A new paper in Nature by Jean Vannier et al. reports the unusual finding of a parade of trilobites—a group of the ancient arthropods—apparently killed and fossilized while walking in tandem, like an invertebrate conga line. They’re 480 million years old, from the Lower Ordovician, and were found in Morocco. (The paper can be seen by clicking on the screenshot below, the pdf is here, and the reference is at the bottom.) This weird lineup of trilobites suggests some kind of collective behavior—the first such find documented by paleontologists. But what kind of behavior? The authors have two hypotheses, and I’ll discuss them briefly.
First, some photos of the species, Ampyx priscus, which had a hollow “glabellar spine” in front and two “librigenal spines” going backwards. The white scale line is 1 cm long, so these things were, including the front spine, about 6 cm (2.3 inches) long. The spines might have enabled the trilobites to sense each other and thus maintain contact while moving in line, much as spiny lobsters do when moving across the sea floor in line as I show below. (Any “communication” must have been tactile as these trilobites were blind.)
All captions are taken from the Nature paper. Here are the individuals at hand, with some close-ups of their spines:
Here are the fossils of the lined-up trilobites, which are remarkable, along with schematics showing the nature of the relief of the stone in which they were preserved.
Here’s a video of spiny lobsters migrating in line, much like these trilobites:
So why were these ancient arthropods marching in line? The authors reject two hypotheses. First, that they were “mechanically accumulated along linear submarine reliefs (e.g. between ripple marks)”. This is the hypothesis that they were blown into grooves in the ocean floor and accumulated there, explaining the lines. That, however, doesn’t explain the consistent alignment rather than some being blown in backwards. The authors reject this because there is no indication from the fossil strata themselves that there were these reliefs.
They also reject the hypothesis that these trilobites were lined up in burrows underwater and then trapped and killed by sediments. Their rejection is based on the absence of “any colored outlines or disturbances in the sediments surrounding trilobites.” I’ll trust the authors on this since knowing how to detect ancient burrows is above my pay grade.
Rather, the authors proffer two hypotheses to explain the alignment. The first, shown on the left below, is that there were underwater storms or currents that made the trilobites orient in one direction, and then they “found” each other by tactile signals (or perhaps also by chemical signals), forming a line that served a protective function. As the authors say, “Such mechanical contacts [as in the lobsters above] appear to be essential for group cohesion and for optimal coordinated locomotion.” Marching in a line reduces drag, saves energy, and, say the authors, “reduces the probability of detection and attacks by predators by creating confusion in their [predators’] visual perception.”
The second hypothesis, shown on the right below, is that the trilobites emitted chemical signals like pheromones as a way of detecting each other and coming together for sexual reproduction, with the lines presumably indicating a migration toward spawning grounds. As the authors note, both explanations could be operating together.
As for how they were buried together, that’s a bit of a mystery since trilobites, when stressed, are supposed to have curled themselves into balls like modern isopods, and these didn’t do that, as you can see above. Here’s one scenario that explains the successive strata in which lines of trilobites were buried.
First, subject to periodic storms that disturbed the waters, the trilobites joined up in a Big March. (Or, as I noted above, they could be marching for mating!). Then, the storm quickly deposited sediment atop the marching trilobites, preserving them in situ. There could have been two other events that preserved them quickly: “water poisoning,” like the release of hydrogen sulfide gas or, more likely, the upward movement of oxygen-poor (“anoxic”) sediments, which killed the trilobites quickly from lack of oxygen as well as protecting the carcasses from scavengers.
You can see one instance of preservation in panels a-c below, and then another line of trilobites forming in panel “d”:
Now much of this is speculative, as it must be with limited information about what happened 500 million years ago. But it certainly looks as if, like spiny lobsters, these trilobites were marching in line, probably following each other using tactile cues. And so we get a rare window on invertebrate behavior from the distant past.
______________
Vannier, J., M. Vidal, R. Marchant, K. El Hariri, K. Kouraiss, B. Pittet, A. El Albani, A. Mazurier, and E. Martin. 2019. Collective behaviour in 480-million-year-old trilobite arthropods from Morocco. Scientific Reports 9:14941.
Very interesting. I wonder what Richard Fortey (paleontologist, and writer) has to say about this. Trilobites were his specialty, and I loved his book Trilobite! (the exclamation mark is part of the title. But I also loved the book that much!).
I have that book and bought it for the exclamation mark. Whenever I mention it I say it with that emphasizing punctuation as well!
A groovy book! There’s only a handful of PopSci books worth reading twice & that’s one of them! Very comfortable writer to read – like a worn in pair of slippers! And funny too!
Rumour has it that the “Professor with the Thousand Trilobite! Stare” is working on another book.
Not sure exactly what Richard’s take on this might be but I can refer you to a particularly thrilling paper on a possible sexually selected role for the anterior spines on _Ampyx_ (the genus fossilised here) which I wrote with him a few years ago: Knell, R.J. & Fortey, R.A. (2005) Trilobite spines and beetle horns: sexual selection in the Palaeozoic? Biology letters, 1, 196–199.
