Presentation on theme: "Sedimentation and the Fossil Story. The evidence supporting evolution comes from many sources. You could think of it as a jigsaw puzzle which produces."— Presentation transcript:
Sedimentation and the Fossil Story
The evidence supporting evolution comes from many sources. You could think of it as a jigsaw puzzle which produces a complete picture only when you put all the pieces together. Of those pieces, one of the most important is the fossil record.
I’ve never seen anyone with a sense of curiosity who didn’t find fossils interesting… at least for a few minutes. (LOL.)
Fossils have been a strange curiosity throughout history. But it may surprise you to learn that only in the last 300 years or so has anyone understood what they really are. With society in Europe locked into believing the teachings of the Church, at the threat of being a heretic, no one would have dared to suggest in the Middle Ages that fossils were actually a record of once living creatures.
The prevailing belief, was that fossils, were representations or remnants of creatures that no longer existed. They either represented creatures that were still alive (somewhere out there…) or they were something else. But to suggest that any creature ever went extinct would be heresy and against the teachings of the Church. Nothing created by a Supreme Being, you see, could ever disappear from the face of Earth, since they had to be perfect creations of a perfect Creator.
So, let’s examine what fossils are and what they mean, scientifically. It’s important in understanding fossils to know a bit about how they are made and that’s the focus of this powerpoint. You are going to see fossils in the classroom over the next few weeks and you ought to know how they come to be and how we use them scientifically. Scientifically, we’ve come to different conclusions about the origin and meaning of fossils in the last 300 years or so…
There are two major types of fossils - body fossils and trace fossils. Both are the remains of living organisms. Body fossils reveal the body structure of the organism while trace fossils reveal the activities of these organisms. At the right is an example of a trace fossil. Below is an example of a body fossil.
The process of fossilization involves three main components. First, there is the death of the organism.
Next the organism has to get covered with something like mud or sediment before it totally decays or is destroyed by scavengers.
Not a bad place to die.
Finally, there are certain processes that occur after the organism's body is buried. These processes result in the different categories of fossils.
If we are talking about the typical fossil that is a rock, then its formation involves sediment, water, pressure and a lot of time. When something dies and floats to the bottom of a lake or ocean, if left alone and buried quickly, minerals in the sediments and water surrounding the body of the creature enter small pores and cavities. Over time, minerals are deposited and often times replace the original organic materials. Under high pressures, these minerals chemically interact and become solid. In effect, the fossilizing organism becomes rock over a long period of time. Very little if any of what remains is the original organic material. Amazingly, an impression or structure of rock and mineral is left that almost perfectly represents the creature’s form and features when it died.
A number of factors can affect fossilization and whether it will occur. First and foremost… What is the original organism made of? Does it have any soft or hard body parts? An organism that has some hard body parts has a much greater chance of leaving an impression or of having it’s hard parts mineralize and turn into rock. The process of fossil formation takes a long time and if the organism dies in an environment where bacteria can decompose it quickly, the process of fossilization will not take place. There wouldn’t be anything left to fossilize, after all.
Organisms that are made entirely out of soft body parts have very little chance of leaving a fossil. They are decomposed by bacteria too quickly. Hard parts, on the other hand, give the process of fossilization the needed time to work. Under extraordinary circumstances, however, if a “soft” organism dies in a location where there is little oxygen to support bacteria, then fossilization of even soft body parts can occur.
Not all fossils are made of rock. For example some can form when an organism is trapped in a substance that preserves their original body. Amber is a tree resin… (the sticky stuff coming of trees) and an insect that accidentally gets trapped in this sticky substance can end up being preserved in whole.
Another way that an organism can be fossilized is by being frozen. At the right is an example. A baby mammoth fossil was found thawing out of the frozen tundra in Siberia. The mammoth is made of organic material that is part of the original organism.
Two things to consider from what we’ve stated above. The fossil record preserves only certain types of organisms under certain types of conditions. That is to say, fossilization is a rare event. Furthermore, only certain types of creatures have any chance of being preserved. So, in effect, the fossil record is NOT a complete record. Not every creature that has ever lived has been fossilized. There will never be a perfect record of every species that has ever lived. What the fossil record gives us, instead, is a series of glimpses of the story of life. It’s like someone is watching a movie and you aren’t paying close attention. Every now and then you peek in and watch just a snippet of the show. Often times, you can get a good idea of what the show is about because you’ve seen parts of it along the progression of the story. The same is true of the fossil record. We don’t know the entire plot but we know enough to be able to make assumptions and scientific conclusions about how the history of life has played out.
How long have people known about fossils? As we said before, in the Western world, the answer is that only recently (in the last 300 years or so) have people really understood what fossils were. They had some odd ideas before that. In China, people finding dinosaur bones thought that they were the remains of dragons. Actually, if you think about it, the Chinese were not far off when you look at a T. Rex.
