This transcript was generated automatically and lightly edited for readability.

Oliver Strimpel: This is Geology Bites with Oliver Strimpel. In several previous podcasts, we talked about dating rocks using the radiometric decay of isotopes trapped within crystals. But long before radiometric dating appeared on the scene, geologists and paleontologists had defined the geological time scale in terms of the sedimentary rock record. A key aspect of the stratigraphic time scale is the time spans defined by the presence of specific fossils in the record. The fossils of long-lived yet rapidly evolving species have played an especially important role in distinguishing the various periods within the stratigraphic time scale, and none more so than the trilobites.

Richard Fortey is former head of arthropod paleontology at the Natural History Museum in London, and was until recently Visiting Professor of Paleobiology at Oxford University. When he was 14, he developed a passion for trilobites, and he has devoted much of his research career to their study, and to the study of other ancient life, ever since. He's written an engaging book on the subject, simply called Trilobite! Richard Fortey, welcome to Geology Bites.

Richard Fortey: Hello. Nice to be here.

Oliver Strimpel: What is a trilobite?

Richard Fortey: A trilobite is an arthropod. It's an extinct arthropod. There were thousands of different kinds, so when you try and encapsulate them, you have to remember there's a huge variation. But what they share is, as the name implies, a body that was divided lengthways into three lobes, hence "tri-lobe-ite." They have an exoskeleton, that's the hard part, made of calcium carbonate, which very few arthropods have, and that means they fossilize extremely well. They had a pair of eyes at the front that also had calcite lenses, which is practically unique in the whole of the animal kingdom. And their bodies were composed of a head, which carried the eyes; a flexible thorax composed of a number of segments; and a tail, a pygidium, as it's called, at the far end. All of these operated together to allow the animal to enroll, which many of them did. So they could curl up tightly into a ball, which I think resembles the animals that, in the UK at least, we call woodlice.

So they're quite complex animals, and what they did was experiment in extravagant ways with the things you can do with your exoskeleton, with the hard parts. Some of them are as smooth as boiled eggs, others as spiny as porcupines. So they're a most extraordinary group of extinct marine arthropods. They never got onto land. They never ventured even into freshwater. And sadly, they're no longer with us.

Oliver Strimpel: But even though they weren't on land or in freshwater, presumably some of this enormous variety reflected the differing environments in which they prospered?

Richard Fortey: That's right. I spent quite a lot of my research life investigating that particular aspect: where trilobites lived. Were they related to things like water depth, oxygenation? Were they all on the sea floor, which is what people thought originally? I've shown, I think quite satisfactorily, that many of them learned to specialize, existing as part of the pelagic fauna. Which is a short way of saying that trilobites radiated into just about all the ecological niches the ocean has to offer. Some of them were predators. Some of them did boring stuff, like just eating mud. And I think some of them were also probably filter feeders. So they ran the whole gamut of what crustaceans do in the modern ocean, and that's probably why there are so many species of them.

Oliver Strimpel: And when did they emerge in the geological record?

Richard Fortey: They appear very near the base of the Cambrian. That's part of what has been dubbed the Cambrian evolutionary explosion. They appear all over the world, so they're known from China, North America and the UK in Lower Cambrian strata. But when they first appear, they're already quite differentiated, which common sense tells you means they must have had an earlier history, perhaps one without an exoskeleton, I don't know, to allow them to differentiate so widely around the world. So that's when they appear, which is, well, the date always changes slightly; I think it's 542 million years ago, something like that.

Oliver Strimpel: So right at the base of the Cambrian, perhaps just a little bit after the first signs of the explosion. And they lasted an extremely long time, didn't they?

Richard Fortey: Yes. From that early start, they go out at the Permian mass extinction, which I think is dated at about 250 million years ago. So they have a very long history, lasting nearly 300 million years. I don't like the terminology about dominant animals, but at one time people would have said trilobites were the dominant form of life in the Cambrian. We now know that isn't the case, because we've discovered soft-bodied animals in profusion, preserved in very special localities. They're not normally preserved as fossils, and that shows there were lots of other arthropods living alongside the trilobites. But nonetheless, they were extremely successful animals that invaded virtually all the ecological niches.

Oliver Strimpel: I love that passage in your book where you speculated that if you were a sailor in Ordovician times and you just stuck a scoop down into the mud in a tidal area, or wherever under the ocean, it probably would come up crawling with trilobites.