That’s amazing- i wonder how extensive a burial it could have been, and if there are more fossils nearby these trilobites.
Typo : “sentiments” should be “sediments”
Fixed, thanks.
“Parading in column” is better or since it’s in water how about “parading in line astern”?
My favourite bit above is:
I have a mental, mental picture of that happening – these trilobed critters being crushed under the weight of Hallmark Condolences Cards & smothered by the ingestion of coloured vinyl “I Care About x Cause” wrist bracelets. 🙂
I was thinking it was a bunch of other trilobites trapping other trilobites as they expressed their opinions about things.
Very cool to witness examples of paleo-behavior. Thanks for posting.
Ichnology – the science of studying and understanding footprints and other traces of the behaviour of “trace-making” organisms – has a reasonably long (1850-something onwards) and distinguished history. As is often stated, these fossils are the marks of living organisms, not of dead ones.
For statistical purposes, please note that I read, enjoyed and was educated by this science post.
🙂
Ditto!
I read this piece and found it enlightening as well.
Let this count as my comment, so that it doesn’t appear these science pieces are drawing insufficient attention for our host to continue posting them.
I wish to associate myself with this remark.
Me too! I’m going to look at how to work this into one of my lectures on fossils. Thanks.
I’ll go with that, too.
I concur, just to weigh in.
Me too.
Me too.
I align to this conga line.
Watch it man! Start at the end of the line just like everybody else. 😎
Since the last article posted from Nature was about the postmodern constructions of scientific interpretations, I feel the need to snark that this mating and defense centric view is clearly a reflection of male Whig culture and that the meaning of why they were marching in line changes depending on the observer. If science were more Woke, we would realize they were protest marching the appropriation of their look by horseshoe crabs, which they saw coming a few million years away.
Hmmmm. As I recall, the direct lineage of the horseshoe crabs goes back to the Jurassic only. They’re often mis-described as if they were descendants of the trilobites, but that’s only true in crude morphology. They’re arthropods, for certain, but the biramous limbs have a different structure, and there are other anatomical differences too. Particularly, as I recall, in the feeding appendages – but that’s stretching my memory.
These Lower Ordovician trilobites – close to 500 Myr BP – had to look a long way forward to see the first documented members of the horseshoe crab group. 350-or so Myr.
I wouldn’t put money on mammals even existing 350 Myr from today.
Since the last article posted from Nature was about the postmodern constructions of scientific interpretations, I feel the need to snark that this mating and defense centric view is clearly a reflection of male Whig culture and that the meaning of why they were marching in line changes depending on the observer. If science were more Woke, we would realize they were protest marching the appropriation of their look by horseshoe crabs, which they saw coming a few million years away.
Maybe they celebrated St. Patrick’s Day
and threw a parade.
Fascinating. Perhaps like the spiny lobsters, they were migrating? Or perhaps they were all standing in line for the buffet table.
Whenever I contemplate these ancient mysteries, a line from Gerard Manly Hopkins comes to my mind. “These things, these things were here and but the beholder wanting.”
Oh, and I wanted to say the librigenal spines on this species of trilobites look like they evolved precisely for the purpose of co-ordinating this sort of parade.
Yeah, way cool too. It makes sense that a trilobite needs fellows behind as ahead. More safety in numbers.
Jerry,
DarwinWins,
Such spines are very common across the trilobite clade. They’re a useful, rapidly varying, set of features. But their main use is interpreted to be as a defensive structure. If you take the head structure (the cephalon, including the central rounded mound, the glabella, which has a spine in this species (but not in many) and the two genal spines from the “cheeks” at either side, then “roll” the segemented body around it, placing the rigid armoured pygidium (tail unit, the name may be related to “pyramids”, etymologically) across the feeding structures on the underside of the head, you get a fairly hard to eat ball of armour plates. If you now grow the spines – glabellar and genal – your armour-plated mouthful rapidly gets a lot less appetising.
Another common location for spines to grow (and hence dozens more genera of trilobites) is on or near the middle of the several segments in the thoracic region – a “trailing” spine that develops here will protrude out of the plane formed by the cephalon and pygidium folding together. Even less appetising! And you’ll need an even bigger mouth to actually grab it.
Trilobite spines – arthropod spines in general – are important clues to behaviour.
I wonder if they were just fascists. Fascists like parading in a line, and we have no real way of knowing the precise political inclinations of trilobytes. So the explanation could be as simple as that.
The authors reject the hypothesis that the trilobites were lined up along features of the terrain because they found no fossil evidence of such features. But suppose the features were formed of soft vegetation that didn’t fossilize? Could the ‘bites have been following submarine game trails that weren’t preserved?