The person credited with the beginning of paleontology, or the study of fossils, is Nicolas Steno. Steno lived in the 1600’s and was initially a scientist and doctor. He travelled widely around Europe and met some of the best scientific minds of the time. Strangely, he later became a priest while living in Italy and he gave up his scientific pursuits. Steno travelled at a time when science was gaining acceptance as a way of understanding the world. Scientific interpretation of the natural world was gaining in acceptance.
Steno had a huge breakthrough when he was working in Italy as a scientist. Local fishermen had caught a shark and had preserved its head. They asked Steno come take a look. Steno spotted something that no one had noticed before.
Steno noticed that the teeth of the shark resembled stones that people often found embedded in rock all over Europe. The popular belief was that the stones were the tongues of snakes or dragons. After looking at the shark’s teeth, however, Steno concluded that these odd rocks were actually those of ancient sharks that had lived long before. This was a huge leap in scientific understanding. No one could imagine, before this, that rocks could be the remains of once living creatures. Furthermore, what were they doing on dry land if they belonged to sharks?
Compare for yourself. The conclusion is rather obvious. Dragon teeth? Or shark’s teeth?
Steno also had another powerful idea. From his travels and collecting he knew certain types of fossils were found in distinct and different layers from other types of fossils. Steno concluded that fossils were laid down in layers mixed with sediment. Sediment build up from bottom to top. Therefore, the oldest fossils were on the bottom and the newest were on the top. This later was called “Steno’s Law of Superposition.” In effect, the layers preserved different periods of time, oldest on the bottom and youngest on top. Each layer has fossils of creatures that were alive at that time but then went extinct. So each layer has it’s own unique fossils.
Using this idea, one can read the fossil record like peeling away layers of an onion. Deeper layers represent periods that are older in time. The story this gives us is the history of life, and indeed patterns arise when you compare fossils from different layers to each other. Simplest organisms that don’t resemble much of what is alive today are found the deepest. Organisms more similar to what is alive today are found in younger layers.
These ideas in geology were later rediscovered and expanded upon in England by later in the 1700 and 1800’s by geologists such as William Smith. He was a surveyor and travelled extensively throughout England mapping out sedimentary layers. He found he could find the same layers with the same fossils in a predictable sequence. In essence, Smith has confirmed the ideas of Steno.
Around this same period of time, in the late 1700’s, geology was becoming a formal science. James Hutton from Scotland played a significant role in the early days of geology. Based on many observations he made in England he believed that one could understand the present landscape and geological features of the earth based on gradual processes that have been at work on earth for a very long time.
His idea was called “uniformitarianism.” Let’s look at an example of this principle. Look at this river valley. Hutton would say that the valley was created by the process of erosion that occurs over a very long period of time.
Why there are no dinosaurs Many people of Hutton’s time subscribed to the idea that all of the earth’s features were created by one or several events described in the Bible as a flood. This would explain why fossil creatures like starfish or clams could be found on mountain tops. They were washed there by floods. It could also explain how some organisms are no longer alive… They were “wiped out” by the sudden flash floods described in the Bible. According to this line of thought, the features of the earth were relatively young. Whole river valleys could be created in a matter of days.
This idea was, of course, held by those who believed that the events described in the Bible were literally true.
In the early 1800’s the most current ideas about earth formation were written about by Charles Lyell in one of the most influential books on Geology ever published. In fact, Lyell is now called the “Father of Geology.” His book brought together all of the current thinking in the science of geology and included the ideas of Hutton and Smith and Steno. This work made the science of geology available to everyone and the study of geology took hold and became fully established as a science. This laid the groundwork for the later discovery and scientific interpretation of the fossil record.
Darwin was given a copy of Lyell’s book by Captain Fitzroy, the captain of the HMS Beagle. This was the ship Darwin sailed on, beginning in 1831, and which took him around the world as the ship’s naturalist. The Beagle’s voyage was a military mapping operation and also a commercial and scientific exploration of South America and the South Pacific about which little was known Darwin was one of the first who could take Lyell’s book and apply what it said to what he saw around the globe. He found a great deal of evidence that agreed with uniformitarianism and also with the ideas of Steno and Smith on the arrangement of fossils in the sedimentary record. At the time, Darwin was actually thought of more as a geologist than a biologist.
One last development in geological science needs to be mentioned. In the late 1800’s and early 1900’s major discoveries were made about radioactive particles. Madame Curry and others discovered that some rocks and minerals occasionally give off bits and pieces of their nuclei. As you know, proton number determines what an element is. If an atom loses protons as a result of radioactive decay, then it changes into a new element. Curry and others discovered that this process occurs at a highly constant rate over long periods of time. In some cases, billions of years.
The elements that decay (called the “parent” element) turn into another element (called the “daughter” element.) For example, one of the things that uranium turns into is lead. The age of a radioactive material can be determined by comparing the amount of parent material to the “daughter” product. The result is… you can date certain rocks if they contain radioactive elements within them.
