The Science of Everything Podcast - Episode 156: Fossils and Dating Methods

Episode Date: November 30, 2025

An introduction to the techniques used to study the history and ages of geological speciments. We begin with an overview of chronostratigraphy,  covering principles of relative dating, way up indicators, and biostratigraphy. We then discuss geochronology, including a review of early attempts to determine the age of the Earth, the principles and assumptions of radiometric dating, and the radioisotopes most commonly used for dating. Finally, we discuss processes and mechanisms of fossilisation, and review some of the most important classes of index fossils. Recommended pre-listening is Episode 74: Minerals and Rocks, and Episode 77: Nuclear Physics. If you enjoyed the podcast please consider supporting the show by making a PayPal donation or becoming a Patreon supporter. https://www.patreon.com/jamesfodor https://www.paypal.me/ScienceofEverything

you're listening to The Science of Everything podcast episode 156 fossils and dating methods. I'm your host, James Fodor. So today we're going to discuss the processes by which geologists are able to date different rocks and sediments and place them in relative chronological ordering. This will be preparation for the next episode we're going to do looking at the geological time scale. But before we discuss what we know about the temporal sequence of events and history, we need to know a little bit about the mechanisms and the processes by which we come to this knowledge. And so that's why we need to talk about dating methods. And one of the most

important dating methods is fossils. So I'll also be talking about the processes of fossilisation, what fossils are, and how they're important, how they're used for dating purposes and establishing relationships between sediments in the geological timescale. Recommended pre-listening is episode 74, minerals and rocks. So let's make a start by distinguishing a few commonly confused terms, particularly I want to distinguish between chronostratigraphy and geochronology. These terms are very similar and they're often confused, and it's also true that different scientists have used the terms differently at different times, and you can actually find articles talking about the proper usages of these terms. I'm going to simplify things a little bit here, just for ease of explanation. Basically, chrono stratigraphy, if you think about breaking up that term,

chrono relates to time and stratigraphy relates to the study of layers of sediment. So, chronostratigraphy relates to the science and study of relating strata, so like layers of sediment, to historical time or periods of time. And what this amounts to in practice is using principles of relative dating to determine which layers, sedimentary layers, were deposited before and which sedimentary layers were deposited after other layers. So the relative ordering of different layers. Now, geochronology is the scientific study of the age of rocks. Now, that might sound like the same thing, but remember here, we're talking about, in geochronology, dating, dating, the age of rocks. It's not about ordering strata. It's about dating a particular rock or type of rock. So obviously these two different disciplines interrelated and they both important for establishing the geological timescale. But essentially the way that we're going to think about them here

is that chronostrography, which we'll talk about first, is the principle of using relative dating methods to determine the ordering that different sedimentary layers were and also other types of rock formations, as we'll see, were laid down relative to each other. And then geochronology, which we'll talk about second, relates to methods for dating these rocks in terms of their absolute year ages. And mostly that relies on radiometric dating, though there are some other techniques as well. Okay, so having said that, let's begin by talking about chronostratigraphy or principles of relative dating. So these are methods that were developed in, the late 18th and early 19th centuries, which enables scientists to determine, as I said,

the ordering in which different rock formations were formed, particularly sedimentary layers, but there's also principles that are relevant to others as well. So let's start with the simplest, original horizontality. This is the idea that most sediments were originally laid down flat relative to the Earth's gravity field at the time. Sedimentary layers only later are tilted as a result of geologic events, such as tectonic plate movement or the formation of mountains or erosion, things like that. The next principle is superposition. The oldest rocks will be at the bottom, assuming that there's been no inversion, and younger sediments will be deposited on top of older ones. This obviously relates directly to the first principle of original horizontality.

So this is pretty straightforward, but these are two very foundational principles that are used to determine relative ordering, you can determine initially that sediments were laid flat and that older is on the bottom. Anything else that changes, that must have been a subsequent geological process. So the next principle is called lateral continuity. So what this means is that most sedimentary beds do not abruptly end when they were originally laid down. Sometimes they can, for example, due to geographical constraints. But in general, beds tend to become gradual. thinner or grayed out and progressively become a different sedimentary form. So if a rock bed ends abruptly, this is usually an indication that there was some weathering or erosion or other process

after the original formation. So again, this is a principle that allows geologists to determine where the boundaries of different beds are likely to have been and when there was likely to have been subsequent geological activity or some weathering process. The next principle relates to cross-cutting relationships. So a geological feature that cuts through another must be the younger of the two features. If you think about that for a second, it makes perfect sense. So if, for example, there is a fault or an inclusion or erosion through a layer of sediment or multiple layers of sediment, that fault or inclusion or erosion must have occurred after those initial sediments were laid down. So this helps us to determine the relative order in which those geological

events took place. This is related to the next one, the law of inclusion. So fragments of rock that are inside of another rock must have been formed first. So this is common if there's been, for example, well, any type of sedimentary rock will involve the initial sediments, which were derived from previous rocks, and which are then fragmented and are dispersed over some area, moved and then dispersed over some area and deposited again. So the fragments are older than the rock that they, the sedimentary rock that they subsequently become part of, obviously because they must have been sediment before they became the sedimentary rock. And that's true for any type of inclusion where there's pieces of one rock inside of another.

An important example of this principle is when you have igneous inclusions. So this is when you have, say, a series of sedimentary strata, and then as a result of geologic activity, there is igneous rock that forms as an inclusion. So basically a projection into those layers of sediment. So igneous rock is formed from magma, which then cools and solidifies and becomes rock. So if you're excavating a particular area and you notice that there's this very hard igneous inclusion that extends across a number of vertical layers of sediment, this igneous rock must have formed after all of those sediments were laid down, because otherwise there'd be no way for it to cut across those layers of.

