Concrete Sidewalk Impressions: What's A Fossil?

In which I, once again, may be accused of obsessing about issues involving trees while missing the spirit of the forest.

For a reason that now escapes me, I ended up on the Nova Scotia Museum’s website blog, specifically on the post for March 25, 2020, the one titled Urban Geology.  The post deals with the impressions captured by chance in concrete sidewalks, specifically, those imprints left by plants (mostly leaves) and animals (often birds or humans) in the concrete when it was still wet.  The author (Tim Fedak, I think) labels these as urban fossils with no explanation as to why they are fossils.  The post includes photographs of sidewalks showing interesting random traces of the passing natural scene.  Since I am easily sidetracked and entertained, I spent the next few of my local outings in search of these delicate and evocative sidewalk impressions.  Here are a few of my finds discovered while I, once again, spent time staring at the ground where the paleontologist’s gaze is so often fixed.  I provide no identifications for the images of leaves, given no likely suspects standing in the immediate vicinity or, at least, not ones I could identify.

It was an enjoyable time fritter.  By the way, the last picture shows various impressions of the treads of running shoes.

The Nova Scotia Museum’s blog post steered me to retired middle school science teacher Kenneth Fuller and his Science Can Be Fun! website.  Here Fuller lays out various science activities for students and their teachers, including a paleontology unit that focuses on these sidewalk impressions, which he calls Sidewalk Fossils.  These are potentially engaging activities that, as he structures them, might instill some worthy habits in student participants, ranging from close observation of phenomena to recording what is observed to fashioning hypotheses about the origins of these phenomena.

Significantly, though, he asserts that concrete impressions in sidewalks are actually fossils.  He writes:

By definition a fossil is any remains or trace of a living thing preserved in rock.

He then states that the concrete impressions of leaves and animal tracks meet that definition:

The concrete of which a sidewalk is made is rock. . . .  Since the concrete is rock, remains or traces of living things preserved in the concrete are fossils.  Since such fossils are most often seen in the concrete of sidewalks, we refer to them as "sidewalk fossils" wherever we find them.

Posit an erroneous definition and all sorts of mischief can ensue.  The central error of his definition is made clear below.  Further, there is an unsettling note of authority in this.  Who’s the “we” calling them sidewalk fossils?  “Scientists” is the implication, I think.  He later distinguishes these fossils from natural fossils with no attempt to explain the differences.  I also don’t know what to make of his suggestion on another page of his website that, when teachers fashion a definition of fossil with their students, that they have “some real fossils to show [which] would help at this time.”  Real?  As opposed to what?  Sidewalk fossils?  More than a bit confused.

Later I read an entertaining piece by science writer Jessica Leigh Hester on the Atlas Obscura website titled Fresh Concrete Turns Paw Prints and Bird Tracks Into Urban Fossils (September 18, 2020).  At times she refers to them as fossils, modern trace fossils, and concrete impressions, and notes they have “prehistoric counterparts.”  The article profiles Carl Mehling, a specialist in paleontology at the American Museum of Natural History, who has had a longstanding interest in these sidewalk impressions, amassing a collection of several hundred photographs of them, particularly, of animal tracks.  The photographs included in Hester’s piece are quite beautiful.  Of the origins of his interest, Mehling says that, as a second-grader, he came upon a leaf impression in a sidewalk.  “I regarded it as a fossil, and was scheming about getting a hammer and busting it out.”  

Hester posits that animal trackways preserved in concrete pose questions of interpretation like those that bedevil paleontologists working with trace fossils (ichnofossils) of dinosaurs and other ancient animals.  Among the questions raised by both kinds of trackways are:  What animal made these marks?  What was it doing?  How many were involved?  Were all of these marks made at the same time?  She writes, “It’s a slippery task to try and draw too many conclusions from the trackways – which actually makes them similar to prehistoric prints, Mehling says.” 

Though pursuit of concrete sidewalk impressions makes for an enjoyable hunt, the literature I've cited clearly troubles me.  Do these authors believe these are actually fossils or am I missing the gentle joke here?  Is the label fossil in these texts applied with a touch of whimsy or poetry and a gentle wink to let the reader know quotation marks surrounding the word are implied?  I would have let slide their use of fossil in this context had any one of them explained that the trace fossils studied by paleontologists are somewhat similar to, but critically different from, say, a bird’s trackway captured in wet concrete.  They don't.

