How does a soft, organic, squishy organism turn into a fossil that can survive earthquakes, rising mountains, closing oceans, and colliding continents for millions billions of years? Just what are the chances of some organisms becoming a fossil?
Formation of Fossils
To be honest, the chances of anything becoming a fossil are pretty slim. The fact that we have fossils at all speaks to the sheer numbers of individuals and species that have existed through time. With their countless billions, it would only take a tiny fraction of them, though, to fossilize to leave us with a substantial record in the rocks.
But let’s consider those that actually do make it. What factors do they have in their favor? How do you maximize your chance of becoming a fossil? As my real estate agent would say: location, location, location. Just as being in the right place might maximize your chances of making a killing on the housing market, so being in the right place increases your chance of becoming a fossil.
To form a fossil, you need to get your body buried as quickly as possible, out of the way of the scavengers and preferably sealed from oxygen, or in at least reduced oxygen conditions. Now, this just isn’t going to happen on an open plain, but if the subject in question happened to live close to a body of water—a river or a lake—you now have a chance to being in an environment where you might be able to bury your corpse with sediment.
This is a transcript from the video series Introduction to Paleontology. Watch it now, on The Great Courses Plus.
As being buried in sediments is probably your best bet for becoming a fossil, it follows that organisms that live in aquatic environments like lakes and rivers will have a greater chance of becoming a fossil. It also follows that, on the whole, aquatic creatures that live in the oceans and other water bodies that are receiving vast quantities of sediment via rivers and streams will have a greater preservation potential than terrestrial, land-based organisms.
Life Forms that Make Good Fossils
So where you live is important, but there’s another important factor to consider as well, and that is what you’re made of. Soft-bodied, squishy organisms like this octopus will have a much reduced preservation potential compared to, for example, one of its close relatives, the nautilus. Of course the body parts of the nautilus will decay just like the octopus, but a significant portion of the original creature, its shell, will have a greater chance of preservation. This is why the fossil record of octopi is considerably poorer than that of their shelled relatives.
Any creature that has a significant development of hard parts, therefore, will have a greater chance of preservation and a greater representation in the fossil record. This is a persistent bias paleontologists have to be aware of when reconstructing ancient ecosystems from fossil sites, especially when, in some settings, soft-bodied creatures lacking any skeletal components may have made up a large part of the animal assemblage.
Learn More: Taxonomy: The Order of Life
Minerals Come Into Play
But, considering hard parts for a while, life has used a variety of materials for production of structural support. Calcium carbonate or calcite is a very common mineral used by many organisms, including bryozoans, corals, brachiopods, mollusks, and many arthropods and echinoderms like sea urchins. Examples of silica-secreting organisms include sponges and the radiolarians—tiny marine protists about 0.1 to 0.2 millimeters in size that secrete these exquisite ornament-like structures out of biological glass. Calcium phosphate, usually in the form of the mineral apatite, is used for the skeletal elements—that’s bones and teeth—of vertebrates and the feeding apparatus of the extinct chordates called conodonts.
Now, the varied skeletal components of these creatures will behave differently under different environmental, rock-forming—what we call diagenetic—conditions. Even slightly different forms of the same mineral can react very differently to the processes of fossilization. Consider these 2 fossils: an ammonite cephalopod, relative of the nautilus; and a brachiopod, a creature that might look like a clam but is actually more related to the bryozoan moss animals. Both creatures use calcium carbonate in their shells, but not all calcium carbonate is the same. The ammonite is composed of a mineralic form called aragonite while some brachiopods use the more typical calcite.
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The main difference in the 2 forms of calcium carbonate is in the arrangement of the atoms that make up the crystal lattice. This might sound like an esoteric difference, given that chemically they’re very similar, but when the 2 substances are buried they behave quite differently. Aragonite is only really stable at surface temperatures and pressures while calcite is stable over a much broader range.
Structural Components of Specimens
Insects also have a modest preservation potential, but because of their sheer abundance they have a better fossil record than would be predicted from their exoskeletal durability alone. But it’s not just the durability of the materials—calcium carbonate, silica, calcium phosphate, cellulose, and chitin—that we have to consider. Another factor is how the material is organized. For example, arthropods consist of numerous exoskeletal body parts called sclerites. These tend to separate after death, giving us vast quantities of components of specimens in the fossil record.
Corals, however, secrete a single robust skeletal element, and as a result the preservation potential of the entire organism is better than that of the sponges.
The same is true for sponges, which are composed of discrete structural elements called spicules, some as large as 2 millimeters but many less than 60 microns in size. As a result, the various scaffolding units of sponges are also common in the fossil record—more common than fossils of the original complete organism. It’s a bit like an archeologist finding the bricks and rocks that make up a large building rather than the building itself. Corals, however, secrete a single robust skeletal element, and as a result the preservation potential of the entire organism is better than that of the sponges.