sediment like that. They had to have been there first, right? And then, for example, you might have further faults or folding that's occurred across that or along that igneous inclusion as well as the surrounding sediments. Well, then those must have occurred after both of those things. These principles seem relatively simple by themselves, but what becomes interesting is when they're applied in combination with each other. And obviously, in real world, geological examples, it becomes quite complicated quite quickly and it takes a lot of expertise to put the pieces together and determine which types of rock or which samples are the oldest and then the relative ordering from there. Because it's very common for there to be significant faulting, folding,

fracturing, inclusions and then further fractures on fractures on inclusions and folds and also tilting and inversions of actually tipping upside down of whole rock layers is not that uncommon either. Plus there's all sorts of erosion that happens. You can have laying down of sediments for millions of years and then as a result of a change in sea level, for example, or volcanic activity or all sorts of different processes, you could have millions of years of erosion after that. So there'll be sediments laid down and then erosion of some of those sedimentary layers and then renewed deposition. So all of these different tools have to be brought together in order to determine the relative. ordering of these different samples of rock and to determine periods of time that might separate instances or periods of deposition. So gaps in the deposition record essentially. And these are known

as unconformities I've discussed these previously. So there are layers or periods of time during which there was no deposition and therefore we don't have any evidence of what happened geologically at that time in a particular place. Or when there was initially deposition, but it's subsequently being eroded away. So if we don't have any evidence of what happened geologically, eroded away. So for whatever reason, there's a discontinuity between layers that are currently next to each other. Now, I mentioned that the law of original horizontality says that most sediments are initially laid down flat and only tilted later. This, to be useful, requires this, of course, to know whether the sediment has subsequently been tilted, particularly whether it

subsequently become inverted. And in order to determine this, we have to have some, ways of determining which way is up. And so these are called, funnily enough, up indicators or way-up indicators. So these are very important for us to determine how rocks have been tilted over time, particularly, as I said, because they can be completely inverted. So we need to know which way was originally pointing up when the rock was, when the original sediment was deposited. So the first thing that can be checked is we can look for cracks within the rocks. So cracks will tend to fill up from the bottom to the top. In particular, what we tend to find in these sorts of fractures or cracks in the rocks is that the bottoms will tend to fill up with sediments,

whereas the upper layers will tend to form crystals as a result of crystallization, for example, from water. So we can look at the fractures in the rocks, the cracks between different segments, and see which parts seem to have filled up first and which parts had crystallization. form that gives us a clue as to which direction was initially up and down. Now another principle is that of graded bedding, which again I've talked about previously, this is the idea that at least for certain types of sedimentary rocks, the grain size varies systematically from the base of the bed to the top. So certain types of, often the heavier sediments are deposited first and then the lighter ones, though that can be

inverted in certain instances. So the relative size of the the fragments of sediments that then comprise the sedimentary rock can be a clue as to which direction was originally up. In stream beds that have been gradually filled by sediment, there is usually a characteristic U-shape at the bottom of the bed. That's the shape that tends to be eroded by the force of the water. Again, I've talked about that in previous episodes as well, and so this U-shape can give a clue as to which direction was initially up, because it will be sort of the open part of the U. Physical indicators like mud cracks and water marks, such as from raindrop impressions or ripple

marks from water that was rippling across it, because mud cracks tend to have a characteristic V shape. So all of these sorts of physical indicators can provide evidence of which direction was initially up at the time that the sediment was laid down. Some types of rocks, particularly such as igneous rocks, may form bubbles, so they may trap air in it. So for example, lava can trap air bubbles, which are usually found near the top of the lava. They're usually fewer at the bottom. And so if there's igneous rocks that have bubbles in it, you can count which facing of the layer has relatively more bubbles, and that can be an indicator

of which direction was initially up. And finally, fossil evidence can also be an indication of which direction was up. So tree stumps, footprints, animal burrows, and root traces, all will give some indication of which direction was upwards. So these tools or techniques won't always be relevant, but often at least one or more of these methods will be useful for determining whether a particular sample was facing up as the same direction as presently or whether it's been inverted at some point in its history. So basically what we've been talking about so far is principles that can be used to determine the relative ordering of different types of rocks and particularly to determine which way was originally up, that being essential in terms of establishing,

again, which layers were laid down first, and then which layers came subsequently. Now, a final area that I want to talk about is biostrategraphy. So this is a branch of stratigraphy, which focuses on assigning ages of rock strata, relative ages, based on the fossils that are contained within them. So, again, this is not about, like, dating the fossils using radiometric dating or something like that. We'll talk about that in a moment. This is actually an older technique, which predates radiometric dating. So radiometric dating rarely only became a thing around the mid-20th century,

but use of fossils for dating different layers of rock dates back for at least 100 years before that, probably longer. We'll talk about the definition of fossils a little bit more in a moment, but a fossil is essentially some kind of remnant or trace of an animal or plant that existed in the past that has been preserved in some way. So it's a rock remnant of a living organism. would be a simple way to define it. Now, because fossils provide evidence of organisms that existed in the past,

they also provide evidence about the conditions that prevailed at the time that rock formed or around that time. Most useful of all are what are called index fossils. So these are fossils that only formed, like the organisms were only around, for a limited time in Earth's history. So if you find a fossil, one of these fossils, in a particular, in a particular sediment, then that restricts the range of dates that that rock must have formed. Good index fossils have a limited range of time, but also are relatively easily preserved

and can be preserved in lots of different environments. So that makes them more useful because they're more readily found. The more readily found they are, then the more cross-referencing we can do to determine how common they were in different time periods, right? So we can establish from many finds around the world, for example, that a certain organism lived maybe only from 34 million years ago to 30 million years ago, and so we can constrain any rocks that contain that fossil within that fairly narrow range. Now, another important thing about fossils

is that they tell us not just when the rock formed, but also what conditions prevailed at the time the rock formed, because most creatures can't survive in all types of environments. They require certain temperatures, humidity, certain plants, certain levels of moisture available, So the fact that a fossil is present in a particular location tells you about the weather and the ecology of that particular time and place when the rock was laid down. And these are called ecological indicator fossils, because they tell you about the ecology at that time that the fossil initially formed. So the way that chronostratigraphy works is by using these tools of relative dating, way-up indicators and index fossils, and studying many, many sites all around the world.

we piece together, in a very gradual sort of piecework way, a picture of the Earth's past. So no individual site is going to give you all of the information. It's very rare that a site will be sufficiently, a geological site will be sufficiently complete and well preserved to give you a very long period, to give you information about a very long period of time. But the way that scientists study this is by finding good sites, in one location, which then overlap with a site in another location, then partially overlap with another site and then so they progressively piece together through these different overlapping,

temporally overlapping portions, a view of the Earth's past. And of course, that varies by location as well. Contents move over time, and so there's a need to establish the historical relationship of the rock samples, not only in terms of like when they were laid down, but also where was this rock relative to other rock samples or like other sites at different times in history. So the further back you go, the more things could have potentially moved relative to the present day. And it's interesting looking at some of the maps here. There's still quite an over-reliance on European and North American sites and a relative

lack of geological sites, particularly in Africa and many parts of Asia. And there's increasingly more work being done in these non-traditional sites, often in developing countries, but there's still clearly a need for greater study of geological sites in these locations to give a more representative sample of the Earth's past. Okay, so now let's move on from chrono-strategomy. We'll start talking about geochronology. At this point, what we have, or what we've been talking about is how geologists are able to describe the relative ordering of when different layers of sediments were laid down, when different, and the temporal sequence of rock strata in different locations, and so they can say that, like, you know, this fossil is only found at this particular layer, which came before this one and so on and so forth, and they combine all that together.

