Understanding the Fossil Record Bias, Part I: Fossilization’s Influencing Factors

Yes, there is a bias in the fossil record. We may have millions of specimens from across the geologic timescale, but that doesn’t mean the record is complete or unbiased. Fossilization is a delicate and intricate process, and is influenced by many factors. As a result, not every organism gets left behind for us to find. But what factors are at play here, and how do they impact our understanding of ancient life? 

In Part I of Rex Machina‘s “Understanding the Fossil Record” series, we examine the answers to these questions and their importance to the field of paleontology.

What Influences the Fossilization Process?

Fossilization is the result of a set of physical and temporal forces acting on the remains of an organism. Sometimes luck is the deciding factor, but a number of conditions can improve an organism’s chances.

Environment

Certain environments lend themselves to preservation. However, the main factor in any environment is deposition. If an organism lives in an environment where sediment is not being deposited, its remains cannot be fossilizedRiver valleys, deltas, floodplains, ocean basins, etc. are ideal locations for this. The energy in flowing water pushes sediment along and deposits it into basins or depressions. These fluvial environments cover an organism’s remains with sediment, protecting them from scavengers and bacteria; the faster the remains can be covered, the better the odds of preservation become.

Image of the Yukon River Delta, taken from Copernicus Sentinel-2 Mission
River deltas are areas of high sediment deposition, as illustrated by this image of the Yukon River Delta in Alaska, USA. Photo Credit: European Space Agency (CC BY-SA 3.0 IGO)

Arid environments are also ideal preservational areas. Dry, hot deserts, for example, will desiccate an organism’s remains, making them less attractive to scavengers. Windblown sands can also cover the remains quickly and preserve them from other natural forces.

In contrast, you rarely find fossils in lush jungles and on mountaintops. Jungles have lots of of scavengers and bacteria breaking down carcasses. And, like mountaintops, jungles have limited depositional opportunities. More extraordinary circumstances are necessary in these environments for an organism to fossilize.

Geology

Even if the environmental conditions are right for preservation, geology can threaten a fossil’s long-term survival. Exposed to the elements and time, a fossil can either can degrade to nothing or past recognition and utility for scientists. Likewise, geologic activity can push remains down past where humans can access them, and/or destroy the remains completely. 

Certain geologic processes can work in a fossil’s favor, however. For example, if lithification–the process of converting loose-grain, deposited sediment into rock–occurs early, the fossil is able to withstand more pressure and changes in temperature. While this doesn’t protect a fossil from being deformed, early lithification can prevent crushing and further destruction.

Image comparing two Tyrannosaur skulls.
Notice the differences between the skull of SUE [Tyrannosaurus specimen FMNH PR2081] on the left v. Stan [Tyrannosaurus specimen BHI 3033] on the right. Stan’s skull has been reconstructed, while SUE’s has retained some of the deformities associated with the geologic processes acting on it over time. Photo Credits: Geoffrey Fairchild [left] via Wikimedia Commons (CC BY 2.0), Steve Jurvetson [right] via Flickr (CC BY 2.0)

The Organism

Three key factors of an organism influence its ability to fossilize: size, tissues, and abundance/diversity.

Size

Size can work as an advantage in either direction. Typically, the bigger the organism, the greater its chances of preservation. The remains of small animals are more susceptible to natural forces or destruction by larger animals, whether through consumption, trampling, and so on. Because scavengers and bacteria have a more difficult time breaking down an elephant than a rodent, for example, larger remains get more time to fossilize intact. 

However, size also has its disadvantages. Larger organisms require more depositional energy and sediment; otherwise, they may not be buried fast enough. Smaller organisms require less, provided they can survive other natural forces. This is especially true for small invertebrates (bivalves, gastropods, etc.). Their remains are small enough to behave like another clast–a fragment of pre-existing rock, mineral, or other material–in sedimentary environments.

Tissues

Squishy bits don’t really make it through the early steps of fossilization. Scientists often refer to what typically fossilizes as the “hard parts,” and these are most often bones. We sometimes get lucky with skin impressions or feathers, but these are the exception rather than the rule. In rare cases, soft tissues are preserved due to calcite or other carbonate materials replacing them during fossilization. For this to occur, specific conditions within the organism’s remains must be present. Bacteria appear to have some sort of control/influence as well.