Hester's article is the one that puzzles me the most.  With a paleontology professional at hand, she never once quotes Mehling weighing in on whether the label fossil is appropriate for the concrete trackways whose pictures he has amassed.  I’m sure he could have opined knowledgeably on the subject.

A discussion of what the common or scientific definitions are of fossil seems appropriate at this juncture.  Regarding common usage, my Compact Oxford English Dictionary (revised 2003) provides, as its first definition: 

the remains or impression of a prehistoric plant or animal embedded in rock and preserved in petrified form.

Certainly of no help to the Nova Scotia Museum, Fuller (despite the emphasis on rock), or Hester.  The use of the adjective “prehistoric” categorically disqualifies concrete impressions.  (The second definition is “an antiquated person or thing” which also doesn’t come to their aid.)

Turning to how the word is defined in the science devoted to the study of fossils, I will cite paleontologist Richard Fortey whose definition is more succinct and broader than the OED's:

Fossils are the remains of prehistoric animals or plants.

He immediately adds: 

Usually they are some hard part of an extinct organism, resistant to decay, that has been preserved enclosed in sediment – past life that has been buried in the rocks and entombed inside them.  Fossils could equally be the record record of activity of animals:  fossil footprints, perhaps, or the tubes and trails of soft-bodied worms that otherwise leave no trace.  (Fossils:  The History of Life, 4th edition, 2009, p. 8.)

Fortey’s definition starts with that succinct statement and then acknowledges the complexity of what is embraced in that terse phrasing.  Of most importance for my purposes is that telling adjective “prehistoric.”  Again, this precludes these sidewalk markings and trackways as being fossils.

Anyone who has had the briefest exposure to paleontological literature will realize that the challenge in defining what is a fossil lies in the many ways in which science explains that animals and plants (and traces of their activity) become fossils, that is, are fossilized.  Fortey covers all of the outlier processes with the adverb usually.  Indeed, several of those ways do not involve being "preserved in rock."  Sorry, Mr. Fuller.

Although I could rustle up other contemporary paleontologists’ definitions of fossil and see if concrete impressions might fit into one of those definitions, it would mostly be a fool’s errand.

Mehling himself makes that abundantly clear in an essay he wrote for Fossil News:  The Journal of Avocational Paleontology titled Collecting My Thoughts:  A Paleontologist Ruminates on What Makes a Fossil a Fossil (Volume 20, Number 1, Spring 2017).  This is a fascinating, albeit meandering and very long, exploration of the various definitions of fossil that have been employed over time.  In the course of this essay, Mehling describes a host of different ways in which animals and plants generate fossils, and suggests that any satisfactory definition should embrace as many of these fossilizing processes as possible.  He acknowledges that a fixed, rigid, “bullet-proof” definition is likely to be counterproductive and certainly not universally embraced.  Besides, our understanding of what is a fossil has evolved over the past several centuries as our knowledge has grown.  As a result, definitions have changed.

Mehling takes the reader through many definitions and the multitude of things that have at one time or another been called fossils (of note, sidewalk impressions are not among them).  He highlights the utility and, more importantly, the deficiencies of these definitions, and finally comes to rest (well, mostly) on a definition satisfying many of his concerns:

For now, here’s a useful definition:  “fossils are evidence of life that is at least 10,000 years old.”  That conforms reasonably well to a hazy intuition, leaving a few loose ends and poetic traps.  While increased qualifications may fortify definitions, they may also merely dilute the intent of the word into oblivion.  (p. 38)

He notes that a definition resting on an age distinction (such as “prehistoric” or “at least 10,000 years old”) “incorporates more of what are most commonly considered fossils and excludes those that make most paleontologists grimace and shake their heads.”  (p. 34)

Certainly, Mehling would dismiss the notion that sidewalk impressions are real fossils.  They fail the age test.  Still, he would understand the impulse to attach the word fossil to them, an impulse reflected in how often the word has been co-opted in the past.

In closing, I must cite the clearest definition Mehling considers, one nestled in the last footnote of the essay:

My sister Ellen offers the following definition:  “Old bones and shit like they have in the museum.  You know, dinosaur bones!  You know what I mean.”  I guess I should’ve checked with her first and saved myself all of this trouble.  (p. 38)