Richard Fortey: Yes, probably, really in all sorts of habitats. People think fossils are rare. Of course, if you go to the right place, they're not rare at all, and I have been to places where trilobites litter the ground. But of course, like most other arthropods, they molted as they grew. So a lot of the material you have to work with is fragments, the molted fragments of the trilobite. They split into perhaps a dozen pieces, and it's very hard to do the jigsaw if it's a new kind of trilobite. But that means that their fragmentary remains get distributed more widely. Rock beds full of nice, complete, whole trilobites are actually rather rare, which means that the really beautiful ones, and some of them, in my view, and I don't think I'm biased, are extraordinarily beautiful fossils, now have a monetary value attached to them, thousands of dollars in some cases. That's to be welcomed on the one hand, because it means people are going to look after them. But on the other hand, it means poor old public museums, like the one I worked in for so long, find it difficult to keep up with the market. So it won't surprise you to hear that some of the best collections of trilobites are in the hands of people with rather a lot of money who work in the oil industry. Some of them are star items at the fossil fairs, which have become quite big business.

Oliver Strimpel: So they first appeared in the Cambrian and lasted till the end of the Permian, about 300 million years later. Did they flourish for all that time, or did they have an apex, if you like, and then start petering out over a long time?

Richard Fortey: Well, you can argue about when the apex was. They are much less numerous at the end than they were, say, halfway through the Cambrian or halfway through the Ordovician. They increased very rapidly through the Cambrian in terms of their diversity of shape. It's a harder one to argue, but they were probably doing more interesting things by the Ordovician. In terms of bizarreness, you've got to go to the Devonian period, where there were trilobites like no others that lived before. Some of them were covered in spines, others bore tridents on the front of their heads; any imaginable shape and size evolved. So you might say that was the peak of the trilobites.

What's perfectly clear is that after a mass extinction near the end of the Devonian, there are fewer trilobites. In what we call the Carboniferous period, the Mississippian broadly, the trilobites were not so varied as they had been; several families had disappeared by then. And then the ones in the Permian, well, they're relatively rare. But they hadn't given up even then. One of the last bits of research I did was on the discovery of new Permian trilobites in the Middle East that seemed to be really going great guns. They were affected greatly by mass extinctions. There was one at the end of the Ordovician, very well known, one of the big five, or six if you include ours, and that set a lot of trilobites back rather severely. But they popped back again, though into not quite so many ecological niches, in the Silurian and onwards. So if you said it was a life-or-death question, when did the trilobites reach their acme, I'd probably say the Ordovician.

Oliver Strimpel: So what makes them so incredibly useful as chronometers, as a means of dating the sedimentary rocks in which their fossils appear?

Richard Fortey: Well, several things. First is that they've got quite complex morphology. Now, there are other fossils around at the time, clams, brachiopods and so on, and they are still useful, but their features are not so varied as those of the trilobites. So if you find a species of trilobite which has some very distinctive specific character, it might be that it's covered with coarse tubercles or something, you can be pretty sure that you're dealing with the same species wherever you find it. And that, of course, is very useful for correlating rocks over long distances. Then some trilobites, as I briefly mentioned, probably took to living in the open ocean. They were swimmers, or part of the plankton; my own view is that I incline towards swimming. That meant they really got around, from one continent to another, which meant they could be used internationally as well. Then if you add into that the fact that they evolved fast, so that one individual species probably didn't last more than a couple of million years, that means you can divide the time scale quite finely on the basis of trilobites.

And it's surprising how quickly, following the work using index fossils, as they're called, that William Smith pioneered in the British strata, everybody twigged to the idea that they could use trilobites to solve problems about geology, and in particular to characterize the Cambrian to Silurian periods, with their own particular faunas. So historically, they played a very important part in constructing the geological time scale, and time scales that are written really in the language of extinct species. Then all you need to do is collect lots of them.

Oliver Strimpel: So does that mean we can effectively get a temporal resolution of, well, what is it, about a million or two years?

Richard Fortey: Yes, that would be about the finest you could do, although claims have been made for still finer-scale subdivisions. The Cambrian is really quite finely divided in many places. What you need to set up the time scale, really, is a series of strata that ideally hasn't suffered too much tectonic disturbance, so they're still in order. And there are many places, particularly the western US and parts of China, where there are superb undisturbed sequences, where you can read the geological textbook, if you like, stratum by stratum, all the way through the Paleozoic. Historically, though, a lot of the early work was done here in Britain, where the rocks are much more disturbed. In the early literature, there were big arguments raging about the age of this bed or the age of that bed: is it older or younger than that one? Because many of the beds have been folded together and separated from their nearest neighbors. So the early history of trilobite studies was quite complicated, just from the point of view of where people were working. Once the brilliant sections in America opened up, for example, it helped clarify things considerably.

Oliver Strimpel: So you said this is a global time scale because the trilobites spread through the oceans. But once the continents separated, can we in fact track the drifting of plates around the planet by the different kinds of trilobites that evolved independently in different parts of the planet? Or is it truly global? Does it always define a global time scale?

