Years ago I met Richard Fortey at the inaugural meeting of Spain’s new evolution society, and found him an affable and lovely guy. He’s a paleontologist and writer, and I had the pleasure of reading and giving a positive review to his first book, Life: A Natural History of the first Four Billion Years on Earth, which is well worth reading (he’s written several other books, including Trilobite: Eyewitness to Evolution (also a good read).
And it’s four trilobite species that are the subject of Fortey’s new paper coauthored with Alan D. Gishlick, a geophysical sciences professor at Bloomsburg University, in PNAS, a paper you can read for free by clicking the title below (it’s free with the legal Unpaywall app., the pdf is here, the reference is at bottom, and judicious inquiry might yield a pdf if you can’t see the paper). Trilobites are common fossils, and were marine arthropods that went extinct without leaving descendants.
The upshot is that Gishlick and Fortey analyzed fossils of one species of trilobite found in Morocco, deriving from the Devonian (400 million years ago). This species, Walliserops trifurcatus, had a long trident attached to the front of their bodies, and tried to figure out what it was for. They also found one adult individual whose trident was a bit deformed (see below). Their conclusion is that these were weapons used by males to fight with other males, almost surely to compete for females. They are, posit the authors, the arthropod equivalent of reindeer horns. The other possible functions (feeding, digging, etc.) were largely ruled out.
Read on:
Here are four species of Walliserops, shown below. All specimens bear a rigid cephalic trident. W. trifurcatus has a slightly recurved trident that bends upwards, while the other species have tridents more flush with the surface of the sediment (all captions come from the paper):
The obvious question is: what is this damn thing for? And there are several hypotheses, all assuming that the structure was molded by natural selection (which includes sexual selection). The authors find evidence against all but one possible function. Here are the alternatives (of course, it could have been used for several things, but it’s likely that selection was wholly or largely on one function). Indented bits are quotes from the paper. The rest of the discussion concerns W. trifurcatus:
A.) Defense. Perhaps the structure could have been used to ward off predators, like the spines found on other trilobites. Here’s how the authors rule this out:
However, such a function would have been difficult given the overall anatomy of the trident and the trilobite. The trident is rigidly attached and cannot be moved independently from the cephalon; it could only be flexed in a dorsal-ventral plane by the trilobite raising and lowering its cephalon. This would create further difficulties since the long genal spines limit how high the head could be angled without lifting the entire body. The trident, therefore, could not be employed in a versatile way, nor be presented as to defend from a predator attacking from above or behind. This morphology is not consistent with a defensive structure.
B.) A feeding structure. Doesn’t seem likely:
A second possible function for the trident would be as an aid to feeding. Like all members of the Phacopida, Walliserops was probably a scavenger/predator, and it might be considered as a possibility that the trident was a comparatively sophisticated sensory device concerned with early detection of prey species—such as buried annelid worms—which could then be grasped by the endopods of the ventral limbs.
C.) Sensory detection of the environment. This is also deemed unlikely from inspection of the structure:
However, examination of the trident in optical and scanning electron microscopy failed to find the arrays of cuticular pits or tubercles usually indicative of the presence of sensilla in fossil arthropods. Most groups of trilobites include species with exterior exoskeletal pitting that is preserved even if the intracuticular canals have been removed by calcite reorganization—and there is no evidence of such exterior pitting on the trident of Walliserops. The absence of evidence for specialized organs on the tines makes it unlikely that it was primarily a sensory apparatus.
D.) A spear to pierce prey: Unlikely because the structure was inflexible, so the animal would have no way of accessing speared prey.
E.) An apparatus to dig, perhaps for prey. The way it’s shaped and angled seems to preclude this (remember, it’s slightly recurved upward; see below):
Another possibility is that the trident may have been used to agitate sediment to disturb prey items, which could then be trapped by the limbs. It is difficult to conceive of W. trifurcatus digging into sediment because to engage sufficiently with the substrate the cephalon would have to tilt at an angle greater than would be allowed by movement on the posterior occipital margin. Equally, if the thorax was arched, the pygidial spines themselves would dig into the sediment.
