Q2 W7 - LESSON Fossils
Students,
The main point of this chapter is that the many layers of earth directly below our feet have remains of ancient organisms that tell us much regarding what lived in the past and what the earth was like millions of years ago.
Students, scientists use the fossil record to reconstruct events in Earth’s history. For example, when the same fossils were found on the western coast of Africa and the eastern coast of South America scientists theorized that the two land masses where once joined. Or scientists have discovered whale bones in the Andes mountains in South America which indicates that those mountains were once under the ocean. Or fossils of ancient trees in the Mojave Desert indicate that the land once was a forest.
Fossils indicate the past events in the Earth’s geology. To learn about this let’s first examine what are fossils, how are they made in nature, and how scientists determine their age.
The Dynamic Changing Crust
Always throughout the earth there is weathering and wearing down mountains and rocks. The process of erosion carries and deposits rock and sediment to resting places like rivers, lakes and oceans. Hence, there is always places on the earth where new layer of sedimentary rock is formed. Likewise there are volcanoes that spew out new layers of igneous rock. There is magma rising toward the surface to create new layers of granite and other igneous rock that at first lies underground but later through erosion at some point becomes surface rock.
On the ocean floors sediment is building and new layers of the earth are formed and millions of years later these layers of the ocean floor rise in elevation and become the surface ground we live and walk on. The earth's crust is in a constant change that is hard to comprehend as it ocurrs over millions of years. Geologist study these earth changes and the study of the layers of rock tells scientists about the earth’s ancient past.
In this sense, ancient geological structure and activity and the remains of biological records (fossils) can be used to reconstruct Earth's history.
One of the primary laws of nature used in geology and paleontology (the study of fossils) is the Law of Superposition. Let's now turn to that law.
The Law of Superposition States:
Although not in every case,
the deeper a layer of rock exists underground, the older that layer is compared to the layers above it.
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The Earth is always in slow but constant change, acted upon by dynmic forces of nature. |
The Earth’s Crust as Layers of Rocks
The earths crust below our feet consists of many layers of different kinds of rock. All these layers at one time were on the surface of the earth and organisms, animals and plants, once lived on it. Some layers of rock formed on the ocean floor but later became land. In every case as scientists investigate the fossils remaining in the rock layers they learn what life was like millions of years ago.
The layers are stacked one upon the other and in general the lower layers are more ancient in time and the upper layers are more recent in geological time. A fossil of an organism found at a lower layer should be of an animal that existed further in the past than a fossil found in a layer above it. This is called the Law of Superposition in geology.
Dinosaurs were NOT the first fossils and therefore were not the first animals. Dinosaurs came later and have left and new species of animails and plants emerge and then they become exstinct, and so forth.
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Students in the above images, you see that older layers are on the bottom and therefore the animal fossils found in those older layers are therefore older animals. More recent organisms are found on the top layers. The word "recent" in geological time may mean 20 millions years ago. The word "older" in geological time may mean 300 million years ago. The dinosaurs became exstinct only 65 million years ago.
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Organisms Leave Fossils Behind
Notice from the image on the left, the many organisms that lived long before dinosaurs. Fossils are a record of all the biological life on the planets history. |
Geologists study rock formations and Palentologists study the fossil records found in rock formations.
Paleontologists study fossils to reveal what animals and civilizations were like in ancient times.
Paleontologists discover ancient artifacts and serve museums. The
picture in the middle is of discoverying dinosaur foot prints in stone.
1. The layer of rock at the bottom of a mountain face that has several rock layers above it should be a) the oldest or b) youngest rock
2. Dinosaur fossils are the oldest fossils on Earth. True or False?
3. What is the difference between a geologist and a paleontologist?
The 7 Ways Fossils are Preserved
Mineral replacement
It may sound strange but organisms that die (plants and animals) can be turned into mineral stones. When a tree falls and starts to decay water can saturate the entire tree. This water has minerals in it and those minerals start to precipitate, that is harden, into stone in the very cells of the tree. The minerals harden in the exact likeness of the tree. The stone looks just like the tree did. All the tree fiber and cells are gone but the minerals that replaced thsoe cells and fiber have an appearance just like the tree it replaced. When this happens to a tree it is called petrified wood. The tree decayed away and minerals replaced it.