But what's lacking here is any method of providing absolute dates to this sequence of geological events. And this was a major point of controversy in the late 19th century, because by this point, there'd been a lot of study of fossils and a lot of development of techniques for understanding, chronostrography, so different temporal relationships and sequences of Zed imagery layers. But no one really had any idea about the absolute age of any of these specimens, rock, or fossil, nor did they have a clear idea about what periods of time they were talking about, like from the oldest fossils to the present day. And there are a number of different attempts to calculate this. One of the most famous calculations was made by Lord Kelvin on the basis of assumptions about the rate at which the Earth was cooling over time. And many of these different

types of thermodynamic calculations, there were also some that were based on erosion and other assumptions around this time, the sort of late 19th, early 20th centuries, arrived at dates in the order of like tens of millions to maybe 100 million years old as the age of the earth. And that was much, much shorter than the biologists generally thought would be sufficient to account for evolution. I mean, they didn't have a clear idea either, but the general idea was that tens of millions of years or even 100 million years would not be sufficient to account for the diversity of life, that the gradualness of evolution would require much longer periods of times, billions of years rather than tens or even 100 million. And so this was a puzzle for a

long time. It was really only with the development of radiometric dating, with the initially discovery of radioactivity in the early 20th century, and then over the coming decades the development of dating techniques. So it was in the 1950s that the first dates of the age of the earth and the oldest rocks were published. And these pointed at an earth that was much, much older than many people thought. The earth wasn't 50 or 100 million years old, but was in fact billions of years old, about four and a half billion years old, which is still essential the currently accepted date. So the history of the dating itself is quite interesting. And the reason why many of these earlier calculations went wrong, such as Lord Kelvin's calculations based on the rate of cooling of the

Earth, is because he was unaware that the Earth wasn't just cooling as a result of losing the energy acquired during Earth's gravitational collapse, which heated up the Earth and then was gradually being radiated out. But also, Earth was continually being warmed by energy absorbed or heater absorbed as a result of radioactive decay. And Kelvin didn't know anything about radioactive decay at the time, so that's why many of these earlier calculations were so far. far off because they didn't know about radioactivity. It's sort of ironic that radioactivity was both the cause of why these calculations were wrong, or at least one important cause, and also would turn out to be the method by which correct dates would later be determined. Okay, so let's talk about radiometric dating and how it works and how it can be used to determine the age, particularly of rocks, and thereby provide an absolute dates to the chronostrography results that we've been discussing. I've talked about radioactivity in the previous episode.

You can look back to episode 77 on nuclear physics if you would like some more context for this. But here I'm just going to talk about, I'm going to focus on the process of radioactive decay purely in terms of how it's relevant to dating. As a brief reminder, there are many different types of atoms, differentiated by their atomic number, which is the number of protons that they have, and also their mass number, which is, determined by both the number of protons and the number of neutrons that they have. Different elements which have the same number of protons, but different numbers of neutrons are called isotopes of that element. So many elements have stable isotopes that exist for hundreds, thousands, millions, or even billions of years and don't really change. But there are some types which are radioactive, and these are unstable isotopes. So essentially they spontaneously fall apart or decay into usually two what are called daughter nuclear.

so the parent nucleus decays into daughter nuclei, which may then further decay into other products. So an individual decay is unpredictable. You don't know when a single nucleus is going to decay. However, if you have a large number of them, you can predict very accurately what proportion of them will decay over a period of time. It's statistically very, very reliable. The half-life is the time for half of the population of parent atoms to decay into their daughter products. A longer half-life means that it takes longer for those atoms to decay. Or in other words, they're more stable.

More unstable atoms have shorter half-lifes, and they're said to be more radioactive because they decay more quickly and release energy much more quickly. Now, there's a very important fact about the half-life of a particular radioisotope, which makes it useful for dating purposes. And that is, as far as we know, the half-life of a given isotope is always the same.

It's not affected by the chemical environment in which that isotope exists, or the physical environment, or magnetic fields, or pressure, temperature, anything like that. It's always the same. This makes it very useful for dating purposes, because the main problem with trying to date things that are very old is that any type of other chemical or physical process occurs at different rates depending on various factors, like I just mentioned, like the temperature and the pressure, or the chemical environment, or the state of the atmosphere at the time, or how much water was present, or what other minerals are involved or chemical reactions of other sorts.

And because you don't know those conditions, or at least you don't know them throughout the entire history of the sample in the vast majority of cases, there's no real way to determine using rates of these processes how old the sample is, because you have too much missing information. The processes vary depending on these factors, and you don't know all of the relevant variables that would allow you to make that computation.

So effectively, you can't use most physical or chemical processes for dating purposes in general. Of course there are exceptions, but for the most part, it's too unreliable. Now, radioactive decay is special because half-life is fixed and does not change. And effectively, this is because the half-life is determined by the nuclear force, the strong and the weak nuclear forces technically. I won't get into that. But it's not affected by any chemical processes, so it's not affected by the chemical bonds. And most processes that occur, like geological processes, biological processes, are chemical in nature. that they involve exchanges of electrons.

But none of that affects the half-life, and nor, as it turns out, is temperature or pressure. The only real processes that can affect the half-life are those that involve nuclear fusion or fission, which effectively involves changing something into a different isotope or a different element. And those are extremely rare, and it will be very obvious if those have occurred, right?

And so, for the most part, once a radioactive isotope forms, it will decay at a fixed rate, which is always constant, which means that if we know how much of the parent isotope was initially present, and then we can measure how much is left now in the present. So we know how much there was initially whenever it was formed, and then we know how much is there now, we can, with very simple math, fit an exponential function

and apply the known half-life and determine how many half-lifes have passed since the sample was formed. We know how long the half-life was, we can then give a date in years to that sample. Now, for this method to work, certain assumptions do need to be satisfied. So, firstly, the rock or mineral that we're trying to date must be closed to both the parent and the daughter isotopes. Now, what is meant by closed here is that the mineral structure is sufficiently sort of fixed and rigid such that there is no significant diffusion of the parent or daughter isotopes out of that mineral environment, that crystalline structure, into the external environment. Now, because rocks and minerals are solid, usually there is a fairly rigid lattice of bonds that connects the different atoms together, the different isotopes, and prevents the parent or daughter

isotopes from leaving, from escaping from the mineral. However, this only holds insofar as the mineral remains a solid and is sufficiently, well, solidified essentially, sufficiently rigid to prevent this sort of leakage. If the rock heats up too much, the bonds will become destabilized and isotopes will be able to escape. And this is known as the closure temperature. It's the temperature below which there's no significant leakage of parent or daughter isotopes. The closure temperature differs between different minerals and it can be determined using laboratory experiments. So basically what we need to do is for a given mineral or type of rock sample that we're interested in dating, we need to, using laboratory experiments, determine its closure temperature,