Before radioactive dating, the determination of the age of the earth and fossils found with rock layers was left to “guestimates” of how long it took to form sedimentary deposits. Needless to day, these guesses could be very inaccurate…. Off by many millions of years. “Radiometric dating” of materials allowed scientists to put very accurate dates on rocks and rock layers. Why is this important?
There is one slight problem with dating fossils. Sedimentary rock, of which most fossils are made, often do not contain radioactive minerals, so they can’t be dated directly. Sedimentary rocks in which fossils are usually found are made from… you guessed it… sediment. That is to say, minerals that have precipitated out or fallen out of sea water, lakes, streams etc..
The bits and pieces that form sediment can come from material that has been washed into the body of water by erosion.
Or it can be from minerals collected from sea water by organisms. Many sea creatures use calcium and carbonate ions in sea water to create their shells or hard parts. Often these creatures can be microscopic like the foraminifera shown at the right. When they die, they fall to the bottom of the sea. After long periods of time, these particles from both land and sea build up thick layers on the floor of the lake or sea and essentially “glue” themselves together chemically after being buried and placed under tremendous pressure deep in the earth.
After millions upon millions perhaps this material rises out of the water through geological processes and the material on top of it weathers away. The rock you see lying around you might look like this… But as we said before, it often lacks radioactive minerals. So, how do you date a fossil formed like this?
One way to date a fossil is through “relative dating.” You can date a fossil if it is found in a position in a rock layer between or near other fossils whose age you know. How do you know the age of those fossils? Well, geologists have worked out the position of fossils in layers. Within any given layer of rock one can find certain “indicator fossils” that are ALWAYS found in a particular layer. That’s because those organisms lived at a particular time and died there. If you know the approximate age of the indicator fossil then any fossil found with it must be about the same age. So, for example, if you know the age of this fossil, you could say, with some degree of certainty, that the fossil found right next to it is the same age.
Relative dating works, and its what scientists used before radiometric dating was available, but its accuracy, at least initially was way off. After all, you efforts at dating something depend upon your estimate of how old something else is… and if you don’t know for certain how old that other things is… well….
Another way is to date material using radioactive materials found near or in the fossils themselves to get a precise date. So, for example, let’s say I find a layer of rock that contains these fossils… and I don’t know their age.
And somewhere you find a layer of rock above and below containing radioactive rocks where you find this layer of fossils. If you can now date the radioactive material above and below the fossils and you have a very good idea how old the layer of sedimentary rock is. Dated 320 m.y.a. old Dated 350 m.y.a. old Must be between 320 and 350 million years old.
On occasion, some radioactive materials do actually seep into fossils during formation and they can be directly dated using radioactive dating methods. Also, some organic materials do actually contain radioactive isotopes. Actually, all living organisms contain a certain amount of carbon-14, which is radioactive. So if you can find something that still has original organic material in it, you can date it fairly accurately. An example would be the baby mammoth shown to you earlier.
For example, this mummy of a man preserved in ice in the Alps was recently found and dated using Carbon 14 dating methods to 5300 years ago. What was he doing there? No one knows for sure but they did find an arrow head buried in his shoulder. An ancient murder mystery?
Also, using radioactive dating, scientists have been able to come up with a very accurate date of the Earth’s age. This was a huge piece in the puzzle of evolution. Today, we know from radioactive dating that the Earth began to congeal or come together into a large structure from the dust orbiting our sun around 4.2 billion years ago.
The earth’s age was and still is a major issue in evolution and also the fossil record. This is because the question is… are fossils really the remain of creatures that have long since gone extinct? For organisms to evolve and change from one species to another, it was believed that large amounts of time would be required. One early criticism of the idea of evolution was that the earth was considered to be too young for this to occur. An Irish Archbishop, named William Ussher, in the early 1600’s came up with an estimate of the earth’s age using the description of events in the Bible and other texts. His date was that the earth was only about 6,000 years old.
Ussher’s ideas about the age of the earth were widely accepted, especially by those who believed that the earth was created as described in the Bible. This age of 6,000 years did not leave enough time for evolution to have occurred. After all, species don’t change perceptibly in a person’s lifetime. How could they change so much in 6,000 years? Until radioactive dating came along and established scientifically the age of the earth, scientists were left with estimates based on sedimentation rates and the rate of cooling of the earth from a molten state. Some of these early scientific estimates put the earth’s age at a couple of million years… still not long enough for evolution. Establishing the earth’s age in the early 1900’s at 4.2 billion years old made plenty of time available for the vast changes life has undergone.
In sum, we now know from fossils… 1.What many (not all) extinct organisms looked like. (i.e. We have a pretty good idea of the appearance of a T-Rex. 2.We know when they lived. (i.e. T. Rex lived around 65 million years ago.) 3.From their anatomy and position in the fossil record can make judgments about how they relate to each other. In effect, we have a history. (i.e. We know that fish appear before amphibians and that amphibians evolved before reptiles and that they, in turn, came before mammals.) 4.The fossil record is an impressive body of evidence that supports the theory that life has changed on Earth.