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heavy petrified stones that have the identical look of the tree they mineralized.
Well animals and plants can have this done to them too. This is called mineral replacement. That is a good name for it. Any organism when dead if it is covered with sediment and saturated with water can have its bones and teeth and even in rare occations its interal organs mineralized by the minerals in the water precipitating, hardening, right in its cells and taking the very exact shape of the organism it is replacing.
Hence, many fossilized bones are minerals in the identical shape of the bone. But more recent fossils that have not mineralized are the actual bone itself, providing the bone is well preserved.
Permineralization
Permineralization is very similar to mineral replacement. Mineral replacement seems to be more thorough and can even mineralize the soft tissue of animals. But this does not happen too often. In permineralization minerals in water enter the bone and encases the bone material to preserve it instead of replace it. For our purposes we can think of permineralization and mineral replacement as pretty much the same thing.
These bones have been mineralized as stones but
still in the orignal shape of the organism.
Carbon Remains - Carbon Films
Carbon film is another way ancient remains are preserved. Dead organisms that are at the very bottom of a sediment layer will have intense heat and pressure on it. The carbon body of the organism looses all liquids and gasses and the remaining carbon material is turned into a film, which is thin layer of carbon in the form of a silhouette of the organism. Literally the organism was carbonized.
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Coal is a fossil that is carbonized but it is classified more as a fossil fuel and not as organism remains. Plants are carbonized in the heat and pressure and their atomic structure changes into coal.
Molds and Casts
Molds and casts are a means to record ancient organisms. A shell falls into sediments, is dissolved by acid and leaves a mold. A mold is an open space that represents the exact empty shape of the organism that occupied it. You have probably used molds in plaster art projects.
Water will fill up this mold space and the minerals in the water will precipitate (harden) out of the water filling the mold space. Then the hardened mineral is in the shape of the shelled organism that once existed there. The hardened minerals taking the shape/form of the shell is called a cast.
The empty space or impression is the mold. The filling the empty space with minerals is a cast.
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The illustration on the right shows a shell dissolving, leaving a mold space
and minerals precipitating in the space and lastly a mineralized shell of the
identical organism. The minerals that filled in the shape of the shell is the cast.
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Here you see a cast of an an ancient shell. The shell deteriorated long ago, leaving and empty space, a mold. The mold filled in with minerals that form in the very same shape as the original shell. |
Original Remains - Body Fossils
On rare occasions an ancient organism can be preserved entirely even its soft tissue. In other words the entire body is preserved. Mosquitos and insects in amber (tree) sap is preserved with soft tissue in whole for millions of years. Tar pits and frozen ground can preserve the whole body of an organism, like a tiger or mammoth.
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Both organisms are of their original tissue well preserved in tree sap.
Ice also will preserve an entire organism. In 1991 a human body was found on a glacier near the Austria and Italian border. The body was 5,000 years old and was preserved in the ice to maintain good condition to study. Do a search for "Ice Man" to learn more about it. Also do a search for whole mammoth fossils. Their pictures are unsightly to some so I did not post them here. The first video you will watch for this chapter has a mammoth find in it.
Trace Fossils
Trace fossils are footprints in the mud that later hardens into rock and remain for millions of years. There are many examples of this from the era of dinosaurs.
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Index Fossils
Index fossils are a special species that lived for a short period of geological time but were abundant on the planet and did not change (evolve) during its time. Index fossils are used by scientists are markers by which they can gauge the age layer of rock is compared to other layers without the index fossil in it. This is called releative age.
Remember the age of a layer of rock is also the age of the fossils found in it. Scientists have the earth history designated into certain Periods or Eras as we will learn next chapter. Index fossils help determine these eras.
A trilobite is an index fossil as they
were common everywhere at
certain eras in history .
