Making your Bones:
The Fossil Record and Taphonomy
Michael F. Forlenza, P.G.
Editor, HGS Bulletin
The fossil record is a window to view the vast scope and range of life on earth. The earth is more than four and a half billion years old and the earliest fossils, such as stromatolites, are more than three billion years old. But abundant fossils assemblages are not found until we look at rocks that are around 530 million years old. Sediments that were deposited at this time contain the fossil evidence of the wild diversification of species known as the Cambrian explosion.
Critics of evolution cite gaps and discontinuities in the fossil record and suggest that there is an inadequate representation of transitional forms to support the current macroevolutionary understanding and the resulting phylogenetic tree. It is true that the there are gaps and discontinuities, it could hardly be otherwise. The development of a comprehensive catalog of life is unrealistic. Only about 250,000 of the more than 1.6 million existing species have been identified. And paleontologists estimate that more than 99 percent of all species that have ever existed are extinct.
But how does an animal or a plant become a fossil and how representative is the fossil record? Taphonomy, a sub-discipline of paleontology, involves the study of the processes affecting decaying organisms over time and how the remains might become fossilized. The term taphonomy, from the Greek taphos meaning burial, was introduced by the Russian scientist Ivan Yefremov in 1940 to study the transition of the remains of organisms from the biosphere to the lithosphere. One of the objectives of taphonomy is to better understand the potential biases present in the fossil record.
Many taphonomic processes must be considered when trying to understand fossilization. These include processes that affected the organism during life, the transferral of that organism, or a part of that organism (e.g., leaves, spores, etc.), from the living world (biosphere) to the sedimentary record (lithosphere), and the physical and chemical interactions that affect the organism from the time it is buried to the time it is collected in the field.
Any organism must successfully pass through three distinct, and separate, stages in order to be seen in a museum display. These stages, spanning the entire time from death of the organism to collection, are:
Necrology - death or loss of a part of the organism.
Biostratinomy - interactions involving the transferral from the living world to the inorganic world (including burial). Burial plays an important role in potential preservation of the organic matter. Very specific chemical and physical conditions must exist in the burial environment to allow preservation in a recognizable form.
Diagenesis - processes responsible for lithification of the sediment and the chemical interactions with interstitial waters.
Only a tiny percentage of all of the earth’s fossil is, or ever will be, accessible for collection and study. While tectonic processes can destroy fossils, these mountain-building forces are necessary to uplift fossil-bearing rock that can later be exposed during erosion. The quantity of fossiliferous rocks beneath ground far exceeds those available at the surface. Nevertheless, there are far more fossils than paleontologists.
The Triassic redbeds of the Wolfville Formation in Nova Scotia, composed primarily of alluvial and fluvial silt, sand, and gravel, were deposited in an arid rift valley. A modern analogue is the alluvial fans and braided streams of the arid valleys in the basin-and-range region of the American West and Death Valley in particular. Field work for my master’s thesis involved logging sedimentary sections along the 10-meter-high sea cliff exposures of the tilted redbeds along Cobequid Bay and the Bay of Fundy.
The 15-meter tidal range submerged the outcrops twice each day. At low tide, exploration of the jumble of red rocks that had fallen from the face of the sea cliffs, would, on rare occasions, turn up a white fragment in the rock. These were typically rod-shaped, approximately one-quarter inch in diameter with the biggest pieces being an inch or two long. The color and shape contrasted sharply from the red rock matrix. These were the fossilized bone fragments of Mesozoic reptiles.
The arid Triassic rift valley was not a favorable environment for the preservation of fossils. No complete fossils, or even any articulated bones, turned up. Yet, the unlikely preservation of even these tiny fragments was enough to testify that these reptiles walked the sandy river banks of those streams 210 million years ago.
Not every organism that ever lived can become a fossil. Olivia Judson, in her December 2008 column for the New York Times, The Wild Side, notes that, "It’s hard to become a fossil, to leave a tangible record of your presence on the Earth millions of years after you died. Most of us swiftly get recycled into other beings. After all, the competition for corpses is fierce. Species of bacteria, worms, ants, flies, beetles and even some butterflies have a taste for rotting flesh. And that’s without mentioning larger scavengers, like vultures, hyenas and mongooses."
Dr. Judson is an evolutionary biologist and award-winning science journalist and writer. She received her doctorate in biological sciences from Oxford University and is a research fellow in biology at Imperial College London. To illustrate how rapidly a body can disappear, she cites Pat Shipman’s 1981 book Life History of a Fossil: an Introduction to Taphonomy and Paleoecology, indicating that in the tropical forests of the Congo, an adult male gorilla — all 330 pounds him — will be reduced to a pile of bones and hair within 10 days of his death, and within three weeks, there will be nothing left but a few small bones.
Clearly a rapid burial is needed to become a fossil. That means that death must occur in area of deposition and at the right place and at the right time. The right time could be during a flood or during fallout of volcanic ash.