The Fossil Record - Online Tutor, Practice Problems & Exam Prep

Fossils, the preserved evidence of past organisms, are crucial for understanding Earth's history. They can be categorized into four types: those in sedimentary rock, amber, trace fossils, and those found in ice or bogs. The fossil record is biased, favoring organisms from sediment-rich habitats, those with hard tissues, more recent species, and common organisms. Dating fossils involves relative dating, based on sediment layers, and radiometric dating, which uses the decay of radioactive isotopes, such as carbon-14, to provide specific ages. This knowledge is essential for studying evolutionary processes and the history of life.

Fossils

As we think about the history of life on Earth, we now want to note that the history of life is told mostly through fossils. We just want to spend a little bit of time talking about what fossils are and how they're made. So, fossils are defined as the preserved evidence of organisms from the past. Normally, when I think about a fossil, I think of something like a bone. However, here, we're defining them as preserved evidence. It doesn't have to be the organism itself; it can just be something that shows us that the organism was there. Paleontologists study fossils, a fact known even by many children, as at some point many of them want to become paleontologists and dig up dinosaur bones. I know I did.

As we discuss fossils, we categorize them in different ways. Here, we're going to group them into four groups based on how these fossils are made. First, we have fossils made in sedimentary rock, which is normally what comes to mind when thinking about fossils. Sedimentary rock is formed when soil or mud sediment is laid down in layers and compressed to form rock. If an organism is buried in the sediment, it can turn to rock along with that sediment, like the Archaeopteryx we see here. However, that's not the only way fossils are made. Fossils can also be found in amber when tree resin captures entire organisms intact. This process preserves small organisms, such as insects, and can be incredibly valuable.

We also have trace fossils, which are essentially the preserved evidence we talk about. These include things like burrows or footprints. For example, a dinosaur might walk through mud or volcanic ash, and we can later see that footprint and even put our hand in it. Trace fossils can be valuable as they may show something about how the organism lived, which we might not learn just from the structure of its bones.

Our final type are fossils found in ice, frozen soil, or acid bogs, like the baby woolly mammoth found in the tundra in Siberia. When things are fossilized this way, it can be quite valuable because it preserves soft tissue, such as skin, hair, and muscle. Additionally, this allows us to retrieve biomolecules like DNA, which means we could even perform genetics on these fossils. However, the downside is that ice, frozen soil, and acid bogs do not last long in geologic time; environments change, ice melts, and those fossils can be lost. Typically, these conditions preserve remains in the tens of thousands of years, not in the millions or hundreds of millions of years that you can see with other types of fossils. With that, we'll spend more time on fossils coming up. I'll see you there.

Biases in the Fossil Record

As we think about the fossil record, we want to be clear that the fossil record needs to be interpreted. And that's because we're going to say the fossil record is biased. And by that, we just mean that certain organisms are more or less likely to be represented in that fossil record. So we're going to look at here 4 ways the fossil record is biased. For each one, we'll go through and we'll think of an organism that's more likely to be represented and one that's less likely to be represented.

So let's go ahead and do it. We're going to say that it's easier to find fossils if the organism, we'll start here, lived in habitats with sedimentation. So marine habitats like this, right? Sand and mud are sort of always getting laid down. It's getting compressed to form that sedimentary rock.

It's much more likely that you find fossils in that environment than say a forest environment, right? That just doesn't make sedimentary rock as easily. Now for that reason, we have a really, really excellent fossil record for marine environments. There's just a lot more sedimentation, a lot more things becoming fossils than in many land environments. Alright.

Our next one here, we're gonna say it's easier to find fossils if the organism had hard tissues. Right? If you're interested in the evolution of snails, you are in luck, right? That snail shell is hard, which means that it doesn't decompose easily, which means that it's around long enough after it gets buried to become a fossil. Now you compare that to this worm.

If you're interested in the evolution of worms, you're kind of out of luck, right? This worm has no hard tissues. So when it dies, it all just decomposes before it can become a fossil. Alright, we're also going to say here, it's easier to find fossils if the organism was alive more recently. Alright, so if you're interested in, say, woolly mammoths or brachiosaurs here, well, the woolly mammoth was alive just tens of thousands of years ago.

There's going to be a lot of fossils around from that organism. Now this brachiosaur, it's much, much older. So those fossils are just much, much older and old things break, right? Geology happens.

Weather happens. Whatever it is, old things, well, they just don't last forever. There are fewer fossils around. Alright. Finally, what do you think this one is?

We're going to say it's more likely to find the fossil easier to find the fossil if the organism was, we're going to say common. Now if you're interested in human evolution, which a lot of people are, you might spend some time walking around looking for fossils on the ground in Africa. I've actually done this. Now when you do that, what do you find? You find a ton of antelope.

It's hard to miss those fossils. They're everywhere. What you don't find many of or you only find if you're extremely lucky, is a fossil of a human ancestor. There just are way more antelope fossils because there were more antelope living at the time those fossils were made. Those human ancestors, they were kind of rare in the environment, so it's just harder to find those fossils today.

