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WHAT IS A FOSSIL?
“Fossil” means different things to different people. For some, it’s a handbag. For others,
it means an old guy, as in “old fossil”. Literally, it means “something dug up”. The word
derives from Latin: fodere means “to dig”, and fossilis means something dug up. In
paleontology, however, it means “ evidence of pre-existing life that has been dug up”, although
the digging might have been only to liberate partially exposed “remains” from near the surface of
the ground. While we’re at it, “paleontology” comes from the Greek and means the “study of the
existence of ancient things”. So what we’re really about is the study of fossils to learn how
ancient animals and plants lived.
I put “remains” in quotation marks in the first paragraph because most fossils are not true
remains, as a body or a skeleton is. Many fossils are EXACTLY like the original, as is fossil
oyster shells. Those remains have been replaced by an imprint of the living being (perhaps a tree
or a dinosaur) or a mineralized substitute for the original organic matter. Paleontologists
generally speak of fossils as being of four types. Let’s look at each in turn.
1. Body fossils: most or all (rarely) of the body of the animal or plant is preserved, such
as the mammoth in the permafrost, the bug in the amber, or the tree in the peat bog.
Focusing on animals, we observe that usually only the skeleton (and not the tissues) is
preserved in mineral form, discussed below.
2. Trace fossils: called “ichnofossils”, these are evidence that the animal existed rather
than evidence of the animal itself. Usual examples are footprints, of which the
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impression in mineralized mud remains; nests, again in mineralized earth or mud (and
extremely useful to study behavior of dinosaurs); or coprolites, every docent’s
favorite, which of course are mineralized dung.
3. Mold fossils: These are mineralized exteriors of an organ such as an egg, which was
mineralized after the young departed it. In our hall, there are eggs on display which
have original shell material filled with cemented sand. The sand inside forms an
interal mold and the sand outside is an external mold.
4. Cast fossils: These are mineralized interiors of an organ, such as the brain, which
dissolved during putrefaction and was replaced with rock granules, such as sand or
finer particles. The finer the rock particle, the more detail shows in the cast.
The ways in which fossils are created are usually identified in five categories as follows:
1. Permineralization: This refers to two processes. The first is the insertion of minerals
suspended in water into the spaces (lacunae) in bone tissue such that the eventual
destruction of the bone tissue leaves behind a , web-like mineral structure. Most dino
bone has agate filling the marrow spaces and forms a quite robust part of the fossil.
These fossils rarely survive intact. The second process is true petrification, in which
the molecules of bone tissue are replaced with minerals. The result is a nearly exact
replica of the bone tissue in mineral form. This is the most common form of bone
fossil that can be displayed in museums. In the next section we’ll discuss how this
process occurs.
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2. Refrigeration (freezing): Here the entire animal may be preserved, as some arctic
mammoths were. The process requires a quick freeze to arrest bacterial action
leading to putrefaction, and continuous freezing thereafter.
3. Dessication: Here again the entire animal may be preserved, this time by drying in a
dry climate and burial in a saline soil. Early Egyptian royal remains (and those of
cats) were naturally preserved by burial in the hot sand, an effect later sought through
mummification.
4. Inclusion: In this type of fossilization the remains are encased in an anti-bacterial
medium such as asphalt, hence the numerous Pleistocene fossils recovered from the
La Brea tar pits, now in downtown Hancock Park in Los Angeles. Volcanic ash
works nearly as well, and amber (polymerized sap or resin) does a fine job, especially
with insects, spiders and small lizards.
5. Carbonization: This refers to creation of trace fossils. Leaves and soft body parts of
animals, such as fish, reptiles and marine invertebrates, decompose and leave a
carbon trace in rock.
Now that we have our types and categories identified, we can look at the circumstances in
which permineralized fossils are created. These are the most common dinosaur fossils
other than tracks (trace fossils). Again we have a list, this time of five steps:
1. Fossilization: The animal dies in or near water and is quickly covered by water, thus
retarding decomposition. Bacteria consume the soft body parts.
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2. Sedimentation: The body is buried in subaqueous sediment, the quicker the better.
Landslides and mudslides help. The finer mineral grains preserve more of the cellular
detail, and chemicals in the muddy minerals affect the hue of the fossil (iron produces
a reddish color, phosphates a gray-black).
3. Permineralization: The weight of deepening sediments (think thousands of years)
adds pressure to form rock. Water seeping through the sediment carries minerals into
the cellular structure of the animal in order for full petrification to occur. Most of the
ancient wood that we find in Texas has been permineralized, i.e. silica fills the empty
parts of the cellular structure, preserving the carbon from further bacterial attack. If
you put a piece of Eocene wood from Bryan into hydrofluoric acid, a piece of wood
would remain. (I have done it).
4. Uplift: Over millions of years the tectonic plates making up the earth’s crust shift and
collide, thus creating upthrust faults; these cause the deeply buried fossils to rise
toward the surface of the landmass (think Olduvai Gorge and Awash Valley, the two
Great Rift sites for early hominids).
5. Erosion: As the fossil nears the surface via uplift, wind-blown sand erodes the rock
and reveals the fossil to the lucky hunter. Undiscovered fossils (perhaps the majority)
simply erode away.
Fossils come in many forms, but all are rare. Trace fossils are relatively common, but the
others are very rare because they require (1) ideal water (or ice or desert) conditions for
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preservation, (2) rapid covering, (3) avoidance of scavenging predators or strong marine
currents that scatter remains.
Sources:
www.fossils-facts-and-finds.com
http://drscavanaugh.org/dino/fossils_and_fossil_types.htm
Bob Blanc
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