and then we need to examine the particular sample that we're interested in dating and determine whether or not it was likely throughout its geological history ever exceeded that closure temperature, that is after its formation. If it did, then it's likely that there will have been loss of parent-hand or daughter isotopes, and therefore it won't be able to be dated, at least not prior to the time when its temperature exceeded the closure temperature. And there's a, there's a usually ways of doing this, particularly because when rocks are subjected to high temperatures, as well as pressures, that tends to disrupt and deform the latter structure and resulting in metamorphic changes. And so these can usually be detected. Of course, how difficult that is depends

on the exact type of rock and the exact minerals and other conditions. But this needs to be investigated on a case-by-case basis. But usually there'll be a way to tell if that closure temperature has been exceeded since the original formation of the rock. So the first assumption, again, is essentially that there's been no loss of parent and daughter isotopes over time to the external environment. Now, the second assumption is that we are able to determine what the initial ratio of parent-daughter isotopes was, or in other words, how much of the initial parent-isotope was there. Now, this is not always possible because sometimes you can just have different proportions of the parent-and-daughter isotopes both present at the same time, in which case you won't know

what the initial proportion was. However, there are certain cases, where we know, because of the processes involved in the formation of certain types of rocks or minerals, we know what the initial ratios will be. So for example, uranium very easily dissolves in water, whereas thorium precipitates out almost immediately. So any rocks that formed directly from ocean water will have essentially no thorium in them, because it will have precipitated out, before that point. And therefore, we can use uranium thorium decay to determine accurately the date or the number of half-lifes since the formation of that uranium rock. Because there shouldn't be initially any thorium or will be all uranium. Any thorium that has formed will have been formed by radioactive decay in the period since the formation

of the rock. In other cases, this forms because of different melting temperatures. So for example, in potassium argon dating, feldspar rocks will form with potassium, but they tend not to have any argon in them because that's a gas and escapes. The only argon that will be found inside those rocks will be argon that has been trapped in there as a result of forming as a result of radioactive decay of the potassium in the period since the rock was formed. And essentially, you know there won't have been any initial argon there because it would have escaped before the rock had time to cool down. So in cases like this where we're able to determine what the initial ratio would be, and usually that means that we know that there won't have been any door to isotope present because it would have

escaped or it would have dissolved away or something like that. So we know that there's only parent isotope initially present, and then we also know from assumption one that none of that parent isotope or any of the daughter isotope as well could have escaped to the external environment because the system remained closed. Now, if we have those two requirements satisfied, the third key assumption is that we must be able to accurately measure the amounts of parent and daughter isotope that are present. And this is done using a mass spectrometer, which essentially determines the mass to charge ratios of different chemical species, and that's a very accurate process. But the main limitation

there is you need a certain amount of substance in order to accurately, in order for this to be done accurately. And so if the rocks are too young, there'll be two little daughter atoms, daughter isotopes present, and so you won't be able to accurately measure the quantity. Likewise, if the rocks are too old, then there'll be two little parent left, because essentially all of it will have decayed. And so you need to have this sweet spot in the middle where there's enough of the parent and the daughter isotopes in order to accurately date the sample. The final assumption here is that the radioactive half-life must be constant over time. Now, this is not something that's really an issue, because as I've discussed, we have

a detailed understanding of radioactivity and extensive evidence that half-lifes are constant and don't change, either due to local chemical or physical properties or just over time in general. This is sometimes brought up by people who are skeptical about radioactive dating, such as young as creationists. But in fact, we can verify this assumption that half-lifes are constant over time. And so uranium decay is at the same rate now as it did four billion years ago. We can determine that in many different ways. So, for example, we can observe the decay rates of isotopes that have very short half-lifes, and we can just do that in the lab right currently. And we don't see any deviation over time there. We can measure the rates

to decay with extreme precision, such that if there was change, we should be able to detect it. Of course, one could always argue, well, it's so small that we can't even detect it with the extreme precision that we have, but there's been no evidence of any such changes in the half-life being detected, despite the extreme precision with which we can measure such things. Another technique that can be used is to examine the decay products of very old nuclear processes, so there have been some on Earth as a result of large accumulations of uranium, for example. Or distant stars, which we can see because obviously the speed of light being finite, it takes a long time for the light from distant stars to arrive.

And so when we're looking out at the universe, we're looking back in time as well. And then we can see, we can collect the light spectrum and analyze the proportion of different elements, including radioactive elements and their isotopes. And the results that we have there of dating exactly agree with exactly what we do expect from the assumption of constant half-lifes. A third method we can use is the fact that we have a fairly good, not a perfect, but a fairly good understanding of the underlying physics of radioactive decay, particularly relating to the weak and the strong forces that I mentioned before. And we can do quantum field theoretic calculations that estimate how long such decays are expected to take. And again, the half-lifes that we determine are in line with fundamental theory, at least for where we can make the comparisons.

I don't think that's always possible, but for those in which we can make our calculations, the observed half-lifes are in line with the expectations. So there's a conciliance between theory and experiment there, and quantum field theory is extremely well experimentally verified, and so something would have to be wrong in that theory in order for it to be the case that half-lifes change over time. And finally, and perhaps most straightforwardly, we can simply use different dating methods to date the same samples

and then see if they are in accordance with each other, in the margins of error. And once again, we find that that is the case. And it's very hard to see why that would be the case if different half-lifes changed over time, that they would change in such a way that they still preserved the relative order, like the ages of different sample. So it's essentially impossible that given the evidence that we have, that half-lifes really could change over time, there'd have to be massive holes in throughout our understanding of physics in order for that to be the case. Of course, you can never say absolutely never in science, But as far as we know, there's overwhelming evidence that radioactive half-lifes are constant in time

and are not affected by physical or chemical processes. So that means that in order to determine the age of a rock sample, the main requirements are that you know the processes involved with forming that rock sufficiently well such that you can determine how much of the daughter isotope was initially present. And I just mentioned some examples of that before, because say thorium precipitates out of water in the uranium case or argon is lost into the atmosphere in the potassium case. And then secondly, you can ensure that that rock system was closed to any escaping of parent or daughter isotopes throughout its geological history since it was formed.

