The most common fossil
I am not a fan of house sparrows. I have a number of bird feeders in my small backyard and would love to see a variety of birds at them. But nearly all I get are house sparrows and I have witnessed them intimidating and chasing away other birds, such as goldfinches. Similarly, even though the occasional opossum or rabbit shows up, nearly all of the mammalian visitors are squirrels.
We all recognize that some species are incredibly common. Travel to any city, you will see pigeons (and rats). Rural areas are overrun with deer; red-winged blackbirds are ubiquitous along road sides. These common species are not only important numerically, their sheer numbers give them major ecological importance.
For a number of years, I have been interested in the question of which species are most common in the fossil record; that is, what are the pigeons of paleontology? Working with my colleague Peter Wagner, we used an extensive online compilation of all known fossil collections, the Paleobiology Database (paleobiodb.org), to determine which genera show up most commonly in fossil collections. We did this by counting occurrences; the number of collections that contain a specific genus. Some of our results surprised us; only a small fraction of all the genera account for a large percentage of all the occurrences. What was not surprising was which genera these were; they were the ones that we and every paleontologist (and geologist) learned from our very first paleontology class. They are the genera pictured in almost all introductory historical geology books and are found in even the most basic teaching collections. They are even among the very first genera given a scientific name.
By far the most common fossil, based on the number of times it occurs in collections, is the snail Turritella, which is not only found almost everywhere since the Cretaceous, but is often quite abundant within each collection. But what I am going to argue here is that by whatever criteria one wants to use to measure commonness, Turritella is far outstripped by something called Chondrites. What is Chondrites? It is not a type of meteorite, although the spelling is identical. Instead, Chondrites is a trace fossil (aka ichnofossil), the class of remains that includes tracks, trails, burrow, borings, and everything else where the activity of an organism has left some sort of mark behind. Shells, teeth, bones, etc. are, in contrast, known as body fossils. Probably the most familiar trace fossils are the numerous tracks of dinosaurs which, like all trace fossils, give valuable clues to their maker’s behavior. Overall, however, compared to body fossils, trace fossils are unfortunately overlooked when we discuss the history of life.
Chondrites is easily recognizable. It is a preserved burrow in firm marine sediments and, as described in the very useful site put together by Stephen Hasiotis and his students at the University of Kansas (http://ichnology.ku.edu/), is made of: “Large complexes of small root-like tunnels and shafts in dendritic patterns… Sometimes, look like bird feet.” Of course, you might be asking, “what made Chondrites and why?” As is often the case with trace fossils, there are no definitive answers. Based on its shape and comparison with burrows made by modern marine annelids, the trace maker was certainly some kind (or kinds) of worm-like animal. It is highly unlikely that the trace maker is a single species, since almost no species survive for 500 million years. It is more likely that it is a group of perhaps unrelated species, all with a worm-like body form, that moved in a similar way and left behind geometrically similar traces. And why was it moving this way? The reasonable consensus among trace fossil specialists (ichnologists) is that the animal was feeding on organic materials buried in the sediment, a life habit known as deposit feeding. The branching pattern represents an efficient way of mining a localized concentration of resources.
Why do I argue that this may be the most common fossil of all time? First, Chondrites first appears in the Cambrian and has been found in deep sea cores of modern age, so it has existed for more than 500 million years. Second, it is found in wide range of sediments and environments, from shallow to deep water, including settings with lower than usual oxygen concentrations. And finally, and what has always struck me, is that there are a lot of them. I know of sites in the Ordovician of Illinois were there are acres and acres of rock containing Chondrites. Since these represent small exposed sites of what was once a vast shallow sea covering much of North America, the total number of these traces must be well up in the billions. Considering how numerous the trace makers must have been, their total impact on the ecology and environment of the oceans must have been tremendous.
There are other extremely common trace fossils. They represent the best record of those organisms, such as the myriad kinds of worms, that would otherwise leave no indication of their existence. They attest to how abundant these forms of life must have been. And they tell us that a history of life based only on body fossils is woefully incomplete.