Photo of a crinoid and a seastar fossil in impeccable preservation.
This level of preservation is extremely rare and requires unique circumstances. Photo Credit: James St. John via Flickr (CC BY 2.0)

Non-Anatomical Factors

A species’ potential to be preserved in the fossil record also depends on three non-anatomical conditions: abundance, geographic range, and temporal survival. Species that are more abundant often get more opportunities to fossilize. For example, with their low population numbers, tigers may have less chances than feral domestic cats (whose numbers are up past 60 million) to enter the fossil record. A species’ abundance occurs as a result of complex population dynamics influenced by both the species itself and external forces, and differs between species.

How a population spreads out geographically also impacts fossilization potential. Moving out to a broader range of geographies increase a species’ chances of entering regions where fossilization conditions are more favorable. Limited spreading does not mean a species is out of luck, however. If a species can survive on a longer geologic timescale, exposure to environmental changes, sudden depositional events, and other unique conditions for fossilization increases.

Impact of Preservational Bias

The factors influencing an organism’s chances of becoming a fossil are not evenly distributed. As a result, what remains does not fully represent ancient life and ecosystems.

As mentioned earlier, we typically only get the hard parts of organisms. Things such as bones, shells, claws, and teeth are usually tough enough to sustain natural forces acting on them; fleshy bits–not so much.

Photo collage showing progression of decomposition of a pig's remains over time.
As you can see from these images taken during a decomposition study, little-to-no soft tissues remain at the end of the decay process for this pig. Photo Credit: Hbreton19 via Wikimedia Commons (CC BY-SA 3.0)

This is the rough deal invertebrates are dealt. For example, we’ve observed large communities of invertebrate and soft-bodied organisms compose about two-thirds of the total species and individuals in nearshore ecosystems. The fossil record doesn’t reflect this, however. Soft-bodied organisms require the highest-quality depositional environments to fossilize. 

Lack of soft-tissue preservation also limits our understanding of the organisms that do have hard parts. A lot of paleoartists talk about the bias through the idea of the “shrink-wrapped” dinosaur. One particular debate involves the spines of animals like Spinosaurus. When looking at skeletal remains, the vertebral spines of Spinosaurus look similar to those of bison and other animals known for humps on their backs. There’s no soft-tissue available to study from Spinosaurus, so it’s impossible to fully argue which interpretation is right.

Things like plumage, fat, skin texture/thickness/color, cartilaginous structures (like our noses), and reproductive organs aren’t available either. Scientists and artists go back and for about things like feathers on Tyrannosaurus rex, the mating habits of stegosaurs, and how sauropod necks worked because of preservational bias. Some of this data we can try and model or make inferences about based on other fossils and modern animals, but questions always remain.

Skeletal reconstructions of the oviraptorid Ajancingenia taking into account soft tissues.
As you can see, preservational bias gives us one particular image of ancient life; if we had soft tissues, it’s possible we’d get something closer to the bottom-right. Image Credit: Jaime A. Headden via Qilong (CC BY-NC-ND 3.0)

Human Factors Play a Role, Too

Human biases and behaviors influence the fossil record as well. Our self-interests and natural shortcomings can and do impact the fossil record in ways we might not always be consciously aware of.

Academic and commercial interests are some of the biggest influences on the fossil record. Commercial fossil hunters, for example, look only for coveted and rare fossil specimens they can sell. Sometimes–like in the infamous case of Halszkaraptor–unique and important fossils end up in private collections as a result. Mistreatment by private collectors can also jeopardize rare specimens, stunting our greater understanding of ancient life.

While academic excavation is a bit more noble in its pursuits, it is not bias-free. Paleontologists may have trained themselves to only identify certain types of fossils. As well, academic investment in their own work may also influence what they dig up.