Richard Fortey: That's a very personal question. A lot of the research I did in my earlier career was trying to establish principles on which you could identify ancient continents, which may have had a completely different configuration from the continents of today. How can you use trilobites to help recognize an ancient continent? Basically, the principle would be that shallow-water trilobites, which included rather a lot of them, were adapted fairly obviously to the ambient conditions, the temperature and so on, pertaining where they lived. So, for example, if you had a continent around a pole, you would get cool-water trilobites adapted to those particular conditions, whereas if at the same time there was another continent on the equator, you wouldn't expect any of the shallow-water trilobites to be common to the two. In other words, you have a handle on where this continent was at that time. And I fought a big battle, I remember, back in the '80s, about the position of Baltica, the whole Baltic continent, relative to Europe back in the Ordovician.

So, if you like, the shallow-water trilobites could be regarded as a bit like postage stamps issued by that continent. But as you go off the edges, you get into deeper water, into different biofacies, deeper-water sites where different trilobites lived, and many of those are more widespread, so they could be the ones that give you more international correlation. On the other hand, they're not going to be so sensitive to which continent they were on. So that's where arguments have broken out. But generally speaking, at least at the paleocontinental level, the trilobites have been pretty good predictors of where the ancient continents were and where they lay in relation one to another. And of course, that feeds into all kinds of things, like which minerals you can look for, and so on. So to that extent, knowing about trilobites is not just contributing to the time scale but might contribute to really practical problems to do with mining and resources.

Oliver Strimpel: As you said, trilobites have been used to define time over an enormous span of geological time, about 300 million years. But I'm sure they didn't simply appear in a single massive, continuous pile of strata that revealed one trilobite after the other in perfect succession. So how hard was it to piece together the different forms to create the trilobite time scale?

Richard Fortey: The first thing to say there, probably, is that the trilobites have been most critically useful, for the longest time, in defining the Cambrian. By the time we arrive at the Ordovician and Silurian, they've been joined by other extremely useful fossils, graptolites. Graptolites are even better for international distribution; they can be found all over the world. And then they are joined by microfossils, like acritarchs and so on. So the scale gets richer as you go up. But traditionally, at least, it's been in the Cambrian that the trilobites are the ones on which most correlations have been based, on the different sorts of trilobites in successions of strata.

And initially it was done piecemeal. It was done first, as I mentioned, in the type Cambrian, which is, of course, as the name implies, in Wales, where the Lower Cambrian and the Middle Cambrian and the Upper Cambrian are not easy to unscramble, and mistakes were made. But once the Scandinavians, who have a beautifully undisturbed rock succession, started putting their sequence in order, I think an idea of the succession of trilobite faunas through the Cambrian began to harden up. And in America, there was particularly the great American paleontologist Charles Doolittle Walcott, Doolittle being the most inappropriate middle name ever given to anyone who's done so much. Walcott wrote huge monographs, published mostly by the Smithsonian, about North American trilobites, which really cemented their position as timekeepers for the Cambrian.

So it was not without battles and arguments, which probably still carry on at a relatively fine scale, but it was put together over, let us say, 50 years or so, to give us the basis of the scheme we have now. And of course, it was obvious to the people at the time that these successions of faunas required quite a lot of time to account for them, one above the other. But the time scale couldn't be solidified, as it were, or related to absolute time scales, until radiometric dating of one kind or another came into play in the 20th century. And since then, it's been a matter of progressive refinement.

Oliver Strimpel: As I mentioned in the introduction, the stratigraphic time scale was pieced together long before radiometric dating became really practical and we started to know what the absolute ages were in millions of years. But when we did actually get a lot of radiometric dates, how well did the stratigraphically determined ages match up with the radiometrically determined ages? Did we actually get it right, in fact, based on the trilobites?

Richard Fortey: Well, you must remember that when radiometric dates first came out, the margins of error were initially sort of plus or minus 13 million years. There were big margins of error. And as time has gone on, those margins of error have shrunk considerably. The trilobite scale is now less accurate, if you believe modern error bars on things like zircon dates, which could be down to perhaps 100,000 years or so. I don't think we'd ever claim that sort of accuracy. On the other hand, I don't know of any example where the radiometric dates have come along and said, "Hey, you trilobite boys have got it all wrong; this fauna comes before or after that one." And so in general the stratigraphical succession has proved really quite robust. If you're a paleontologist and you go through a rock section and you find a little tiny trilobite called Glyptagnostus reticulatus, it certainly is terrific to be able to just whack the rocks and say, "My goodness, it's Glyptagnostus reticulatus; we're at this particular point in the Cambrian." And if the absolute time scale is compared all around the world, these sorts of time instants, as it were, seem to be pretty well supported.