Alright. So with that, we're going to be talking about fossils more going forward. We've got examples and practice problems to come. You check them out.

The Fossil Record Example 1

For this example, we need to use some of that knowledge about the biases in the fossil record. So let's start. It says for each pair below, we want to circle which fossil would be more likely to be represented in the fossil record, and we want to cross out the fossil that would be less likely to be represented, then we need to briefly explain why. So let's go through them. Our first pair here, a says a sea sponge from 2,000,000,000 years ago, and b is an early mammal from 200,000,000 years ago.

So looking at that, which one do you think is more likely to be represented in the fossil record and why? Well, I'm going to circle b, an early mammal from 200,000,000 years ago. I think I am more likely to find that, and I'm less likely to find a sea sponge from 2,000,000,000 years ago. So I'm going to cross that one out. My reasoning, early mammals have bones.

Bones are hard parts. Right? And hard parts fossilize more easily. And also, I'm going to say that early mammals from 200,000,000 years ago, that's much more recent than 2,000,000,000 years ago. And more recent fossils are easier to find because there's more of them, older things break, there are just fewer fossils around from a very long time ago.

Alright. I just do want to know that sea sponge, what it does have going for it, it's an area where there's more sedimentation. But because of the other reasons I said, I still think that early mammals can be easier to find. The next one we have here is a, a bee on a tree that produces resin or b, a dragonfly on a cactus in the desert.

Which do you think is more likely? Well, I'm going to go with a. Right? That bee on a tree that produces resin. I'm going to cross out the dragonfly, my reasoning, while I'm going to say resin equals amber.

Right? And we talked about fossils being found in amber. Now most bees that land on trees do not get fossilized in amber, but I can at least see a route that can happen. That dragonfly on a cactus in the desert, it's just I see a less route for that to become a fossil. Right?

It's not an area with a lot of sedimentation, and so I just don't see that happening where a, I see how that could become a fossil. Alright. Our final pair here, we have a, a snail in the intertidal or b, a slug in the forest. What do you think is more likely and why? Well, I'm definitely circling that snail in the intertidal, and I'm crossing out that slug in the forest.

The reasons well, two of them, that snail has hard parts. Right? That shell is going to fossilize relatively easily if it gets buried in sedimentary rock. And the second part relates to that, that intertidal, I'm going to say there's sedimentation. So there's just a lot more sedimentary rock being made in the intertidal, because it's always laying down sand, mud, etcetera.

Things are getting buried. That slug in the forest, nothing hard, in an area with not a lot of sedimentation, just much less likely to become a fossil. Alright. With that, we got more practice after this. Check it out.

Which of the following would you think is most easily found as a fossil by a paleontologist?

• A. A tiger skeleton.
• B. A mineralized leaf.
• C. An early prokaryote.
• D. A shark tooth.

What is an advantage to studying fossils that are frozen in ice or soil?

• A. Frozen fossils are more likely to contain intact DNA or other biological molecules.
• B. Frozen fossils are extremely common compared to other fossils.
• C. Frozen fossils are less delicate than other fossil types and, therefore, last longer.
• D. Frozen fossils are more likely to contain minerals that have replaced soft tissues.

Dating Fossils

Fossils are great because they tell us all about the organisms that were alive in the past, but often we want to know when they were alive. For that, we need to date fossils. So, we're going to say here the fossil record can give us two basic types of dates. Now the first type of date we're going to call a relative date, and this type of date is pretty straightforward. It's just based on what layer in that sedimentary rock a fossil is found in.

Right? So, in our image here, we have this sedimentary rock. We see all these different layers here. And, in simple terms, we know that this trilobite fossil has to be really old because it's found deep in the sediment. All these other layers were laid down after that trilobite was alive.

Now this dinosaur had to be alive more recently than the trilobite because we find it in a layer that is closer to the surface. Now this woolly mammoth, it's even higher in the sedimentary rock still, so it must have been alive relatively recently compared to those other fossils. All right, so that's great, but often we're giving pretty darn specific dates for fossils. So how do we get those? Well, we need to use what's called radiometric dates.

Radiometric dates can give us a pretty darn specific date based on the decay of radioactive isotopes. Now you have radioactivity all around you in the world. There's radioactivity in you right now. It's just at very low levels, low enough levels that it's not harmful. But because radioactivity decays, over time it goes away.

If we know how much radioactivity we started with, we can see how much is left and figure out how long it's been because radioactivity decays at a very predictable rate, and we measure that rate in something called a half-life. So the half-life is the time it takes for half the radioactivity to decay and how long that half-life is depends on the specific element. Every specific radioactive element decays at a slightly different rate. It has a slightly different half-life. Alright.

So, how would this work? Because we know what the starting ratio is for all sorts of different elements, we can take the radioactivity that's left in the sample. We can compare it to what we know that starting amount of radioactivity would have had to have been, and we can use that concept of a half-life to calculate the date. Now to look at this, I'm going to move over on the screen here, and you can see we're looking at the decay of radioactive isotopes over time.