F.) A combat device on males molded by sexual selection mediated by male-male competition for mates. The authors consider this most likely, especially because the tridents resemble the structure of male dynastine (rhinoceros) beetles, which use them to fight for females.
Here’s a picture of three of those beetles which have similar projections as do the Walliserops trilobites (the one at the extreme right).
The authors did a complex morphometric analysis of body and horn shape of W. trifurcatus, comparing it with living rhinoceros beetles to see if the trident could have been used for shoveling/prying, grasping, or fencing—the three types of male-male combat seen in living beetles. The analysis puts the trilobite in the group of living rhinoceros beetles whose males fight by fencing/shoveling: jousting with the structure in front and then trying to shovel the opponent over onto its back. I won’t go into the gory statistical details, which involve principal-components analysis, but the recurved structure of the trilobite’s “trident” is similar to that of shoveling, prying, and fencing beetles (left column: observed means of fighting of living beetles; center: the cephalic structures used; right: the species name [trilobite at the bottom]).
As you see, and as the statistical groupings show, W. trifurcatus is similar to the structures used in rhinoceros beetles for fencing, prying and shoveling. Here is Gishlick and Fortey’s scenario of how the males battled it out in the competition to pass on their genes:
We would hypothesize a fighting scenario in Walliserops similar to that of Trypoxylus. The trilobites would meet and at first spar with their forks, pushing and poking. At some point, they would shift to trying to slide the fork under the other, in an attempt to flip them over. Given the morphology of Walliserops, flipping would be a very effective combat technique. Although the appendages of Walliserops are unknown, it is likely that they were like those of other phacopids in not extending beyond the carapace. This is seen in the Devonian Chotecops, asteropygines Asteropyge, and Rhenops, and recently described in three-dimensional material from the Silurian Dalmanites. Once the trilobite was inverted, righting would not be a simple matter, especially if the dorsally directed spines had snagged in the sediment. An upended trilobite would probably be even more helpless than a beetle in this position and thus excluded from sexual competition.
It might also be dead!
Now the first thing that struck me when I saw this paper was the question that would have occurred to many of you: WHERE ARE THE BLOODY FEMALES?? One of the signs of male-male competition is that the structures used to compete are present in males but almost never in females, as they’re of no use in that sex—and detrimental to fitness if you don’t use them. Male deer have antlers, females do not. Body size, used for combat in elephant seals, is huge in the males, and much, much smaller in females. So if these trilobite horns really were tools used for the “combat” form of sexual selection (the other form, as pointed out by Darwin, is female preference), the females should be around but lack the ornaments. Where are they?
Gislick and Fortey suggest that the females were indeed around, but because they lack the tridents they have not been identified as females of Walliserops trifurcata:
Since the diagnostic synapomorphy [JAC: shared derived trait] for Walliserops is the anterior trident, it would be likely that the female of the species has been classified in a different genus. That leaves two possibilities: either the females of the relevant species are at present unknown, or they are known but placed in another trilobite genus within Asteropyginae.
That mandates a search for trilobites that resemble the males but lack the horns. The authors raise another possibility: the females weren’t preserved or were offstage, living elsewhere, but this seems less likely:
If we extend the beetle analogy further, it is possible that the females are not preserved if some trilobites, like many dynastines, engaged in sex-specific aggregations; in this case, the females were not always present in the same locations as the males, although it is difficult to explain why the latter were selectively caught up in obrution events. [JAC: “Obrution” is rapid burial in the sediments, the way these creatures must have died and been preserved.]
I favor the “females not yet found” hypothesis. There’s one more hypothesis, which is mine: both males and females have tridents. I don’t know why this would be the case, although you could think that it’s used to take other individuals out of action in conspecific competition for food. But that makes little sense.
Finally, the authors found one example of W. trifurcatus with a deformed trident, having an extra spike (a “quadent”?). Here it is on the right. Note that the branching pattern can be asymmetrical in the normal three-pronged structure).
Because the individual on the right was an adult, Gishlick and Fortey suggest that the deformed structure did not prevent the bearer from growing up and thriving, and thus was unlikely to be used for some vital function like feeding. This adds a little more weight to the sexual-selection hypothesis.
