Paleontology: How the Earth Makes a Fossil

We’ve spent a couple recent posts discussing geologic time and the two methods earth scientists use to describe and measure it. We started with a general introduction to earthtime and absolute age dating (Intro to Geologic Time), and then jumped into the other method of describing geologic time in Intro to Relative Geologic Time.

I think it’s safe to assume that absolute time — while giving us actual dates and ages — is too much like science to be something most of us can use without a lot of help and money, and faith that the guys offering the help and money know what they’re doing (and don’t have an agenda that taints their efforts).

Ordering earth history using relative time, on the other hand, is something we can do for free all by ourselves. It’s easy, and follows clues that our senses are already trained to observe and decipher.

Many of these clues are related to sedimentary rocks, and the fossils that can be found within them. Using fossils to organize earthtime is tied to a branch of geology called Paleontology: the study of the history of life on earth that is — wait for it — in large part based upon the creation and interpretation of fossils.

So, what’s a fossil? Well, fossils are the preserved evidence of life that existed at some point in the past, died, and was preserved in a rock — usually of the sedimentary variety. I deliberately use the word “evidence” of life, and not the more expected “remains” of life for a good reason.

Sure, many if not most fossils are limited to the “hard parts” — bones and shells and teeth and such — but others are non-organic and are called “trace” fossils. Trace fossils include such evidence as footprints, nests, burrows, and even fossilized dung, called “coprolites”. (I once had a student do a GeoFantasy on his coprolite collection. Joe — truly a unique individual — wowed us with nearly fifty specimens of fossilized poop.)

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Anyway, making a fossil — although actually extremely rare, considering the number of organisms that have lived and died — is really easy: pretty much all you have to do is remove the dead critter from contact with the oxygen in the water or atmosphere. Beyond that, it’s mostly just a matter of time. Usually lots of time! So, let’s explore how this can happen. We’ll jump into what paleontologists and other earth scientists can do with fossils in a later post.

Keeping the “no oxygen” requirement at the forefront of our brains, most — but definitely not all — traditional fossils are incomplete skeletons and other hard parts — very few organisms can get entombed fast enough to protect the skin and hair and internal organs. But some fossils do also include unaltered soft parts. These can be a lot of fun, and may even incorporate the entire organism. There are several ways the earth can do this.

If you saw the movie Jurassic Park (as is often the case the book is even better, but I digress), you already know how sap can ooze down the bark of a tree and entomb a creature. This is then buried, and hey presto: after a span of years that only the earth can measure you have a fossilized bug in amber. But that’s the problem with tree sap and amber: it’s really only good for bugs and pollen and insects and seeds. I mean really — can you imagine a panda bear holding still long enough to be coated in sap?

Freeze-drying can work, even with the big stuff like mastodons and such, but when the weather warms up and the ice melts, there goes your fossil (hopefully they are storing Ötzi someplace that’s really chilly). But, if the frozen organism can stay where it’s cold, some truly remarkable remains can be preserved.

Dehydration can also get the job done — think Egyptian mummies — but this one only lasts as long as the humidity stays really low. Like with the freeze-drying, some very detailed remains can be preserved, but the longevity of the fossil is at best measured in thousands of years — not the tens to hundreds of millions of years that are the most useful if the goal is to define the history of the planet.

A special and unique setting is at the La Brea Tar Pits in Los Angeles (made famous in the scientifically-questionable movie “Volcano,” starring Anne Heche and Tommy Lee Jones). One possible scenario: a creature wanders into what it thinks is a pool of water (water floats on tar — it’s a density thing again), gets stuck in the pitch, and sinks — just like in quicksand. But, before sinking completely out of sight, the critter is likely screaming and flopping about and calling for help. All this hullabaloo attracts a hungry predator who — probably thinking something along the lines of “Look at that stupid hippo” — wanders into the pool for some free lunch… and gets stuck itself.

Some truly amazing and complete skeletons have been found.

Since all of these methods have some serious limitations, let’s pivot to the traditional fossils that are generally the remains of what are called the “hard parts” of the organism. These are your basic fossils, and form the backbone of paleontology and relative geologic time.

As said above, the important part is separating the organism from the oxygen in the water or atmosphere. Yeah, yeah, I know — oxygen is good stuff and we couldn’t live without it. All true, but O₂ is also extremely reactive (think how oxygen is required for a fire to burn), and will speed the decomposition of organic remains (and even non-organic — think rust). This is the fundamental logic behind pressure canning, a Seal-A-Meal food storage system, or a snap-lock jar with one of those trick rubber gaskets.

Usually, the “separating the organism from the oxygen in the water or atmosphere” part is accomplished by burying the critter beneath sediments — one reason most fossils are found in sedimentary rocks. Sure, a lava flow will also get the job done, but the heat tends to bake the organism well done.

(At this point in the lesson, I would often show the kids at the high school a video of a slow-moving A’a’ lava flow in Hawaii eating a pickup truck. It was great imagery, but beyond that specific version of a trace fossil, lava flows are routinely destructive to organic remains! To cement the lesson I’d usually follow with the subway scene from “Volcano” where the conductor jumps into the lava to carry his unconscious passenger to safety…)

Since the depositional environment for most fossils is typical of the seafloor, where sediments are constantly raining down, it should come as a no-brainer that the fossil record is heavily weighted towards marine organisms. This brings us to the inescapable conclusion that what we are seeing with regard to past life on earth is a very spotty and incomplete picture of what has existed over earthtime. So it goes.

Lastly, most fossils are not the original organism, but an exact replacement by materials dissolved in the groundwater (or seawater) that comes in contact with the buried organism over the immense span of years that it usually takes to complete the fossilization process. In most cases this material is calcium carbonate. As such, most fossils will pass the fizz test. (Click here for a summary of mineral identification tests from my educational website, along with links to GeoMan’s Mineral and Rock Identification pages.)

However, it doesn’t have to be calcium carbonate. Occasionally — although this is much less common — the replacement material will be silica: chalcedony, agate, quartz, or opal. The general name for this is “petrification.” Petrified wood is the most common result, and, while some of the best is found in the Petrified Forest of Arizona, silica replacement of wood fiber can be found in many parts of the world.

One final point: When the first fossils were identified, they were not immediately recognized as evidence of living organisms that had died and been preserved in the geologic record. Some pretty fanciful things were proposed to explain them, including the perpetual favorite that Satan had put them in the rocks and they only looked like bones and teeth and leaves to “test our faith.” I promise more on what nifty things paleontologists can do with fossils in a later post.