today were also at work in the past. Therefore, we can use modern examples as analogies for what happened in the past. 

The second approach is to study the fossil remains themselves (and the rocks around them) for clues about what happened between death and discovery. This requires scrupulous attention to details. We record the level of articulation (the degree to which the bones connect as they were in life) and the types and sizes of elements that are present. We note whether they show a preferred orientation, and whether there are marks indicating scavenging, trampling, weathering, or abrasion. At sites where we find remains from multiple individuals together, we record the number of elements that occur only once in a skeleton (e.g., the left humerus) to estimate the minimum number of individuals that are present. In many ways, this approach is like a detective trying to solve a murder from the few clues that can piece together from a crime scene. 

Let’s look at an example of the first approach. One way that scientists can study what happens to an organism after burial is by borrowing a technique from the oil industry called a thermal maturation experiment. In this technique, we subject organic material to high heat and pressure on a short time scale to simulate what takes place under lower heat and pressure on a longer time scale.¹⁰ In a 2018 study by paleontologist Evan Saitta and his colleagues, parts of lizards and feathers were compacted in sediment to form tablets, which they subjected to high heat and pressure. The pores in the sediment allowed more mobile products of decay to escape, leaving less mobile products behind, as would occur under a real-life fossilization scenario.¹¹

The result? Most of the soft tissue components of the remains disappeared. But, around the bones, there were thin, dark films preserving the outline of the soft tissues. They effectively replicated the process of carbonization in the lab! When they looked at the dark films under the scanning electron microscope, they discovered that, much like in fossil reptile skin, the only distinct structures they could make out were melanosomes, subcellular pigment-containing structures. Paleontologists have used melanosomes to reconstruct the color of extinct animals, like the dinosaur Psittacosaurus (see the image above).¹²

Now let’s look at an example of the second approach, looking specifically at the taphonomy of bonebeds. Bonebeds are layers containing abundant fossil bones from multiple individuals, which may be from the same or different species. Some bonebeds extend over relatively large areas and contain thousands of individual bones. One bonebed that has been important for paleontologists studying animals from the so-called Triassic “Period” is the Canjilon Quarry near Ghost Ranch, New Mexico. The most common fossils found in this bone bed are bones of phytosaurs, an extinct group of crocodile-like reptiles.

Most of the bones are disarticulated, but there are some articulated skeletons too. Researchers found that the long bones show a preferred north-northwest to south-southeast orientation. These fossils come from a layer of mudstone containing pieces of rock ripped up by fast-flowing water and redeposited. These clues indicate that the bones in the quarry were transported there by a high energy event. The disarticulated nature of many of the bones indicate that some time had passed between death and burial for decay to free the bones, while the articulated skeletons were likely buried soon after death.¹³

Application for Creationist Model Building

All young-age creationist models of earth history require that most of the fossil record formed over thousands (not millions) of years. Thus, we think fossilization occurred rapidly at certain times in the past, especially during the Flood and early post-Flood period. We predict that fossils will generally show signs of rapid burial (often by violent, catastrophic processes) and rapid chemical transformation after burial. We can test these predictions through the study of taphonomy. It is encouraging that conventional paleontologists frequently report evidence for rapid burial in taphonomic studies. We also sometimes see reports that certain chemical processes involved in fossilization occur faster than researchers previously thought.¹⁴

Creationists also have made useful contributions to the field of taphonomy for decades.^{15, 16, 17} Dr. Leonard Brand, one of the leaders in this area of creationist research, thinks our unconventional perspective equips us to make outside the box predictions that lead to fruitful scientific research. In summarizing some of his work in the book Faith, Reason, & Earth History (co-authored with Dr. Art Chadwick), he concludes, “This research … shows how a faith-based research approach can yield new insights that were not found by others who were not asking the same questions.”¹⁸