On the y-axis, we have the fraction of the isotope left going from 0 to just 100%, all of that radioactivity left. So obviously at time 0, you have all of your radioactivity there. Now after one half-life, well, you're going to lose half the radioactivity, so you have half the radioactivity left. After 2 half-lives, well, you're going to have half of what you had previously. So a half of a half, you're now down to a quarter.

After 3 half-lives, well, half of a quarter, you're now down to 1/8 of that starting amount of radioactivity. And then after 4 half-lives, you can probably see where this is going. We're now down to 1/16th, the original starting amount. And again, because we know the starting amount for all sorts of different elements, we can use these half-lives to calculate dates. Alright.

So now a type of radiometric dating that you may be familiar with or you may have heard of before is called C-14 dating or carbon-14 dating or radiocarbon dating. Now this is great because it gives you direct dating of organisms. Right? You can pick up a bone and date that bone because living things are made of organic molecules, right? They have a lot of carbon in them.

Alright. So what we do is we take the ratio of carbon-14, that radioactive carbon. We compare it to the amount of carbon-12, and because we know the starting ratio when something was alive, well, we can use that concept of half-lives and that will give us a date up to about 60,000 years. All right, now that's great but 60,000 years isn't that old in geologic time, right? A lot of times we're talking about 1,000,000, hundreds of millions of years.

Now the thing about carbon-14 is that it has a relatively short half-life. So after 60,000 years or so, there just isn't enough radioactivity left to give you a reliable measurement to calculate a date. So what do we do for older stuff? Well, we use other radioisotopes, right? Things like uranium-238, or you may have heard of potassium-argon dating.

Now the issue is you don't have uranium in your body. Living things don't have uranium. So what do you do? You can't date the fossil itself. You need to date the surrounding rock.

Alright. Now the issue is here though, you need to have rock that you know was new at the time it was laid down. At the time that you're finding that fossil, you need rock that was new then. Well, how do you find that? Well, what we do is we measure these radioisotopes in volcanic.

Rock or volcanic ash, and that can give us those older dates. Because when a volcano erupts, that rock gets laid down or the ash gets laid down and it ends up in our sedimentary rock. You get a layer of volcanic rock that you know had to be new at that time that it was laid down. Alright. So again, that can give us dates back to the beginning of Earth into the billions of years.

Alright. But there isn't volcanic rock, right, at every layer. So how do we do this? Well, we compare our radiometric dates and our relative dates. Right?

So you get a radiometric date. You know something was above it. You can now start to at least get estimates of when that thing was alive. And because people have done this for all the sorts of layers all over the earth with all sorts of different volcanic activity, we can get relatively specific dates for pretty much any fossil you find. Alright.

With that, we've got practices and examples after this. I'll see you there.

The Fossil Record Example 2

Our example tells us that the half-life of carbon-14 is 5,730 years. Then it says if a bone is determined to have one eighth of a normal ratio of C₁₄ to C₁₂, how old would you predict that organism to be? Alright, now just to be clear, if you're a scientist really doing this, there's sort of all sorts of fine-tuning that has to go into these calculations. But for us, just conceptually thinking about how this works, we just have to think how many half-lives it takes to get to one eighth the normal ratio. So, I'm just going to set up a pretty simple table here.

I'm going to say half-lives on the top and the amount of radioactivity that is left on the bottom. Right? And after 0 half-lives, well, right, we should have 100% of the radioactivity left, so that's 1. After one half-life, we cut that in half, so we're down to one half the amount. After two half-lives, well, now we're down to one quarter.

We cut that half in half again. And then after three half-lives, we cut that one quarter in half, and we're down to one eighth. And hey, that's what we were looking for, right? So three half-lives, that’s how long it will take to get to one eighth the normal ratio. Well, how long is three half-lives?

We just have to do three times the half-life of Carbon 14, which is 5,730. So, I do that out. I need a calculator for that, but I did it in advance, and that comes out to 17,190 years, approximately.

So, that's how old our bone is by carbon-14 dating. Now remember, carbon-14 is great because it gives us these really accurate readings, but 17,000 years, that's not that old in geologic times. Remember, for much older stuff, we're going to be using other isotopes. Alright. Without more practice after this, I'll see you there.

What type of fossil dating technique would be most accurate for a trilobite from ~400 million years ago?

• A. Carbon 14 dating of the fossils itself.
• B. Carbon 14 dating of the surrounding volcanic ash.
• C. Uranium 238 dating of the surrounding volcanic ash.
• D. Uranium 238 dating of the fossils itself.

Fossil A is found two layers of strata below a layer that contains volcanic ash. Fossil B is from a different organism and is found immediately above the layer of ash. The ash has been dated to be about 125 million years old. Which statement below is correct about the age of the fossils?

• A. Fossil B is 125 million years old, but we have no information about the age of fossil A.
• B. Fossil A must be older than 125 million years old, while fossil B is less than 125 million years old.
• C. Fossil A must be younger than 125 million years old, but we have no information about the age of fossils found above the ash layer.
• D. Fossil B must be older than fossil A, while fossil A must be less than 125 million years old.