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Click here for the video: http://UnitStudiesByGwen.com/video/fossils/player.html
Slide Notes:
The word fossil is Latin and means “having been dug up.”
Fossils are traces of plants or animals that have been preserved in rock.
Fossils are usually found in sedimentary rock, but are found other places as well.
There are several different kinds of fossils. These include trace fossils, mold fossils,
resin fossils, and body fossils. Trace fossils and mold fossils don’t have any pieces of the original organism still. Resin fossils and body fossils have parts of the original organism.
What are trace fossils?
Trace fossils are fossils that are made because a living organism made an impression
on something like a rock. Trace fossils are called Ichnofossils. Trace fossils include
footprints, tracks, dens, burrows, nests, coprolites, and teeth and claw marks. The
common element of trace fossils is that they were formed while the ancient organism
was alive and they don’t contain the actual organism. Trace fossils can be made from
either plants or animals.
Dinosaur footprints come to most people’s minds when they think about trace fossils.
Dinosaur footprints are trace fossils because there is no part of the dinosaur left at
all, just a footprint it made at one time. There are many more fossilized dinosaur
tracks than there are fossilized dinosaur bones.
Another kind of trace fossil are tracks made by sea creatures.
How are footprints and sea slug tracks fossilized?
Imagine a dinosaur came walking along through some wet dirt. The dirt wouldn’t
have quite been mud, but it would have been soft and a bit wet. The sun dried the
tracks out for a bit, and they became harder and less muddy. Then a different kind of
sediment came and buried the tracks. Maybe some wind deflating another area
brought some small-sized sediment over and filled up the track. Over years and years
and years, the sediment solidified, or lithofied, into rock.
Other trace fossils include
Fossilized plants. Sometimes the fossils can show the leaf’s structure.
Coprolite, or fossilized dung, is also a trace fossil.
Eggs
Tooth and claw marks
Nests
Burrows and dens
Feeding tunnels
Mold fossils are fossils that are just the shape of the organism. Mold fossils are
shaped like the organism, but they don’t contain any of the organism because it
rotted away.
A resin fossil contains a whole organism, or part of one, but it was completely
encased in amber.
Amber is a substance that trees secrete, probably to seal wounds (like if a branch falls
off a tree, there’s a wound that needs to be sealed).
Body fossils are the type of fossils most people are familiar with. Body fossils show the shape
of the organism, and some of the organism’s bones are still in the fossil.
These bones were found in Utah. They were marine animals.
This trilobite is also a body fossil.
The dinosaur skeletons you’ve seen at museums are body fossils.
Bones are not uniform throughout. The periosteum is on the very outside of the bone, and it
is a hard layer. Next to the periosteum is the compact bone. Spongy bone makes up the bulk
of the bone, and marrow is found in the inside of the bone. What should you get from this?
Bones have a lot of empty space in them.
So, the animal died. If the animal was buried right away, parts of skin and other soft tissue
might have been fossilized. If the animal decayed first, the body fossil will probably only have
bones. For the fossil to form, the deceased organism had to be covered in sediment with
water coming later—or it could have been buried in mineral rich water, like the ocean or
mud. The minerals in the sediment or mineral rich water precipitated out of the sediment
and started to fill the empty space in the bones.
Over time, more sediment covered the bones. The weight of the sediment also pushed
minerals into the empty space in the bones.
Eventually, all the empty space in the bones was replaced with minerals. The minerals
hardened in the pressure and formed the fossil—which is kind of a rock.
Most fossils are found in sedimentary rock.
Remember how sedimentary rock is formed? Layers of minerals are piled, and the
pressure turns the minerals into rock. New layers of rock are formed on old layers of
rock, and you can see the different layers in the rock. Each layer is like looking at a
different geologic time period, with the oldest layers being on the bottom of the rock.
So, if a fossil is found, the first thing paleontologists do is make note of where the
fossil was found. Which pieces were found where, and in what layer of the rock were
they found.
This image was made by several scientists in 2008 at the U.S. Geological Survey.
The oldest time periods are at the bottom of the spiral. The original caption for this image,
http://pubs.usgs.gov/gip/2008/58/geotimespiral_text.pdf, posted at the US Geological
Survey, read:
The Earth is very old—4.5 billion years or more according to scientific estimates. Most of the
evidence for an ancient Earth is contained in the rocks that form the Earth's crust. The rock
layers themselves—like pages in a long and complicated history—record the events of the
past, and buried within them are the remains of life—the plants and animals that evolved
from organic structures that existed 3 billion years ago.
Also contained in rocks once molten are radioactive elements whose isotopes provide Earth
with an atomic clock. Within these rocks, "parent" isotopes decay at a predictable rate to
form "daughter" isotopes. By determining the relative amounts of parent and daughter
isotopes, the age of these rocks can be calculated. Thus, the scientific evidence from rock
layers, from fossils, and from the ages of rocks as measured by atomic clocks attests to a very
old Earth.
See USGS Fact Sheet 2007-3015 at http://pubs.usgs.gov/fs/2007/3015/ for ages of geologic
time periods. Ages in the spiral have been rounded from the age estimates in the Fact Sheet.
B.Y., billion years; M.Y., million years. For more information, see the booklet on Geologic
Time at http://pubs.usgs.gov/gip/geotime/.
So, when a scientist finds a fossil, they look at what geologic period the rock came from, and
that gives them a good idea on where to start with figuring out the age of the fossil.
The second step in determining the age of a fossil is radiometric dating.
Estimating the age of the fossil by looking at the sedimentary layer it was found in is called
“relative dating.” Using carbon dating to find its actual age is called “absolute dating.”
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