What is a fossil?

Roy Plotnick
3 min readJan 27, 2020
A caddisfly case from the Oligocene of Montana. The case is made of tiny ostracode shells.

In a Peanuts comic that had a place of honor on my dorm door, Peppermint Patty is taking an exam and is asked to: “Explain World War II.” Patty incredulously responds: “Explain World War II !?” The last panel, further explains: “Use both sides of the paper, if necessary.” I recently had a similar reaction, when science reporters, interviewing me about my most recent paper, asked me to explain in a few words, “when do bones and teeth become fossils?” Although they are orders of magnitude fewer in number than those written about World War II, nevertheless there are many books and many hundreds of academic papers written about the formation of fossils, the science of taphonomy. Not so simple to answer in a couple of sentences. In addition, it was implicit in their questions, and from one asked by a reviewer of the paper, that the process of fossilization must include some sort of change to the bones and teeth, that they must become “fossilized,” that is, changed to some sort of rock (think of petrified wood).

At the heart of this is the misconception of what a “fossil” is. Originally, the term was used to refer to anything dug up from the Earth, including gems and ores. This concept still survives in the term “fossil fuels,” but otherwise the term has become restricted to the preserved remains of living things. Fundamental to the concept, therefore, is that fossils were once part of the biosphere and now part of the lithosphere. Or as I facetiously tell my students, “fossils are dead things that don’t smell anymore.” That still leaves two issues. First, archaeologists also study the preserved remains of living things, although they focus on humans and their associated plants and animals. To try to keep the two disciplines separate, some older definitions of fossil decree that they must be “prehistoric.” The problem with this definition is that what may be prehistoric in one part of the world, such as Alaska, is within historic times in places such as China.

The second problem is the misconceptions is that fossils are shells, bone, and teeth that have somehow become changed from their original composition. In reality, there are a huge variety of types of fossilized remains other than bones and shells. For example, fossil insects and leaves are not uncommon. Post-burial changes in dead remains, known as diagenesis, is common but is not a necessary requirement for something to be considered a fossil. There are cases where soft tissues such as muscles and nervous tissues are preserved. It is rare, but not unheard of, for the original biomolecules to be present after hundreds of millions of years. Fossil feathers and leaves can preserve their original colors. As for bones and shells, they can be preserved virtually unaltered from their original form. There are trilobites that show their original color patterns and ammonites with pristine mother-of-pearl in their shells. Unaltered shells are the source of the isotopes that allow us to reconstruct climate deep into the history of the Earth

So, what is a fossil? I have long-ago adapted and teach a definition written by Anna Kay Behrensmeyer, Susan Kidwell, and Robert Gastaldo (2000, Paleobiology): “A fossil is any nonliving, biologically generated trace or material that paleontologists study as part of the record of past life.” This definition covers all the key bases: fossils are non-living, but they are biologically generated, and they are the only direct record we have of life in past. No assumption of age; no necessity for diagenetic change. Most importantly, it recognizes that something is a fossil if a paleontologist studies it! Paleontologists are students of the history of life on Earth: we study fossils, no matter what their age, composition, or structure, if they allow us to document and understand that history.

Note: If you think you have found a fossil, see my post: https://plotnick.medium.com/i-found-a-fossil-or-did-i-a424462af352

My new book, Explorers of Deep Time: Paleontologists and the History of Life, is now available from Columbia University Press.



Roy Plotnick

Paleontologist, geologist, ecologist, educator. Professor at the University of Illinois at Chicago. Author of Explorers of Deep Time.