And you can test that by determining the closure temperature in a lab and then investigating that sample to determine whether there's any evidence that it exceeded that closure temperature over its geological history. So with those conditions met, you can then take samples of the rock and use mass spectrometry to determine the relative quantities of the parent and daughter isotopes. The more of the daughter isotope there is, the more half-lifes will have occurred or have taken place since the formation of that rock or that mineral. And by counting the number of half-lifes, you can determine, based on the known rate of the radioactive date, you can determine the absolute age in years

of that sample within some margin of error. The margin of error being mostly determined, by the way, by the uncertainty in the quantities measured. It's mostly not due to uncertain. in the half-life because we know those very accurately from physical measurements. Now, as I've been explaining, because of these requirements, radiometric dating can only be applied to certain types of rocks and certain samples. It can't be used to just date anything. Sedimentary rocks are usually not possible to date using radiometric dating because they are made of particles of different ages.

We talked about this before in chronostratigraphy, the law of inclusions. Fragments of rock inside of another rock were formed before that rock. So sedimentary rocks are made up of fragments of rock, you know, sediments, which existed prior to that sedimentary rock, and in general they'll be of different ages, so you can't use this method in general to date a sedimentary rock. Some type of chemically precipitated rocks, like calcium carbonate-based rocks, stalactites, can be dated, but they're the exception. Metamorphic rocks can sometimes be dated, but you have to be careful there because there will both be a time of crystallization initially when the rock was formed, and then the time when metamorphosis occurred,

so when it was subject to enough heat and pressure to chemically modify it. And often when metamorphosis occurs, the closure temperature will be exceeded. And so that will typically reset the radiometric clock, although again, it's going to be dependent on the exact environment in which that occurred, the exact temperatures reached, and like the distance between contact. So metamorphic metamorphosis often occurs when there is metamorphic rock often forms at the interface between when magma comes into contact with existing rock that results in metamorphosis at the interface between those

and so the distance away from that contact can affect how much metamorphosis occurred and whether the closure temperature was reached. So you have to be a bit careful there. Igneous rock, which is rock that forms as a result of cooling down and crystallization of magma, is the best candidate for radiometric dating because it forms all at once in a short period of time, and it will have essentially the same composition, or whatever the composition that the magma had. So most forms of dating work on igneous rock.

However, you can still use that to date sedimentary rock as well. You just have to do it indirectly, and that's where we see the interface between geochronology and chronostrategography. So, for example, if we have igneous inclusion that using the principles of relative dating, we can date as having formed between these two rock sediments, then we know if we can date the igneous layer, we then know that, well, okay, this sedimentary layer is older than that, and this other sedimentary layer is more recent than that. And if we can date a bunch of different igneous rocks or inclusions that exist in a given site,

then that places boundaries on the dates of the sedimentary rocks. And then we may also be able to date fossils, which can then help us to further date sedimentary sites. And that's actually one of the benefits here, because radiometric dating works best with igneous rocks, but fossils are only found in sedimentary rocks. I don't know if there are ever fossils found in metamorphic rocks. I would expect that that process would destroy the fossil evidence, but maybe that's possible. But certainly you don't have rocks in, you don't have fossils in igneous rocks. So there's this nice synergy here that fossils

can be used to date sedimentary rocks as well as the principles of relative dating. Now, most of those aren't as useful or not useful at all for igneous rocks, but we can use radiometric dating for those. And so by combining the different approaches that are useful for different types of rocks, we can then get an overall picture of the dates of a particular geological site. And then, of course, we then compare different sites to each other over smaller regions and wider regions to give a complete picture, or we aim to have a complete picture of Earth's history over time. So before we talk about fossils in more detail, let me just talk about some of the major radiometric dating methods, like the specific methods, and when they're used. So the most well-known

dating method is radiocarbon dating. This measures the decay of carbon 14 in organic material, and is best applied to samples that are up to 50, maybe, I think, 100,000 years that they've been able to push, but only on the order of tens of thousands of years. Now, it's very important to understand that radiocarbon dating is not useful for geological samples. I mean, you can use it to date very young geological samples by dating any organic material in the sediment. But because it only goes back to, at most, 100,000 years, that doesn't cover hardly any of Earth's geological history. And so it's not useful for going back very far at all, geologically speaking.

And also, it's only useful for dating organic materials, not geological materials. So again, you could date a sediment if you could date the organic material in the sediment, but you'd have to establish that the organic material was trapped in there at the time, which often will be difficult to determine. So carbon 14 dating is mostly useful for dating of human remains that are relatively recent, so on the order of hundreds to thousands of years. It's not very useful or really useful at all for establishing a geological time span. But I'll mention it here because it is a form of radiometric dating,

and it is, I think, commonly confused with the other types of dating. Okay, so moving now to more relevant forms of dating here. So uranium lead dating is one of the most widely used dating techniques. It measures the ratio of two isotopes of lead, so lead 206 and lead 207, relative to the amount of uranium in the material. So uranium decays into lead, and so you can compare how much of the isotopes of lead there is to the amount of uranium, and that will tell you how long it has been since the rock formed when there will only be uranium present. Often this is applied to the trace mineral zircon in igneous rocks. I don't know exactly why that is probably due to its properties of formation.

This method can be used for any samples older than about 1 million years and back from that, so it's very commonly used for measuring, especially the age of very old igneous rocks. Probably the second most widely used dating method is potassium argon dating. This technique measures the ratio of argon-40 gas relative to the unstable potassium-40 isotope. This is particularly good because, as I mentioned, before, argon gas doesn't stick around when a rock is being formed. It escapes into the atmosphere. It is only trapped there after the rock has formed and it becomes trapped in the crystalline structure. So it's relatively easy to tell, unless the closure temperature has been exceeded and the crystal

structure has been disrupted, then there shouldn't have been any way for the argon gas to have either gotten there or to have left there except for what was produced since the rock formed, by the decay of potassium 40. So it can be used to date metamorphic and igneous rocks and is useful for materials over about 100,000 years old. So it has a very wide range of applicability. A third type of dating, which I mentioned earlier, is uranium thorium dating.

So it's often used to date corals, carbonates, and fossilized bones, and it can be applied to anything from a few years to up to about 700,000 years. So that's only useful for relatively recent specimens of less than like a million years old. There's two other methods of dating that I wanted to mention. These are not actually radiometric dating, but they are similar in the sense that they can provide absolute ages for samples,

and so I thought I'd just quickly mention them here. So there's one technique called cosmogenic dating. This relies on the production of rare isotopes from, not from radioactive decay, but from collision of cosmic rays with atoms in the Earth's atmosphere or on the surface of the Earth. These rare isotopes can then accumulate in rocks and sediment, or sometimes their products can accumulate, depending on the half-life of these isotopes, and they can be used for dating, effectively determining how long a sample was present on the surface

or something like that based on the presence of these rare isotopes. Because we know that they could have under gotten there through contact with cosmic rays, and that's relatively rare, so you can compute the rate at which that would happen, and therefore how long it was present in a certain location. Again, as usual, certain assumptions have to be made for those calculations to be made, but that can be useful in certain cases. And particularly, I think, when dating things like moon rocks, when there's not like weathering and other things happening.