Oliver Strimpel: Yeah. So as far back as the base of the Cambrian, or the middle Cambrian, where we're talking about 500 million years ago, you can get a resolution of a million years or less with a fast-evolving trilobite. So as a percentage, that really is getting up there, to the kind of 1% accuracy that you can get from uranium-lead dating of zircons.

Richard Fortey: Yeah, a couple of million years would be a reasonable claim. It's fairly accurate, but of course we can't claim to be accurate to within, say, 100,000 years. And there we have some other kinds of marker, like single ash beds that have recorded single eruptive events, which will have a zircon signature but will also be in a rock section that has trilobites in it. And the trilobites will be in the right place at the right time, if that's putting it the right way. So these things have tended to be complementary. It's a weakness of scientists that they regard the invention of new technology as pushing everything into a new place, which means that sometimes the old expertise is perhaps not given the importance it deserves.

You know, sometimes you write a paper that you think is never going to get cited at all. I once described some trilobites from Chile, from a fold belt in the middle of the Andes. They were very big trilobites in very nasty rocks, which had suffered metamorphism. They'd really been through the mill, and all the usual tools for absolute dating weren't present. But I could recognize and date these trilobites, not precisely, because they weren't well enough preserved, but within the Devonian anyway. And that actually served to date a whole swathe of rocks running for miles and miles and miles up through the Chilean mountains. Unsurprisingly, it was published in the journal of the Geological Society of Chile, not a mainstream journal, but it's been cited a number of times, of course, because it's the only evidence for the age of these rocks. So we still have our usefulness.

Oliver Strimpel: You've had a passion for trilobites ever since you were 14. What initially attracted you so much to trilobites?

Richard Fortey: Well, I was one of those born naturalists. In fact, my book A Curious Boy is all about how I eventually became a paleontologist, propelled by an obsession with all sorts of aspects of natural history. But it was a very special moment when we went on holiday to West Wales, to one of the places where the time scale we were talking about was sorted out back in the 19th century. On the geological map I found posted on the wall of our rather cheap hotel, there was this little sign that said "trilobites here," and my eyes lit up. I spent the rest of the holiday breaking rocks and finding trilobites. There's something tremendously exciting about seeing an animal that was so old. You know, I thought, all these hundreds of millions of years, how could it survive? And look, it's got eyes. If I was looking at it, it was probably looking back at me, for the first time in hundreds of millions of years. So yes, that's a visceral sort of feeling. And it never crossed my mind at that time that there would actually be a job looking at trilobites, understanding trilobites, let alone that it would be me who would be doing it.

So yes, it set in early. When I was at school, I used to take buses all the way into the countryside, all the way through to Wales, and walk with a backpack to localities I'd found out about, to see if I could collect more trilobites. And it wasn't till I got to Cambridge University, where the great Professor Whittington, the then authority on trilobites, had just been appointed Woodwardian Professor, that I got a chance to think that maybe I could actually make trilobites my living.

Oliver Strimpel: And what are you working on at the moment?

Richard Fortey: Recently I've become interested in some extraordinary Devonian trilobites that have been discovered in Morocco that have tridents growing out of the front of their heads. Now, there's nothing really like this in living animals, so it throws up a particular challenge: what the hell were they doing with this strange trident? Well, for some years now, since these trilobites were discovered, and that's only a decade ago, I've finished up lectures on trilobites by showing pictures and inviting the audience to say what they think the tridents were for. They say, well, could they spear fish with it? No. Maybe they had an electric current and stunned prey with it? Possible. Maybe they stirred the sediment up with it? Hmm, maybe. Et cetera. So it made a very amusing way of finishing off the talk.

But just recently, well, in 2018 in fact, I was at the Houston museum in Texas. A local collector, a blind collector who has a huge and wonderful trilobite collection, had on display a specimen of one of these trident bearers with not just three prongs, but a fourth one sticking up at an angle. So that was my eureka moment, because I realized that if this had been vital for any sort of feeding function, if it was a very adaptive structure, this trilobite shouldn't have grown to adult size. And the immediate comparison in my mind was with a rather common phenomenon, actually: if you think of deer, in various species deformed antlers are quite common, and the deer grow to adulthood. The antlers only become important, as it were, or maybe important in failure, when it comes to winning a mate or seeing off the next powerful male by a fencing process. And I suddenly twigged that this is a very plausible explanation for the extra tine at a funny angle on this trilobite: that the tridents were used for combat, probably for flipping over another trilobite, or threatening to do so. And so here we have the earliest fencing, or sexual competition, 400 million years ago. This is hot research, if you like, which is still in progress. So the great thing about working on something like trilobites is that it never comes to an end.

Oliver Strimpel: Richard Fortey, thank you very much.

Richard Fortey: It's been a pleasure.

Oliver Strimpel: For more about Geology Bites, as well as pictures and illustrations that support this podcast, you can go to geologybites.com.