1. How many methods of fossilization have paleontologists discovered in nature?
2. Petrified stones look like ancient trees. What method of fossilization is this?
3. A rock with a dark thin image of a leaf on it is an example of which fossilization method?
if a fossil was common and characteristic of a specific Geological time period it can be called an _____ fossil
Superposition - Relative Age of Rocks
Superposition
Superposition is a term that refers to the fact that in undisturbed rock the older rock is at the bottom of the layers and younger rock layers are at the top. Relative age is the age of something relative to the age of something else and therefore relative age can only say a rock layer (and its fossils) is older or younger then another rock layers (and its fossils). But the law of superposition cannot provide the actual age of layers and objects.
An Anology
When you see a family with three children the tallest child is likely the oldest and the littest child is the youngest. This is relative safe assumption and you don't know the exact years of the children. When a geologist studies 3 rock layers (and their fossils) there is the good chancce that the upper layer is the youngest and the lowest layer is the oldest.
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These illustrations show the layers of rock and represent the fact that the lower
layers are older. Yet this law of superposition doesn't happen in all cases or
rock layers.
If rock layers fold over by mountain building their age is determined by fossils and not which layer is on top.
Here is an exception on the right illustration. The older layer
was found on the top because mountain folding forced it
into the upper layer.
Unconformity of Rock Layers
Rock layers undisturbed over long periods of time is the best data to uncover the history of the rocks. But geologists do find areas of rock layers that are disturbed, that is, unconformed or missing layers of rock.
These layers could have been eroded by glaciers or had been upwarped to the surface where the top layers eroded and then new sediment layers formed over the older layers. This is a discontinuity of the layers, called unconformity.
Here
you see older rock formations on the bottom but the upper younger
formation are discongruous with them. The younger layers formed
many years later after the older layers had been
tilted and
eroded
by forces of plate tectonics and weathering. All this means
the earth is dynamic and many types of situations arise.
Matching up Rock Layers
It is possible to find the same sequence of rock layers from one region existing hundreds of miles away in another region of rock layers. These layers are the very same wide-area ancient deposition of sediment and rock. Sandstone, limestone, marble in these layers also seen 300 miles away are probably the very same layers.
Bryce Canyon Park and Canyonlands Park have some of the
very same rock layers in their crust.
Layers of rock spread far and wide in the crust. Layers found in one
area may be found also in a region hundreds of miles away.
Here you see that two rock layer groups where they both include layers 3, 4 and 6, but do not share the other layers.
Absolute Ages of Rock and Fossils
Scientists have found ways to know the actual age in years of rocks and organic matter. There are two methods: radiometric and radiocarbon dating. Radiometric dating is for rocks (inorganic matter) and radiocarbon dating for organic (once living) organisms.
These two processes are based on the following special knowledge of atoms. Some unstable atoms spontaneously decay, that is literally change into another type of atom and when they do they release radioactive energy. Scientists use this atomic process to date ancient objects including fossils.
Radioactive Decay
Radioactive decay is a natural phenomena wherein certain atoms in any substance will "decay" and tranfrom into other types of atoms spontaneously. When atoms decay, energy is released. This energy is called radioactive energy and is dangerous to us if we receive it in large amounts.
Also the rate of the decay is known and predictable. Because scientists know the percentage of decay of certain atoms over a period of time they can determine how old a fossil is by mearusing the amount of radioactive decay that has occured in it. Measuring the amount of decay in an object will tell scientists its actual age, that is, its absolute age in years.
Beta Decay
Beta radioactaive energy is a neutron turning into a proton and releasing an electron. The electron flies off as a beta particle.
A sodium atom Na, becomes a
Neon atom Ne, by one proton
becoming a neutron and an
electron being released as a
Beta ray.
Alpha Decay
Alpha radioactive energy happens when is two protons and two neutrons fly off as an alpha particle. In both cases some energy is released and the atom’s atomic number is changed so it is transformed into another type of atom.