And another class of techniques is called incremental dating. I've actually talked about this in the series that I did on the history of the Earth's climate. Incremental dating refers to periodic events which allow the construction of a succession of points in time that are datable relative to each other, usually at like set increments. So often these are annual, although they don't have to be. So tree rings are a good example of this. So tree rings are formed annually. I think they often form two rings every year, depending on the species.

Like there's winter and summer growth, and so you can distinguish between them. And another example are sedimentary layers, which in certain locations will be deposited like annually, due to flooding or something like that. Ice cores, where you'll have, again, the winter snowfall, which is then visually distinct year to year. And also, over a longer time span, magnetic field reversals in sedimentary rocks, and also that can be used to study. the magnetic field reversals in Oceanic Rock, which I've also discussed in a previous episode. Those occur more sporadically, but they still occur regularly enough that you can use them for establishing increments of time. So these methods are also useful for dating purposes, less so for dating much older samples, but they can still play a role. So to summarize at this point,

we've discussed some of the principles from chronostrategography of relative dating, mostly of sedimentary rocks, but also of other rocks relative to those sentiments. And then we've talked to talked about principles from geochronology, most particularly radiometric dating, which allows us to establish absolute year ages to rocks. And that is then used in combination with relative ordering that we have from chronostrategography to establish ages of even things that cannot directly be dated, as I discussed previously. So now we're going to finish out this episode by talking about fossils. And fossils are important because they are one of the major methods of dating of sediments. so we talked about biostrategography before.

And they're also the primary method we have for learning about the history of life on Earth. And that's what we're going to be talking about more in the next couple of episodes, the evolutionary history of life and the geological timescale and how life has developed over time and changed the planet and the planet has changed life. So we need to learn a little bit about fossils, what they are and how they are formed. The term fossil originates from the Latin term fossilis, which means found by digging. And so, as I said before, it's really just any type of preserved remains, impression, or trace of something that used to be alive.

So it's a very broad term. Fossil can vary in size from the microscale up to huge fossil remnants of whales, dinosaurs and trees, which can be like many meters long and weigh many tons. So there's no fixed size that a fossil has to be. It could be really any size. And also fossils can theoretically be any age from 4 billion plus. years old, when life was thought to were formed, up to, well, that is the tricky part, because there's no real cutoff point for how old something has to be to be a fossil. Sometimes a cutoff age of about 10,000 years is used, which places it before the Holocene, which is the current sort of geological period, although that's pretty arbitrary, and there's not really any sort of firm scientific basis for saying that something has to be a certain

amount of age, a certain age to be a fossil. Usually, though, when we talk about fossils, we do talk about things that are at least thousands of years old and often tens of thousands or even millions of years old. Humans have been aware of the existence of fossils since the beginning of recorded history. We have evidence from art, as well as written documents from various civilizations, ancient Greeks, Romans, Chinese, about their awareness of fossils. However, it was not well understood or really understood at all what they were. was one of the earliest to argue that they were the remains of ancient living creatures, which we now know is in fact, well, it's sort of definitionally what a fossil is.

There are sort of pseudo fossils which look like they might be the remains of ancient living creatures but are actually produced by chemical or geological processes, and sometimes it can be difficult to distinguish those, particularly when you're looking at microscopic fossils. Modern scientific study of fossils developed during the 18th century, and by the time of Darwin it was sufficiently developed to provide a basis for the theory of evolution by natural selection. But as I said, it was only in the last 60, 70 years or so that we've been able to combine

the study of the fossil record with radiometric dating to provide an absolute history in terms of years of life on Earth. Now, one important thing to understand about the fossil record, which just refers to the sort of total selection of fossils that have been scientifically studied and we can use to understand the past, the fossil record is highly incomplete and very biased. So by incomplete, we mean that the vast majority of organisms, that have lived in the history of Earth do not fossilize or do not leave any trace in the geological record. It's only a tiny, tiny fraction of all living organisms that leave fossil records.

And indeed, it seems to be the case that even if you're just looking at like macroscopic species, so forgetting microbes, the majority of species that have existed probably didn't even leave fossilized remains. Obviously, the further back you go, the less likely it is that there's remains that have survived everything that's happened since then, all of the geological processes, as well as being accessible to us. So there are certainly many fossils that likely exist kilometers underground, which it's very unlikely that we'll ever be able to uncover, at least unless we develop radically new technologies. So there's different aspects of the fossil record, whether the information still exists in some form and then whether it's accessible

to us. And we are highly limited in what we can access to be able to make inferences about the past. Now the other thing about in addition to incompleteness is the biased nature of the fossil record. And in some sense, this is more of a problem, because if the fossil record was incomplete but uniformly incomplete, then at least whenever we found information, we'd be able to say, okay, so then here's what this tells us about, you know, life in the past or at this time in the past. But the problem is that some types of organisms and organisms in certain conditions are much, much more likely to fossilize than others. So if we find a particular fossil or a fossil in a particular place or in particular time, we can't necessarily say anything about that time

and place because it might be a highly biased or an unrepresented sample. In particular, the fossil record favours organisms with hard body parts that mineralize more easily, such as arthropods, mollusks, vertebrates and echinoderms. So for example, if we go digging and find, the only fossils that we find are from organisms with hard body parts, we can't then say, oh, well, at this time in history, all of the animals that lived had hard body parts. That would be invalid because we're ignoring the biased nature of the fossil record. There's a selection effect that we're only able to see the organisms that had hard body parts because only they fossilize, or at least mostly they only fossilize. Fossilization of organisms that don't have any hard body parts is much, much rarer.

So we have to be very careful at making inferences on the basis of the fossil record because it's not just an issue of lack of information, but we also have very biased information. which means that it's very hard to know how to successfully generalize from the findings that we do have. And this still leads to a lot of controversies in the study of the history of life. Now, fossil sites that show exceptional preservation, sometimes including preservation of soft tissues, are called Largesteaden, which is German for storage places. And there's only a relative handful of these in the whole world because they're so rare. And these sites are often the result of particular processes that occur quite rapidly, because

fossilization is promoted by rapid burial, as we'll discuss, but often when there's rapid burial of many carcasses in an anoxic environment with fairly minimal bacteria. So perhaps that there's a catastrophic flood in a particular area, for example, which kills a bunch of organisms and buries them in a particular place. Ideally, in an anoxic environment, such as a peat bog, for example, somewhere where there's not a lot of oxygen, and that helps to preserve them from decay. And then what we'll have, you know, a million to years later is this site where we have this great preservation of many different species. So, unfortunately, that is very, very rare.