The Upshot: The authors’ analyses and explanations seem plausible to me, though they’d be even stronger if they could find the females. That might be tough: in living species you could find them by looking at mating pairs or even seeing that the DNA was nearly identical, but this isn’t possible with fossilized trilobites, especially because in some living and sexually dimorphic species the females look very different from males. If the authors are right, and I think they are, then this quote from the paper is correct:
Walliserops provides the earliest example in the fossil record of combat behavior, very likely ritualized in competition for mates. Although fossil life habits are difficult to prove, the consilience of morphology, teratology, and biometric data all point to the same interpretation, making it one of the more robust examples of paleoecological speculation.
h/t: Matthew
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Gishlick, A. D. and R. A. Fortey. 2023. Trilobite tridents demonstrate sexual combat 400 Mya. Proc. Nat. Acad. Sci. USA 120 (4) e2119970120 (in press).
Great, thanks for the alert and explanations. I love Fortey’s books and have now read almost all of them (those I did not yet read are on the shelf). I can recommend them all. Good to see also that he’s still active in retirement.
I was hoping you’d feature this finding! Many thanks. I also really enjoyed Dr. Fortney’s Trilobite book.
Thank you, Dr. Coyne, for summarizing this paper. I found the discussion on different possible explanations for the tridents to be very interesting. I always enjoy seeing HOW scientists arrive at their conclusions. Anyone can make a claim, but I like seeing people “show their work.” Also, I have always thought (ever since I was a boy) that trilobites were “cool” and the quintessential Paleozoic animal. I have one from Utah which is 507 million years old. Just imagining that span of time fires the imagination! Please keep up the science posts, Dr. Coyne!
Trilobites are cool! I have one sitting here on my desk that I got years ago from Ward’s Natural Science Establishment. Not surprising, it doesn’t have a horn. I imagine they get lost a lot.
… in which case you’d expect to see the torn/ broken stem of the trident (or other cephalon ornament) in the preserved carapace.
While these were clearly arthropods, unlike the insects they also frequently mineralised their carapaces (with calcite, it’s polymorph aragonite, and sometimes calcium phosphate), which both improves their preservation potential and also constrains where they are likely to fracture in the ecdysis process (splitting the carapace to shed it as part of the growth cycle). Those fracture (“suture”) lines are a standard part of trilobite description, because they’re fairly well conserved between species and genera and are a very useful set of characters for identification. The suture lines form a network of weak spots in the carapace (it’s their job, at least in the moulting parts of the live cycle) and partial trilobite fossils are frequently found to have broken along the suture lines. (Which rather begs the question of whether this particular fossil is a body fossil, or a part of a shed carapace and therefore a trace fossil, like a footprint or a lizard’s cast tail.)
To be able to extract the living core from a “trident”, as part of moulting, I think the suture lines would have to be along the sides of each leg of the trident, forming a pair of three-legged discards … and I’m wondering where the protuberances rooted into the cephalon, or if they melded seamlessly into it?
Assuming that they moulted (and weren’t completely discarded and re-grown with every moult – I’m not sure if that’s possible), where could they fit into the standard network of suture lines on trilobites. I think it would have to be on the facial suture, but my trilobite “naming of parts” is that far in the past that I need to find my textbooks – which I haven’t seen for years.
That in turn raises the question of whether independent “tridents” are found in the fossil record as partial fossils. An awful lot of trilobite fossils are missing the front-lateral edges of the cephalon where the body split along the “facial suture” separating the “fixed cheek” from the “free cheek”, and the “free cheek” part of the fossil (or moulted carapace) is lost. Similarly, if there were a suture near the root of a trident, you’d expect the “tridents” to be found and in the records as a distinct (possibly “problematic”) structure.
I’m not a trilobite specialist, but I’ve never heard of such a thing.
The trident-bearing genera have been known for a long time, and are so spectacular that Edinburgh’s “Professor Trilobite” (Fortey’s “friend in the North”, Euan Clarkson) would certainly have mentioned them in his undergraduate text book – which we also used. If Prof Clarkson could go on for many pages about the optical mineralogy of trilobite eyes, he’d have found room for trilobite tridents somewhere between the eyes (oh, the eyes!) the non-fossilizable “copulation by guided missile” of modern Nautiloid molluscs.
