Episode 137 – Fossilization

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When paleontologists examine ancient remains, they need to consider not only what they’re looking at, but how it got to be there. This episode, we discuss the various processes, incredible possibilities, and crucial limitations of Fossilization.

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What’s a Fossil, Anyway?

The word “fossil” has had a variety of meanings over the centuries, and there isn’t one universal scientific meaning today. But generally speaking, a fossil is the remains of an organism that lived long ago. Fossils can represent entire bodies, body parts, or traces of animals, plants, bacteria, and anything that was once alive. The oldest fossils are at least 3.5 billion years old, and perhaps older.

Fossils come in many, many forms: they can be frozen in ice or trapped in amber; they can be mere footprints or chemical remains. But in this episode, we focused mainly on the most common form of fossilization: being buried and preserved in sediment.

How to Become a Fossil

Step 1: Get buried. Sediment, be it sand, ash, mud, or something else, is the main substance of preservation for most fossils. So, to become a fossil, organism remains generally need to be buried before too much damage deteriorates them.

Generally, organism remains need to be buried before they can be fossilized. The sheer number of things that can happen to remains before burial makes this pretty rare.
Image: Wombat in Jenolan Cave, Australia.

Not all remains are equal when it comes to being buried. The speed and efficiency of burial in sediment can be influenced by a vast list of factors, including the size of the remains, the environment of burial, the local weather or climate conditions, the resilience of the remains to deterioration, and much more. This leads to preservation bias: a large-bodied, hard-shelled organism in a deep, calm lake is more likely to be buried before deteriorating than a small, squishy organism in a thriving tropical rainforest. Most organisms do not fossilize, and most fossils have already deteriorated somewhat by the time they do.

Step 2: Turn to stone. Once remains are buried, they can begin the long process of fossilizing. Most commonly, this involves “turning to stone” by means of permineralization and replacement.

Permineralization occurs when empty spaces in a fossil become filled with minerals dissolved and redeposited from surrounding rock.
Left: Permineralized Devonian bryozoan. Image by Kenneth C Gass, CC BY-SA 4.0
Right: Petrified wood from Petrified Forest National Park, Arizona. Image by Jon Sullivan.
Replacement occurs when the original materials of an organism dissolves away, and new minerals are deposited in their place. If this happens slowly enough, it can preserve even fine details while transforming a shell, bone, or other material into a different mineral.
Top: A variety of silicified Permian aquatic organisms. Image by Mark Wilson, CC BY-SA 3.0
Bottom: Pyritized Jurassic ammonites. Image by James St. John, CC BY 2.0
Top: Sometimes the original remains disappear completely, leaving only an impression (mold) in the lithified sediment. This is a mold of a Carboniferous bivalve. Image by Mark Wilson.
If a mold becomes filled, it can create a natural cast, hardened sediment in the shape of the original fossil.
Bottom: Carbonized leaf fossils like this one from the Eocene Green River Formation form when soft tissues are squeezed between layers of sediment, leaving behind only a carbonaceous film. Image by James St. John, CC BY 2.0

Each form of fossilization comes with crucial caveats. A permineralized or replaced bone can sometimes preserve extremely fine details, but much of the original material, especially soft tissue, will be missing. Natural molds are extremely common, for example with shells, but they only preserve the outer shape of the shell. Carbonized fossils can preserve evidence of soft tissues of leaves or soft-bodied animals, but they are commonly flattened and deformed.

There are other forms of fossilization, of course, besides the typical Burial + Petrification. Trace fossils can preserve evidence of behavior, though they can be highly varied and difficult to interpret. Ice can preserve entire bodies, soft tissue and all, and even DNA, but generally only captures life over the last few tens of thousands of years. Bog bodies are also great for soft tissue, but bad for DNA, and again, only preserve recent things. Amber is a fantastic source of soft tissue information, but usually only preserves small-bodied organisms.

Top left: Eubrontes, a classic theropod dinosaur footprint. Image by Mark Wilson.
Top right: Lyuba, the famous frozen baby wooly mammoth. Image by James St. John, CC BY 2.0
Bottom left: A human bog body in Germany. Image by Bullenwächter, CC BY-SA 3.0
Bottom right: An ant preserved in Baltic amber. Image by Anders L. Damgaard, CC BY-SA 3.0

For a paleontologist, understanding the method of fossilization is just as important as being able to identify the fossil species or its ancient lifestyle, because that process determines what is left behind and how it has been altered.

A fossil site with exceptional preservation is called a Lagerstätte. Examples include the Burgess Shale and Mazon Creek. This image depicts two fossil fish, Diplomystus and Knightia, preserved at the Green River Formation in Colorado. Image by Photolitherland, CC BY 3.0
Paleontologists can study fossilization by making lab-grown fossils. These are called maturation experiments. In this photo, A and D are unaltered feathers, and B, C, E, and F show the same feathers after being subjected to high temperature and pressure to create “faux” fossils. Image from Maria McNamara et al. 2013

Dig Deeper

How to Turn Yourself Into a Fossil
How Can I Become a Fossil?

Fossilization – How Fossils Form. Lots of resources from the Smithsonian.

Studies on fossilization:
Lab-grown fossil experiment, 2018. And here’s the technical paper.
Molecular preservation in mammoth bones, 2021 (technical, open access)
Molecular preservation in dinosaur bones, 2020, and here’s the technical paper.

If you want a really deep dive into how paleontologists interpret the fossil record, check out another one of our major references for this episode: Principles of Paleontology, 3rd edition

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