It does happen, and we're lucky to have those sites available for study. But that's why they have this special and slightly odd name is because they're so rare. And when they are discovered, it's a, you know, very important find for paleontologists. Now, let me talk about some of the processes of fossilization. Because fossilization is such a broad concept, it's just like any preserved trace of past life, there's not a distinct list of fossilization mechanisms. There are some broad processes which are typically relevant and often important, and so I'll just talk about these in general terms. I'm not going to get into the detailed chemistry, because it's often very specific to the exact makeup of the organism and the environment

in which it's fossilized. However, the first step of fossilization in all cases is death followed by burial. So the organism has to die, and then it must be rapidly buried, typically by sediment. So this could be mud, sand, or volcanic ash. That's actually very effective because it buries very rapidly. And this rapid burial is critical because normally what happens when an organism dies is that it is scavenged by other organisms, which extract nutrients from its tissues and break it apart and digest it and move it around.

So it's scavenged and whatever's left is decayed away by microorganisms, which are also effectively utilizing it for energy. And so there's very little that's left after even a few days to months, to years, right? So that's the normal process of what happens when an organism dies. In order for fossilization to occur, you need to protect the information about the organism structure, what it looked like how it was, you know, what it was made of what parts it had. you need to protect that for preservation in rock. And for that to occur, it needs to be buried rapidly, which protects it from scavengers. And depending on how much oxygen is present, it can also protect it from decay by microorganisms, like bacterial decay.

Now, usually the part of the organism that decays first are the soft parts, so the skin and the organs. Usually they decompose quite quickly. And what's left are the hard parts, bones, shells, or wood, which usually are what is preserved. It takes very, very special conditions indeed for the soft parts to be preserved or information about the soft parts to be preserved. I'll explain what I mean by that in a minute. But technically any remains that provide information about the organism count as a fossil. And so actual physical remains of the organism itself can count as a fossil. Usually, as I've said, we have a cut off of maybe 10,000 years or so so anything that's newer than that doesn't count as a fossil. So if you,

bury someone and dig them up 10 years later, you'll find their skeleton there because the soft parts decay away, the skeleton does not because it's mineralized. So you could in some sense call the skeleton a fossil. We normally wouldn't just because it's not old enough, but of course there are skeletons that are older than that, older than 10,000 years, and so you could call those fossils. In that case, though, what's present is really the original material itself, or at least part of the original material, usually the hard parts, like the bones, the mineral structure of the bones that are left. And so sometimes you just have those parts that are left. with sort of minimal changes. But that's usually only the case for fairly recent burials. I don't

know exactly how long that takes, but talking about thousands of years, typically, that even the hard parts can survive without significant modification. Again, depending on their environment. Now, much more often what happens, particularly with much older samples when we're talking millions or even hundreds of millions of years, is that there is substantial chemical change to even those hard parts, right, even the bones, shells, and wood. So what we have when we dig up, you know, the fossil from the ground is not the actual remains of the organism itself in terms of like the actual materials. Rather, it is traces of what the organism looks like and how it was structured or what it did in life, traces that are left in the geological record in some form. So let me explain

how that works. One of the processes of fossilization is called perimeneralization or petrofaction, it's also called. This is when groundwater seeps into the pores and cavities of the remains, and this groundwater contains minerals, as most groundwater does, and these minerals, such as silica or calcite or iron, precipitate out of the water and fill up spaces within whatever remains of the organism. As the precipitates form, they harden, and thereby forming a structure which can survive very long periods of time. Sometimes this process can occur in very small spaces such as within the cell wall of a plant cell and thereby producing very detailed fossils even at very small microscopic scales. More commonly, that level of detail is not preserved,

but you still see the gross outlines of, say, the bones of an organism. Now, this peri-mineralization, again, typically it occurs just with the harder part, so the bones and shells are preserved. eventually the minerals that formed those themselves will be either displaced or changed chemically as a result of new minerals being introduced in the structure. And so that's typically what happens with something like dinosaur bones, for example. What we actually dig up is not the original bones. It's basically rocks that have formed in the shape of the bones because the original minerals have been replaced. And rocks have formed, which preserve the outline and shape of the bones which were initially there.

But, as I said, sometimes perimeneralization can occur even at a very small scale, and it can preserve even microscopic details of soft tissue structure. It all just depends on the environment and how quickly things form and how much decay there was before mineralization occurred and things like that. Another very common type of fossilization is molds and casts. So this is when the organism dies, the original material decays away, but an impression is left in the surrounding rock. And so here, you can imagine, for example, the organism dies, it's buried,

there's a gap in the sediments where the organism is, right, which then over time is compressed and squashed, the organism decays away, but that material is displaced by minerals or other sediments that sort of leak in, like through the groundwater, for example. And so a cast is formed in the shape of the original organism. There's no original material left, but you can see the impression in the surrounding sediment, or which maybe later then is lithified, so it forms into a rock. you can see the impressions of what once was an organism, even though all the parts have decayed away, you get the impression in the mold that's formed.

Another process is called carbonization or compression fossils. So this is when they're over time pressure and heat, the organic elements are decayed away, they react, and all that's left is a thin film of carbon that outlines where the organism was. This is particularly common in plants and also some soft-bodied organisms. And this is effectively how coal is formed.

So basically ferns from the Carboniferous and Permian periods are buried in relatively anoxic environments so that they don't immediately decay away. Instead, they're crushed and under pressure and heat over millions of years. They're sort of ground down until all that's really left is a film of carbon. And with enough of these different plants being carbonized in the same way,

you get these structures that form, which we dig out of the ground. and coal. But essentially all that's left here is the original carbon from the organism is still there, but it's lost all of its structure. It's really just been converted into like charcoal or like coal. However, that can still provide information about the organism because it can trace out the outline or the shapes of the organism in the surrounding rock. So here, you don't have any necessarily three-dimensional information, but you can have like trace information about the outlined or shapes of what used to be there. Some special types of preservation involve an organism

becoming trapped in amber, tar or ice, and so that can preserve organisms for long periods of time with little to no decay, although there's limits to how long those typically survive. For example, there's no ice on Earth older than more than a couple of million years, I think. There's also something called trace fossils. So trace fossils are not, remains of the organism itself, but there are evidence of activity of that organism. So these are things like footprints or burrows. And so, you know, a dinosaur steps into the mud and that mud then dries and a different layer of sediment falls on top of it, which preserves the outline of the footprint and then over time that solidifies, lithifies, it becomes a rock and millions of years later we dig it up