So you see how a whole number of factors come into play when considering the preservation potential of a fossil: location and mode of life of the original organism, the presence or absence of hard parts, the composition of those hard parts, and whether the organism contributes several cast molts during its lifetime. And of course there is pure blind luck.
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Mode of Fossilization
Given that, let us look at some of the ways in which creatures become fossils. A common mode of fossilization is the production of molds and casts. Molds are negative impressions of an organism that preserve information about the surface of a creature. A mold will commonly be produced when circulating pore waters moving through a sediment dissolve away the original skeletal material. Paleontologists will occasionally inject epoxy resin into the mold and then dissolve the surrounding rock to free the cast. Casts can form in the same way in nature when the mineral-rich waters deposit various minerals into fossil molds.
Fossils can also form by a process called carbonization. Here, a process of distillation caused by the heat and pressure of burial preferentially removes the hydrogen and oxygen of the soft tissue, leaving the carbon behind. This is a common mode of preservation of many land plant fossils but also of these beautifully preserved graptolites that used to float through the Silurian oceans over 423 million years ago.
Some of the most spectacular preservation, though, occurs when mineralizing fluids percolate through sedimentary units. Minerals are precipitated in spaces within skeletal materials of shells and other original structural materials, hardening and stabilizing the fossil. This mode of preservation is called permineralization and can preserve wonderful detail.
The same processes can occur in some circumstances when organic material becomes completely replaced by the mineralizing fluids, a process called petrifaction. This can occur in both plant and animal fossils, but possibly one of the most spectacular examples comes from the petrified conifer forests of Arizona.
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The fossils come from the late Triassic period, about 225 million years ago, when Arizona was humid and subtropical. The trees were living on the edges of streams and rivers, some growing up to 200 feet tall. Occasionally, they would fall into the water and get buried in sediment that contained high proportions of volcanic ash that was rich in silica. It’s this volcanic ash that is the key to their preservation. Fluids carrying this dissolved mineral percolated through the sedimentary pile, seeping into the waterlogged trees and replacing their organic material with silica.
The wonderful colors you see in many of these fossils are due to secondary minerals such as iron oxides like hematite that gives us the reds and purples, and limonite, which gives us the yellow. So numerous are these fossil trees that in the past they’ve been used as building materials. Although internal detail is not always preserved in them, some of the logs and some of the animal bones found in these deposits show detail even down to the cellular level.
There are, of course, other rarer modes of fossilization that can produce spectacular material. Perhaps the most beautiful are those fossils trapped in amber. This is an important mode of fossilization for insects and spiders, but other life-forms like small vertebrates and plants have been preserved, as well. Amber forms when resin produced by a number of trees, but particularly coniferous trees, secrete resin to heal an injury or act as a defense. This oozes down the tree trunk, sticking and trapping creatures as it goes. Once the resin is buried, pressure and temperature will increase due to the overburdened pressure of the sediments, and this causes the organic chemicals in the resin to oxidize and polymerize, eventually hardening into amber, and preserving the creatures it trapped in fantastic detail.
Between 1981 and 1985, through the efforts of entomologist Don R. Davis, the Smithsonian acquired a wonderful collection from the Dominican Republic between 21 and 15 million years old. This collection is housed in the Department of Paleobiology and is a valuable resource today to fossil insect researchers throughout the world. It was collected by retired army major Jacob Brodzinsky and his wife Marianella Lopez-Peña.
The Brodzinsky/Lopez-Peña collection comprises just over 5000 specimens, providing a wonderful window into the early Miocene insects, mites, centipedes, and spiders, but also leaves, flowers, and even a bird feather. It is particularly useful for paleontologists as the amber is very pale, which allows a better view of the fossils. The preservation is just spectacular.
Learn more about what fossils tell us about our planet’s exciting historic migrations
Ancient Geological Fossils
Exceptional preservation is not restricted to fossils from geologically recent times, though. The museum holds some beautiful fossils dating to 286–245 million years of the Middle Permian from what is today the Glass Mountains of western Texas. The Glass Mountains are part of an ancient reef system that formed in a shallow inland sea called the Delaware Basin, now exposed in the Apache Mountains and Guadalupe Mountains. The reefs around the margin of the Delaware Basin record shallow warm-water marine life of incredible diversity—a complete reef ecosystem.
Although only a tiny proportion of life on Earth has become fossilized, that portion still represents an enormous cache of material for future paleontologists to examine. New discoveries of fossil bonanzas and new approaches and techniques in paleontology and paleobiology will likely continue to surprise and delight generations of scientists to come.
Common Questions About Fossils
There are five major types of fossils: pyritized ammonite, insects preserved in amber, cast/mold of clam shells, petrified wood, and compression fossils of fern.
Yes, fossils are always being created if the conditions are there for formation.
This article was updated on 8/29/2019
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