If (if) these are one half of a sexually dimorphic species it wouldn’t be surprising to find a size difference between male and female. But environmental indicators (like the spines extending from the body margin, indicative of living on a soft substrate) between the two are likely to be common. You’d also expect to find juveniles in which the trident is starting to grow and be able to establish a growth series from infants and juveniles to the two adult forms.
A growth series for these would be nice to see, but a lot of collecting would be required. I think the Moroccan deposits of “trident trilobites” were recognised in the 1950s, and after a short period of “are these fakes?” questioning, they were formally recognised and went into the text books. But systematic collecting from the Moroccan sites is … uncommon. The area’s fossil sellers are commercially sensitive about localities. I think these genera have been reported from outside the classic “Morocco” area, but not very often. That could reflect a restricted original distribution. Or just restricted preservation.
– If I recall my plate tectonic reconstructions, the US Georgia – Alabama Atlantic coast (or is GA bordered to the N by South Carolina?) would be the place to look. There or in the middle of the Sahara, whichever is easier to get to.
– As far from the structural mangling of the Appalachians as possible.
– In fairly soft mudrocks (all those spines : soft substrate!) of the lower Palaeozoic (Cambrian, Ordovician, SIlurian) – which probably means low ground.
So there’s a search region. What the local laws about fossil collecting are, I don’t know. But to be useful you’d need precise locations for any finds – both on the ground, and “so far above the Wombat Limestone” or “so far below Some Other horizon”. How good your search area’s exposure is … well that’s one of the things you take into account when defining search locations.
The trident lacks any evidence of a suture (even checked under SEM), so it must have been drawn out during molting. Which is feasible given the internal shape, albeit a bit tricky. Worth more research on growth and development.
There appear to be numerous occurrences of individuals with broken tines, some showing regrowth. The problem is that imperfect specimens are either discarded or reconstructed as the collecting is commercial rather than systematic. There are also smaller individuals reported but again discarded by the diggers. Research on the growth aspect is necessary.
The earliest reports of these that I am aware of are from the late 1990s the first example I saw was in 1998 or 1999 at a show. No other occurrences outside of Morocco are reported. I am also unaware of any rumors in the private trade or amongst the private trilobite fanciers.
From TFP (see my message to Jerry away down the thread for a Researchgate link to the PDF.) :
This is on a specimen that was sectioned in the lab, not naturally broken.
Quite what the comment on “secretion … during growth” means, I’m not sure. Maybe this part of the exoskeleton can change shape and size between moults? Or this is part of the “inflating” response of ecdysozoans between shedding one exoskeleton and the next one hardening.
FAO Roz @ 9 below :
These lived on the seabed (see the laterally projecting spines, increasing the area of mud the weight is spread over) So the backward-pointing dorsal (on thorax and cephalon) spines keep things up in the water column from picking the beast up. The genal spines would still make it a spiky mouthful if it was forced into the water column and folded over.
Growth series
Clearly the authors have been asking about this too :
One specimen does not a growth series make. But it’s what a growth series … well, grows from.
The paper is very much focussed on one aberrant specimen with a 4th time.
But that point about there being three simultaneous “trident-o-bites” in the environment is valuable. In the comparison with the rhinoceros beetles, I wonder how many species of rhinoceros beetles are there in the environment at the time.
Knowing how our friends in the drooling community (Creationists) “think”, it’s probably worth quoting from the Materials section – these specimens aren’t taken entirely as presented :
Specifically the specimen was recorded under UV light, which shows up fillers and adhesives used in preparation of these specimens.
Fascinating!
Sub
I have nothing to contribute except: thank you for another interesting and informative science post!
My comment too, exactly.
I miss trilobites [Shakes fist toward the Siberian traps: Damn you, end-Permian mass extinction!!]
The females might look very different from the males. It may also be possible that females are vermiform slug-like things that don’t fossilize well, committing their anatomy to being sedentary egg-laying machines that are guarded by males.