and we say, oh look, a dinosaur footprint. So that's a trace fossil. So just to say, summarize, the underlying process of most fossilization is rapid burial and prevention, or at least delay of decay, long enough for structure to be preserved of that organism, usually through some sort of mineralization process followed by lithification. So minerals deposit, which then preserves the shapes, which then will become compressed, like between layers of sediment over time, which become rock. Sometimes this results in the formation of casts and molds, or sometimes after enough pressure and heat, it actually completely destroys the structure, but results in a thin film of carbon, which can still give sort of trace information. But in all of these cases, unless the

specimen is quite recent, the initial materials will not remain in the form that they were. They will be remaneralized or replaced by other structures, which just preserve information about the form of the organism. Most commonly, this happens with the hard parts, the bones, shells and wood. sometimes very rarely this can happen with soft parts as well but you need just the perfect conditions for that to occur. Just to conclude, I mentioned before index fossils, which are fossils or types of fossils, types of organisms, which are used to characteristically date particular times in Earth's history. And just at the end here, I wanted to go through and talk about some of the most important types of index fossils, not specific fossils, but like categories of index fossils, which are characteristic of different times in Earth's history.

just to give a bit of a flavor of how this sort of dating process works in biostrategraphy. So we're going to go through half a dozen or so here of the most important varieties. And we'll start off with the trilobites. These are marine arthropods, which are iconic of the Paleozoic. So they live from the Cambrian right through the Permian, which is the end of the Paleozoic. I'll go through these periods in the next episodes. I don't worry too much if you're not familiar with them. But they went extinct hundreds of millions of years ago, but were ubiquitous for a long time before then.

They evolved very rapidly and took many diverse forms and therefore form excellent markers in many of the periods in the Paleozoic, because they're very distinctive and a very common type of fossil. So they're very useful for dating, especially in the early Paleozoic. Now, another type of fossil, which is also useful in the Paleozoic and Into the Mesozoic are Ammonites. So these are extinct marine mollusks, and they have a characteristic sort of spiral-shaped shell, which, preserves very well. It's still rare that it's preserved, but relatively speaking, it preserves well. And once again, they form many different shapes and sizes, which are quite distinctive. And so we can identify particular periods of time based on what type of ammonite fossils are present. And they're especially useful for dating Mesozoic rocks. Graptoites are colonial filter-feeding

organisms, a bit like corals. And they're typically preserved as carbonized impressions in shale. So that's a bit like the compression fossils that we talked about. As colonial organisms, they have very characteristic shapes of different species at different times, and so they're very useful for dating, particularly Ordovician and Silurian periods. So conodonts are also useful for dating during the Paleozoic period. These are tiny tooth-like fossils that were originally possessed by primitive jawless vertebrate fish. And it's interesting when you look at the fossils, the fossils are actually just of the teeth.

Most of the organism hasn't fossilized, but the teeth are very distinctive in their shape, and so it's very useful, again, for dating purposes. So next we have phoraminifera. So these are actually, these are tiny protists that are found in the oceans, and they originated in the Cambrian, and they're still around, so they've been around for quite a long time.

What's interesting about them is that they're highly sensitive to very specific conditions of temperature and salinity and things like that. And they also produce elaborate shells made of calcium carbonate, but they're like microscopic shells. And so their fossils are very useful to determine the environment of ancient marine sediments because they're ubiquitous, and the shells are very distinctive, but also sensitive to the environment. So it's the perfect combination of properties that you want for a really useful index fossil.

Similarly, we have radiolarians. So these are also microscopic organisms beginning in the Cambrian. and still existing to the present, these have silesia skeletons, so made from silicon, and their skeletal remains, again, microscopic, but very distinctive. Skeletons essentially just cover the ocean floor in many places, and they form something called silesius ooze. Because of their rapid change in their shape over time, and the many different intricate skeletal structures that they have, they're once again a very useful index fossil. We then have the brachyopods, so these are also still,

stant. These are bivalved animals that have shells, a little bit like a clam, but they're not actually clams, their own thing. They have, once again, very distinctive shapes and a lot of diversity historically, and so that makes them really good markers for different times, especially in the Paleozoic period. And finally, in the Mesozoic, we have dinosaurs. So obviously, there are many different species of dinosaurs, which lived in different times and places, and so they can be very useful for dating rocks during the Mesozoic period. So one thing you might notice about these different classes of index fossils is that they all have hard either skeletons or shells of some form. And so that means that they preserve much more readily. So trilobites are arthropods,

ammonites of mollusks, the conodons and dinosaurs are both chordates. Brachio pods also have hard shells. And then there are the two microscopic organisms, the forminifera and radularians, which have hard shells or skeletons made respectively of carbon or carbonate or silicon. The only exception here are the Graptoilites, which are not hard-bodied, but form these carbon impressions in rocks. So almost all of these index fossils are hard-bodied, and that illustrates what I was saying before about how biased the fossil record is, that we rely largely on hard-bodied organisms, even though the vast majority of organisms that have existed are not hard-bodied

and have only or predominantly soft parts. Even in the case of the conodons, for example, which are toothed, jawless fish, we primarily know about them through their teeth, which was only a small part of the organism, right? But the fossils are useful because they differ in shape and characteristic. So they provide information about the time and place, even if they don't tell you that much about the organism as a whole. I mean, they tell you a lot of things, obviously, because the teeth are important. But the point is that it's only a small part of the whole organism. But that's what we know because that's what we can study.

So I'm just emphasizing here how although fossils are very useful and informative, and it's one of the best pieces of evidence that we have about dating, particularly sedimentary layers, we still have to be aware of their limitations and particularly the incompleteness and biased nature of the fossil record. So that concludes what I wanted to talk about today. We discussed principles of chronostrogrography, that's relative dating methods and way-up indicators.

We talked about chrono-geochronology, which is mostly radiometric dating, which allows us to provide absolute year ages to mostly in igneous rock samples. And then we talk about fossils, including defining what a fossil is, and talking about the different processes of fossilisation and a brief overview of some of the main index fossils. So what we're going to do then in the next episode is we're going to have a tour of the geological time scale and talk about the different main periods and errors in Earth's history and the distinctive events that happened during those.

So stay tuned for that. It will be a very exciting one, the one I've been looking forward to doing for quite some time. If you enjoyed this episode, you can assist the podcast by giving a favorable review on Spotify or the aggregator of your choice. Or word of mouth is also appreciated. I recommend the podcast to a friend. I've actually had quite a few listeners who have told me that they heard about it from a friend or colleague, so I appreciate that outreach as well. If you'd like to make a financial contribution to the show, you can become a Patreon supporter or make a one-off donation via PayPal. My address there is Fodz12 at gmail.com.

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So thanks very much for listening. I'll talk to you next time.