Political issues also come into play. Certain fossil locations may not be accessible because of the region’s political landscape. For example, locations in places like Iran or North Korea may not be open to American scientists. Additionally, fossil locations once protected may have had those protections removed (*cough* Bears Ears/Grand Staircase Escalante… *cough*). These issues impact scientists’ available area for exploration, further limiting the fossil record.

Photo of a person wearing a unicorn mask
Look, until we get North Korea to open up, we’re never gonna know if that Unicorn lair was legit.

Why Does the Bias Matter?

We have a lot of data about our modern world and the complexity of its ecosystems. All this data is available to us through direct observation–we can watch ecological dynamics play out in front of us. With ancient ecosystems, we don’t have that luxury. We can infer that the ecology of our world today was not too dissimilar to past ecologies based on our understanding of life. However, we cannot directly observe a Cretaceous, Cambrian, or Miocene ecosystem. We’re therefore dealing with an imbalance of data. Factors that influence fossilization and discovery further compound this. By attempting to reconcile what we understand of the present with what we know will influence future preservation, we can go back and more accurately address the discrepancies in the fossil record.

So, how do we go about that reconciliation process? Stay tuned for Part II, where we dive into the study of taphonomy and its importance in understanding the fossil record.

 

Header Image Credit: dvs via Flickr (CC BY 2.0)

Special thanks to Mariana Di Giacomo for her help in putting this blog post series together. Go check her out on Twitter @MarianaDGiacomo or at her website, marianadigiacomo.com!

References/Further Reading:

Allison, Peter A., and David J. Bottjer. “Taphonomy: Bias and Process Through Time.” Topics in Geobiology Taphonomy, 2010, 1-17. Accessed November 13, 2018. doi:10.1007/978-90-481-8643-3_1.

Hendricks, John R. “1. Nature of the Fossil Record.” Digital Atlas of Ancient Life. February 13, 2018. Accessed November 13, 2018. http://www.digitalatlasofancientlife.org/learn/nature-fossil-record/.

Hone, Dave. “Bias in the Fossil Record | Dave Hone.” The Guardian. August 17, 2012. Accessed November 13, 2018. https://www.theguardian.com/science/lost-worlds/2012/aug/17/bias-fossil-record.

Holtz, Thomas R., Jr. “Fossils and Rocks.” GEOL 104 Dinosaurs: A Natural History. University of Maryland, Department of Geology. September 1, 2000. Accessed November 13, 2018. https://www.geol.umd.edu/~tholtz/G104/104Y2K/104Lec03.htm.

Erickson, Gregory M. “What Are the Odds of a Dead Dinosaur Becoming Fossilized?” Scientific American. September 16, 2002. Accessed November 13, 2018. https://www.scientificamerican.com/article/what-are-the-odds-of-a-de/.

Naish, Darren. “Dinosaurs and the Anti-Shrink-Wrapping Revolution.” Tetrapod Zoology. May 08, 2017. Accessed November 13, 2018. https://blogs.scientificamerican.com/tetrapod-zoology/dinosaurs-and-the-anti-shrink-wrapping-revolution/.

Sansom, Robert S. “Bias and Sensitivity in the Placement of Fossil Taxa Resulting from Interpretations of Missing Data.” Systematic Biology 64, no. 2 (2014): 256-66. Accessed November 13, 2018. doi:10.1093/sysbio/syu093.

Starrfelt, Jostein, and Lee Hsiang Liow. “How Many Dinosaur Species Were There? Fossil Bias and True Richness Estimated Using a Poisson Sampling Model.” Philosophical Transactions of the Royal Society B: Biological Sciences 371, no. 1691 (2016).. Accessed November 13, 2018. doi:10.1098/rstb.2015.0219.

“Under What Conditions Do Fossils Form?” American Geosciences Institute. November 17, 2016. Accessed November 13, 2018. https://www.americangeosciences.org/education/k5geosource/content/fossils/under-what-conditions-do-fossils-form.

Walley, Mike. “The Potential for Preservation — Potential for Fossil Preservation of Species.” The Everything Dinosaur Blog. February 11, 2008. Accessed November 13, 2018. https://blog.everythingdinosaur.co.uk/blog/_archives/2008/02/11/3515394.html.

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.