Fossilization
of a particular living organism in sedimentary deposits is a rare
event. Yet, past life is so boundless in number that there exists,
in fact, countless numbers of fossils buried in the earth. You might
say that fossils are both common and rare at the same time. While
they may be countless, finding fossils is generally not easy. Normally,
a fossiliferous layer of sedimentary rock will lie between non-fossiliferous
layers. In the former, the conditions enabling fossilization to occur
existed, and in the latter, they did not. The gaps in between the
fossiliferous layers constitute gaps in the fossil record that could
be hundreds, thousands or even several millions of years.
There
are numerous factors that influence whether a particular organism
eventually becomes a fossil, some of which increase the probability
and some of which decrease the probability of fossilization. Let your
imagination roam, and consider that you are an organism in a population
of like-kind organisms that existed some time between 4.5 billion
years ago and much more recently.
You’ve
probably heard jokes like: You
might be a redneck if, your `huntin dawg' cost more than the truck
you drive him around in, or if you have the local taxidermist's number
on speed dial. Or, you
might be an engineer if, you know vector calculus but you can't
remember how to do long division, or, the salesperson at Circuit City
can't answer any of your questions, or, you think that when people
around you yawn, it's because they didn't get enough sleep. The section
below looks at fossil formation in a similar stylistic manner.
You,
or a close relative of yours at least, might be a fossil if:
- You
were lower on the food chain. - Organisms lower on the
food chain are in greater abundance, have shorter life spans, high
fecundity (make many offspring), and thus have a greater chance
of appearing in the fossil record. Prey always far outnumbers predators,
a balance of nature that always rules population ratios. Hence,
Cambrian brachiopod or trilobite fossils that were prey far outnumber,
for example, fossils of the soft-bodied terror of the Cambrian,
the Anomalocaris that fed on them.
- Similarly,
you might be a fossil if, you were small and weak, rather
than big and strong. – Size does matter, as predators
always choose the smallest and weakest prey available to them. For
example, big, carnivorous dinosaurs outnumbered the herbivores of
the Mesozoic, and small fish have always outnumbered the big fish.
Bigger animals live longer because they can, so nature makes fewer
of them, and there will consequently be fewer fossils of them. Evolutionary
adaptation requires that there are many of the small and weak so
some can survive and pass along their genes. Populations of the
small and weak left more fossils behind. There are some exceptions
to this rule, such as: (1) maybe you were too small to tempt a larger
predator to even bother; or, (2) your diminutive stature enabled
you to hind more effectively (maybe you were a tiny mammal during
the age of dinosaurs).
- You
might be a fossil if you were slow, lacked eyes, were attached
to the sea floor and thus immobile, etc. Just like being
small, being slow or immobile was not good, and evolution would
have compensated by making more of you, and your family would have
made more fossils. She or he who hesitates is lunch, so they say.
- You
might be a fossil if you lived in or near a marine or aquatic
environment, as opposed to on land. – The likelihood
of your fossilization was markedly higher in aquatic and marine
environments than it is on land simply because there exists a minute
yet finite possibility that before you are scavenged or rotted away,
you may have settled to the bottom, and was quickly covered with
muck or silt. If you were really lucky, the mucky bottom would have
a paucity of aerobic bacteria to eat your remains; such anoxic (lacking
oxygen) environments are the stuff of which the famous Lagerstatt
fossil sites (for example, Green River, Solnhofen, Burgess Shale
and Chengjiang) are made, but these are indeed rare.
- You
might be a fossil if you were crunchy rather than soft.
- If you were a real softy, lacking bones and exoskeleton, there
is almost no chance you left a trace of your life in the earth.
This is why the fossil record is almost non-existent prior to the
Cambrian some 550 million years ago. If you were soft, maybe you
left a footprint if you were an arthropod with legs, or an impression
if you were a jellyfish, but, your actual carcass if not gobbled
whole, would have fallen victim to a myriad of bacteria, the predominant
life form on the planet throughout all of geological history. However,
if you were a post-Cambrian organism, and was crunchy, your crunchy
part(s) might have left a trace of your past existence. If you were
an invertebrate, you might have had an exoskeleton that was preserved
because it was already fairly solid and partly mineralized. In fact,
if you were a Paleozoic trilobite, you probably left a legacy before
your final demise of many your calcareous exoskeletons that you
periodically shed in order to grow. If, however, you were a dinosaur-age
insect from the Mesozoic, or any time since then, your thin little
chitin exoskeleton would have afforded your meager carcass scant
protection from the ravages of nature, but at least your little
exoskeleton had a better chance for fossilization than if you were
a softy and lacked one.
- You
might be a fossil if you expired in water or fell in some
water after you expired. – Regardless of where you
lived, where you were exactly at the time of passing would make
a huge difference in whether you would leave a trace of your existence.
Unless you existed before the middle Cambrian, you surely would
have expired in the sea In fact, if you existed prior to Devonian
time when life adapted to living on land, your fossilization on
land would be exceedingly rare, and even then only if you were washed
into a tidal zone. Of course, you may have been washed ashore, or
if you expired ashore, washed back out to sea. If you were any land-based
organism throughout geological history, and expired near or in a
river or stream, chances are good you would have ultimately been
washed downstream, and your bones and crunchy parts hopelessly dispersed
and further eaten. If you expired on an arid plain, your uneaten
bones may have merely dried up and blew away before they could be
buried; your teeth, however, had the best chance of surviving owing
to their un-palatability and mineral constituency. If you died and
fell in a lake, protected from currents and waves, and was buried
in a mucky, anoxic bottom, your chances of becoming a fossil would
be markedly enhanced.
Joking
aside, it is really pretty hard for any given organism to become a
fossil. Moreover, while such things as where you were of the food
chain, how fast you were, your size, being soft or crunchy, etc.,
would have a bearing on the probability of becoming a fossils, but
there are exceptions to all the above. Fossil formation can occur
through a number of processes, each of which is chemically complex,
and not completely understood. There is no realistic way to simulate
in a laboratory the processes that take place over thousands to millions
of years on and in the water and earth that results in the formation
of a fossil.
The unlikelihood
of fossilization results in large gaps in the fossil record. Creationists
like to use these gaps to argue against evolution and promulgate supernatural
views. Darwin particularly worried about the absence of Precambrian
fossils that had not yet been discovered during his time. They have
since been found, but not many, and they are found in very few localities,
since Precambrian animals were simple, small and soft-bodied, and
perhaps not very abundant, the Precambrian fossil record is indeed
sparce.
For
more, see fossil
record across geologic time at Darwin's
Dilemma.
|
