Ancient trilobites used tridents to joust

It's potentially the oldest example of animals engaging in a show of strength to attract a mate.....
20 January 2023

Interview with 

Richard Fortey, Natural History Museum

TRILO BEETLE.jpg

Rhinoceros beetles next to Walliserops

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Dating back over half a billion years, the trilobites were one of the first families of complex animals on Earth. With segmented, flattened bodies, some say they look like a big woodlouse. Some had long appendages projecting from various parts of their bodies, and one species, Walliserops trifurcatus, is notable for the three prongs resembling a trident sticking out at the front of its head. Why these animals had these appendages though, scientists didn’t know. Some thought they helped the trilobite forage for food; others speculated that the trident was an electromagnetic field detector. But now the discovery of a mutant fossil, with a fourth prong, has given weight to a different theory entirely: that these prongs are the first ever instance of animals fighting for a mate, as Will Tingle heard from the Natural History Museum’s Richard Fortey…

Richard - Same sort of thing that happens today with members of the deer family using their antlers for battle was happening already 400 million years ago with the trilobites. So it takes jousting sexual competition of that kind back literally several hundred millions of years older than anybody suggested it before. Some people have suggested that some dinosaurs, which I'm afraid evolved after the Trilobites had gone extinct, some of their head structures might have been involved in similar things, but nothing this old has been suggested before and it shows, if you like, that there's nothing new under the sun. The trilobites got there first.

Will - And your idea of jousting trilobites has been further helped by the discovery, for lack of a better term, a mutant trilobite fossil. It has a malformed trident with an extra tine sticking out the front. How did this specimen alongside the other evidence help further the jousting hypothesis?

Richard - It managed to go to full size. So if it had been the trident had been involved in some critical feeding activity which required perfect adaptation and so on, this animal might have been at a considerable disadvantage and you wouldn't have expected it to achieve full size. But, by analogy with say, those deer, a malformation of those antlers is actually very common. Of course, it doesn't affect their ability to grow to full size, but it does affect their ability to necessarily win in competition with their rivals. This very strange, anomalous trilobite seemed to me to support the idea of this particular function for these tridents in battle. Also, I might say that each individual tine on the trident has a little crest along it. They're flattened, so they're supported by a crest on either side, which is something you very often find in, for example, daggers which are used to have maximum strength. It increases their torsional strength tremendously. So you know everything about it: design of the trident, the anomalous trident and the comparison with the living organisms which suggested that our notion of these as sort of battling equipment was supported. So a mystery, at least if not solved, at least with a plausible explanation.

Will - With all these factors pointing towards the idea that these trilobites use their tridents to fight for a mate, what do you think that kind of activity would look like? How would a trilobite win?

Richard - The second part of our study was to look for a recent analog. Obviously trilobites have no living relatives, but there are arthropods around, lots of them. The nearest analog we could find was a family of beetles, called rhinocerous beetles. They too have an anterior, not perfect, trident like our trilobite, but sometimes bifurcates at the end, for example, which provides at least a model for what the trident might have done. These are used by the rhinocerous beetles for upending their opponent. It's a trial of strength really, in which no party becomes fatally wounded or anything like that, which is obviously ecologically a sensible thing to do. But the defeated party scults off and goes somewhere else. And my co-author and I measured the way these living anterior appendages worked and how they grew and with careful statistical analysis showed that the tridents on the Walliserops grew in exactly the same way. So we made a one-to-one comparison with a living animal that, when they fought, flipped over their opponent and that's what we thought Walliserops probably did when it was a confronting its opponent.

Will - If this is the earliest instance of sexual combat recorded, and there's been nothing since people even thought that it was the dinosaurs a couple of hundred million years later, why do you think then that there has been so little record of sexual combat since? Is it merely because the fossil record is so sparse?

Richard - No, I think probably it's because there are different types of combat that don't necessarily involve something very tangible and very obvious like a trident. The evolution of these things is presumably relatively sporadic. I mean, it's come up in the mammals, as we've said, and in one or two families of beetles, but probably no more. It's just one way of doing things which, as it were, nature invented and then reinvented, which of course is what convergent evolution, that's the technical term for it, is all about. There are many other prising examples in the fossil record of long extinct animals, as it were, anticipating things that are going on in animals that are still alive.

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