Radioactive decay happen only to certain isotopes. Isotopes are different varitations of the same type of atom. A carbon atom for example has 3 isotopes. One type of carbon atom has 6 neutrons. It is isotope Carbon-12. The second type of carbon atom has 7 neutrons, carbon-13, and the third type of carbon atom has 8 neutrons, carbon-14. The carbon-14 isotope is the one that goes through decay (that is, transformation) and releases radioactive energy in the process.
When a Carbon-14 atom decays it changes into a nitrogen atom because it loses a proton. As a manner of expression scientists use the "parent and daughter" analogy. The original carbon-14 atom is called the parent isotope and the nitrogen it changes into is called the daughter isotope. Even the some nitrogen isotopes will decay (eventually) and become a different type of atom, and so on. So there is a progression of decay among atoms. The original isotope is the parent and produces the daughter who is the parent of its daughter etc.
An atom decaying (transforming) into another
type of atom and giving off an alpha particle
in the process. The original atom is the parent
and the atom it turns into is the daughter.
Half Life
The different isotopes decay at know rates. The amount of time for an isotope to decay half of the original amount is called a half-life. Any given amount of Carbon-14 takes 5,730 years for half of that amount turn into nitrogen-14.
When an organism dies, 5,730 years later half of its carbon-14 is decayed into nitrogen. Then 5,730 years later half of the remaining carbon-14 also turns into nitrogen. So the original amount reduces from total to half, to one-fourth, to one-eight, one-sixteenth, etc., every 5,730 years. Conversely the daughter product increases with each half-life of the parent isotope. By measuring the amounts of carbon-14 and nitrogen scientist use this data to determine the actual age in years of ancient fossils.
Radiometric and Radiocarbon Dating -
Two Ways to Find Absolute Age
Radiometric Dating (used on rocks and inorganic objects)
Radiometric age is the age in years as determined by measuring radioactive decay. Measuring the ratio of the parent and daughter percentages of isotopes. This process is done with non-carbon, that is, non organic (not alive) objects.
Radiometric measurements are used on igneous and metamorphic rocks, but not sedimentary rocks as these have a preexistence long before forming as sedimentary. Radiometric dating can measure up to serveral billions of years ago.
Radiocarbon Dating (used on carbon-based organic material)
Radiocarbon dating measures carbon-14 for wood, bones and plants, clothe, and carbon-based organisms. By measuring the amount of carbon-14 in the fossil scientists determine the objects age in years. Carbon dating can measure dating up to 60 millions years ago.
Get a degree in physics and you too
can run equipment like this to measure
the absolute age of ancient artifacts.
The oldest rocks on earth are 3.9 billion years old and
science holds the Earth a 4.6 billion years old.
Uniformitarianism
The very slow earth processes acting today acted the same way and at the same slow speed in the ancient earth. This fact never occurred to scientists until James Hutton realized it in the 1700’s. This principle is called uniformitarianism and it sparked a new way of thinking.
Scientists realized it was not practical to think that the earth is only 5,000 years old. Scientists began to understand that the many geological processes they studied at that time, took millions of years to develop. So scientists started to think differently about the age of the earth. Eventually science has come to know that the earth is 4.6 billion years old and the universe is approximately 13 billion years old. Among the scientific community these time periods are debated in details, but not in general conclusion.
Scientists realized that it takes millions of years for
nature to create the natural beauty we see in the
phyical world. So therefore the earth must be ancient
in terms of billions of years old.
1. Determining which fossils are older than others by their location in the rock layers is called, a) the absolute age, or b) the relative age of rocks
2. The absolute age of rocks is measured in terms of _______ by using precise methods such as _______ ________ dating.
3. In the 1800's geologists came to the realization that the Earth's the formation of the earht's mountains, rivers and terrain must have taken many millions of year to to occur. What is this concept called?
Closure
Students, you have now overviewed what are fossils, how they form how scienctist use this knowledge to contruct an explanation of current geological events. Fossils take millions of years to develop, and when we find them they are like portals to see the past. The earth is very old. In our next lesson we will learn about Geological Time, a form of time vastly longer then our daily time frame of hours or days.