So … the males would need to somehow take food to the females – and remove solid waste. Yeeees. Tell me more about the fossilized trilobite shopping bags – I can’t remember that chapter in my text books.
The closest parallel I can think of at the moment is female angler-fish and their male “hangers on”.
Do angler fish have a fossil record? At all?
Incidentally, my trilobite fossil is in a different genus with members that could curl into a ball like roly-poly pill bugs (Flexicalymene). This is in contrast with Walliserops with their massive head weapons. How much you want to bet Walliserops wasn’t rolling into balls!
A rolled-up Walliserops wouldn’t look much like a ball – “caltrop” might be closer ,https://en.m.wikipedia.org/wiki/Caltrop – but it’s pretty likely that they could roll up enough to cover their abdominal legs and gill-legs (upper ramus of the biramous leg) with the hardened shell of the pygidium (https://en.m.wikipedia.org/wiki/Pygidium) protecting the jaw structures under the cephalic “shield”. Which would leave the large “genal” spines protruding from the back of the cephalon shield, the trident out the front, and a couple of minor forests of pointy bits sticking out around the hinge region.
Even if they didn’t “roll into a ball”, this structural ability of trilobites to protect their soft bits by folding the pygidium up against the cephalon seems to be conserved through the group’s existence. Probably the most challenging question would be to try to “roll up” Isotelus rex – the largest known trilobite, about 70cm from free cheek to pygidium tip into an irregular discus about 5cm thick by 30-40cm across. But that probably wasn’t a trick the adults needed to use, while remaining very useful for infants and juveniles which retained the body form by isomorphous growth.
I enjoyed that very much. But of course there are more than two sexes …. ( I hope the ironydetectometer is in full operation !)
Evolutionary biology courses should be ripe for harvesting by the Wokeratti. You have sexual selection, “survival of the fittest”, and a parade of dead white males to criticize.
A fascinating subject and article; many thanks.
I’m pleased to see other Richard Fortey fans here. I too have a number of his books, including his memoir of his time at the Natural History Museum, “Dry Store Room No 1”, which is highly entertaining.
It is, I loved that book! His biography, A Curious Boy, is on my (soon) to read list (and shelf).
A couple of my colleagues were students of his – at different times – at UCL, and remember him fondly.
Thanks for the science post Jerry, quite interesting.
Cool.
Great post!
I really enjoyed reading Fortey’s Trilobyte a few years ago.
Thanks for highlighting these findings and introducing another science writer I hadn’t heard of. Based on the readers’ enthusiasm for his books, I’ll give him a try.
They are, posit the authors, the arthropod equivalent of reindeer horns.
WHERE ARE THE BLOODY FEMALES??
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Males of almost all deer species grow antlers, used to battle for females. But reindeer are the only species in which the females also grow antlers, and an explanation can be found by looking at bovids, a closely related family including antelopes, goats and sheep.
Many female bovids have horns, used to defend food or territories from other females.
In exactly the same way, female reindeer use their antlers to defend food in small patches of cleared snow. Those with the largest antlers tend to be socially dominant and in the best overall physical condition.
Unlike horns, antlers are shed each year. In males, this happens in late autumn, after the rut.
Females retain their antlers until spring, because access to food is critical during their winter pregnancy. Some scientists therefore argue that Rudolph, who is universally depicted in late December with intact antlers, is female.
In fact, most of the reindeer used to pull sleds are castrated males – they are easier to handle, and have antler cycles similar to those of the females.
Not all females have antlers, however, because growing them costs a lot of energy. In habitats where food is scarce or of poor quality, antlerless females dominate.
https://www.discoverwildlife.com/animal-facts/mammals/why-do-female-reindeer-grow-antlers/
The analogous question for an ecdysozoan (all arthropods that I know of use ecdysis – moulting – as a growth strategy ; unless I’m having a senior moment, I can’t think of a non-arthropod that uses the growth strategy) gets interesting.
– Where on the “trident” could you put a set of splitting sutures?
– How could you connect those sutures to the rest of the body’s suture network without also introducing a weak point into the trident structure?
My senior thesis in college was on vision in phacopid trilobites—a popular topic at the time, and still fascinating. Yeah. These trident-bearing guys are the wackiest trilobites ever.