Footnotes

1. Gould, S. J. (1983). Hen’s Teeth and Horse’s Toes. New York, New York: W.W. Norton & Company, Inc. ↩︎
2. Permineralization and Replacement. (August 16, 2024). Retrieved from https://www.nps.gov/articles/000/permineralization-and-replacement.htm ↩︎
3. Permineralization and Replacement. (August 16, 2024). Retrieved from https://www.nps.gov/articles/000/permineralization-and-replacement.htm ↩︎
4. Impressions and Compressions (including Carbonization). (May 31, 2024). Retrieved from https://www.nps.gov/articles/000/impressions-and-compressions.htm ↩︎
5. Mold, Casts, and Steinkerns. (September 10, 2024). Retrieved from https://www.nps.gov/articles/000/mold-casts-and-steinkerns.htm ↩︎
6. Preserved Remains (Drying, Freezing, Amber, Natural Asphalt). (August 16, 2024). Retrieved from https://www.nps.gov/articles/000/preserved-remains.htm ↩︎
7. Francis, J. W. (2003). The Organosubstrate of Life: A Creationist Perspective of Microbes and Viruses. Proceedings of the International Conference on Creationism, 5, 433-444.  ↩︎
8. Whitmore, J. H. (2003). Experimental fish taphonomy with a comparison to fossil fishes. (Ph.D.). Loma Linda University, United States — California. Retrieved from https://www.proquest.com/dissertations-theses/experimental-fish-taphonomy-with-comparison/docview/288277128/se-2?accountid=25314 ↩︎
9. Esperante, R. (2005). How not to become a fossil—taphonomy of modern whale falls. Paper presented at the 2nd International Meeting Taphos 2005, Barcelona. ↩︎
10. Briggs, D. E. G., & McMahon, S. (2016). The role of experiments in investigating the taphonomy of exceptional preservation. Palaeontology, 59(1), 1-11. doi:https://doi.org/10.1111/pala.12219 ↩︎
11. Saitta, E. T., Kaye, T. G., & Vinther, J. (2019). Sediment-encased maturation: a novel method for simulating diagenesis in organic fossil preservation. Palaeontology, 62(1), 135-150. doi:https://doi.org/10.1111/pala.12386 ↩︎
12. Vinther, J., Nicholls, R., Lautenschlager, S., Pittman, M., Kaye, Thomas G., Rayfield, E., . . . Cuthill, Innes C. (2016). 3D Camouflage in an Ornithischian Dinosaur. Current Biology, 26(18), 2456-2462. doi:10.1016/j.cub.2016.06.065 ↩︎
13. Hunt, A. P., & Downs, A. (2002). Taphonomy of the Late Triassic Canjilon Quarry (Petrified Forest Formation: Chinle Group), North-Central New Mexico: data from new excavations. New Mexico Museum of Natural History & Science Bulletin, 21, 291-296.  ↩︎
14. Yoshida, H., Ujihara, A., Minami, M., Asahara, Y., Katsuta, N., Yamamoto, K., . . . Metcalfe, R. (2015). Early post-mortem formation of carbonate concretions around tusk-shells over week-month timescales. Scientific Reports, 5(1), 14123. doi:10.1038/srep14123 ↩︎
15. Brand, L. R., Goodwin, H. T., Ambrose, P. D., & Buchheim, H. P. (2000). Taphonomy of turtles in the Middle Eocene Bridger Formation, SW Wyoming. Palaeogeography, Palaeoclimatology, Palaeoecology, 162(1-2), 171-189. https://doi.org/10.1016/S0031-0182(00)00111-5 ↩︎
16. Esperante, R., Brand, L. R., Chadwick, A. V., & Poma, O. (2015). Taphonomy and paleoenvironmental conditions of deposition of fossil whales in the diatomaceous sediments of the Miocene/Pliocene Pisco Formation, southern Peru—A new fossil-lagerstätte. Palaeogeography, Palaeoclimatology, Palaeoecology, 417, 337-370. doi:https://doi.org/10.1016/j.palaeo.2014.09.029 ↩︎
17. Snyder, K., McLain, M., Wood, J., & Chadwick, A. (2020). Over 13,000 elements from a single bonebed help elucidate disarticulation and transport of an Edmontosaurus thanatocoenosis. PLoS ONE, 15(5), e0233182. doi:10.1371/journal.pone.0233182 ↩︎
18. Brand, L., & Chadwick, A. (2016). Faith, Reason, & Earth History (3rd ed.). ↩︎