It’s best not to rule out the idea that the females have been discovered and are classified into a different genus. It is not uncommon to classify even different body parts into different taxa. For the longest time, different parts of the Cambrian monster Anomalocaris were classified as belonging to completely different organisms until, lo and behold, someone figured out that all the different “species” in the literature actually belonged to a single animal of—for the Cambrian—enormous size.
In the wonderful world of Trilobite – Anomalocaridid (now “Peyotiid”) relations, the question remains – why are 80% of bite marks from “Peyotia” (the Anomalocaridid mouth parts, the first-named part of the creatures, so the senior synonym for taxonomy purposes) on the right-hand flank of the trilobite. This is true for both benthonic trilobites (live on the seabed) and nektonic/ planktonic trilobites (free-swimming forms).
That’s a four-beer question.
I got a copy of his first book, bought it on eBay. And yeah, us guys are always fighting each other, regardless of species.
In Winnipeg, where I grew up, many buildings were constructed of Tyndall Stone, a regional variety of limestone especially hospitable to fossilization. Here’s an example of a common sight:
http://img.geocaching.com/cache/d8acf5ab-f971-4969-b33b-068fdc173821.jpghttp://img.geocaching.com/cache/d8acf5ab-f971-4969-b33b-068fdc173821.jpg
Your link is borked, but I think you were pointing at this lovely solitary coral : https://s3.amazonaws.com/gs-geo-images/d8acf5ab-f971-4969-b33b-068fdc173821.jpg
Thanks – I missed that one. Read Fortey’s Trilobite book a few years ago.
I agree – no reason why the trait could not be male & female. Do internal organs get preserved in the folssilisation, or only the hard carapace?
Preservation of soft parts for trilobites is very rare, but known. I’d have to check, but I think the mouth parts and biramous legs were recognised … in the 1930s? Thereabouts.
Beyond the trilobites … well the main part of the fuss about the Burgess Shale was because it preserved soft parts of genera previously known only from hard parts. (The Burgess has not got a lot of trilobites though – which has raised all sorts of ecological questions about preservation potential versus fossilisation of moulted carapaces while the trilobite swims away, and lives to moult another day.)
Then there is the Mazon Creek fauna. And the Chenjaing fauna. And the Herefordshire Lagerstätte – which has appeared here at least once.
I’ve had Unpaywall as part of my standard load-out for browsers for a few years now. For me, it’s coming up grey, with no legal options for reading it.
I guess you’ve got some other access possibility (university network? cookies from a pre-retirement subscription?) working that UnPaywall is detecting for you.
The first option I get is $10 – better than some journals.
OpenAthens – nope, not listed under that. GeolSoc.co.uk clearly doesn’t rate PNAS for rock science.
Researchgate : well the paper is recognised there. I’ll need to drop a “request full text” in to – it’s probably author Alan Gishlick who has got the citation onto RG. I’ll see how that goes.
Is Sci-Hub still being blocked at the UK’s border ISPs? Maybe not, but it is saying “Unfortunately, Sci-Hub doesn’t have the requested document: 10.1073/pnas.2119970120″, which isn’t how it normally behaved before blocking.” And it looks as if the process has been blocked : “a full list of Sci-Hub dois updated on 2022-02-12”. Doubleplus ungood.
Getting a PDF from the PCCE@LoC (Library Of Caturday) is a solution, but isn’t a scalable solution.
Well, that was quick!
One of the authors put the PDF up on Researchgate when I asked.
https://www.researchgate.net/requests/attachment/109294435
The authors failed to exclude aesthetic selection by females as a path by which beetle horn size and shape evolve. Dr Richard Prum’s book “The Evolution of Beauty” suggested to me that this was a large oversight.
With some half a million species and not-as-many ecological niches and reproductive barriers (genetic isolation) that leaves aesthetic selection on the table. Rather, Prum explains that aesthetic selection is the null hypothesis that others must be gaged against.
Chauvinism seems at play. ‘Bigger/better weapons’ gets consideration, but not female-mediated selection pressure?
Beauty happens, as Prum puts it.