The sediments in general are coarse silts (20-30 microns) and show various bioturbation features (“bt” in the scenario cartoon above). But they don’t mention the presence of the holdfast (“root”) structures that seabed plants need to stay in place in a relatively high current situation. (20-30 microns is the second-coarsest division of the “silt” grain size ; above 64 microns, it’s considered sand ; the current needed to move such material is one to several cm/s, suggesting that any seabed plants would experience considerable hydrodynamic drag.)
Also, you’d probably expect to see the marks of the putative plant fronds on the palaeo-seabed, which aren’t there.
“’Olfactive and mechanical sensors were probably located on the antennules (pink areas 4,5), and genal and glabellar spines (green areas 1 – 3), respectively.’”
This is very interesting. I thought the six thoracic segments made up the spine at first.
“thorax” in humans would be the chest, particularly the area marked with humans segmentation (ribs). I trilobites, their body plan is divided into 3 lengthwise, and 3 widthwise. The lengthwise division is into the “head” (cephalon, a rigid structure consisting of the rounded glabella and two flat “cheek” regions to either side), the “tail” (pygidium, a rigid structure at the opposite end from the cephalon), and in between the thorax, comprising a number of transverse structures which can move against each other, giving the animal the flexibility to bring pygidium against cephalon to form a protective structure. In some areas you can find many similar specimens in a “death assemblage” (all killed at almost the same time), where the number of thorax sections varies by “several”. The common interpretation is that some genera “grew” by adding a thoracic segment at [each/ some] moulting event. That may have been variable between individuals, species and genera.
The “spines” are grown from new with each moult. The probably have a weak line (“suture line” along which they split during moulting, allowing the new structure to emerge. And yes, there must be some interesting biochemistry going on at that point, to harden the new shell.
This reminds me a bit of a bed of fossilized clams I saw once. They were all going in one direction and you could see trails behind them. The paleontologist explained that they had been inundated by a large amount of sediment from which they were trying to escape. Clearly they didn’t make it.
Ah – the rate of sediment deposition need only be faster than the rate of the animals.
I got it in my head that there had to be some enormous, instantaneous dollop of sediment to freeze the positions. Obviously no.
Makes me wonder if this means, as the sediment gripped the animals, there could be slight deviation from perfect coordination. So they’d be aligned but somehow off by 1-2%.
Yes, I wondered about how fast the sediment was deposited. As I understand it, underwater avalanches happen frequently so it is possible for them to be buried all at once. I’m not sure how fast clams can escape. If we were buried, we wouldn’t last long but a clam’s metabolism is presumably much slower. How long can they last without oxygen and other necessities?
I doubt the trilobites were escaping burial as they were organized. The clams weren’t.
I recall a show where they dug up some clams that were in the vicinity of 150 years old. Growth rings, I guess. I have a bad feeling they proceeded to eat them… the clams, that is… instead of the other way around…
Its name was Hafrun (or Ming, depending on which culture you wish to appropriate) and it was 507 years old, a mere lad (or lass – the clam was “spent” so they couldn’t tell for sure.
https://en.wikipedia.org/wiki/Ming_(clam)
FTFP,
Measuring and recording the relative orientation of each print in a trail (in ichnological terminology) is a standard part of recording a site, once you realise that you’re dealing with a trace fossil as well as a body fossil.
Measuring for such alignments is also routine in studying, for example, “death assemblages”, where you have (or suspect) multiple organisms which died together, or were moved together by some sediment re-working after death.
If they were inundated by a lot of sediment at once, you’d think the animals would be tumbled around quite a bit. I’d say something toxic killed them or stunned them and the burial was relatively slow. Perhaps a mild avalanche which was mixed with a toxic substance.
Possibly – not necessarily though – this I what I’m intrigued by – how much variation about a perfect line could be expected, and how so
Or high temperature could do it
Makes me wonder about other fossils we see – if scattered, perhaps they were in fact tumbled around
Obviously the poor bur sinful creatures were trying to escape from Noah’s flood.
There supposedly was a great flood in the Middle East caused by erosion of a natural dam that allowed a lake to drain into a valley. The thought was that this was the source of the 40-days flood story. I don’t know if it is still considered true. Regardless, my clams were in Montana or Alberta so were not involved in that flood. They were presumably on the banks of the great inland sea that once extended northwards from the Gulf of Mexico.
Another thing that does happen – sometimes associated with storm deposits – is if water of reduced oxygen content is brought in (stirred up by a storm, or displaced by water from somewhere else) which then suffocates the benthonic (living on the sea bottom/ sediment-water interface) organisms.
The low oxygen content of the water and the sediment deposited makes such events prone to relatively good fossilisation, so they’re quite well represented in the literature.
The absence of enrolled specimens – the trilobite’s stress response – in the trail is counted as evidence of very rapid death, probably too rapid for burial to be cause of death.
I have a trilobyte on my desk, but, in truth, I suspect that nothing of the original creature remains. That pleasing and fascinating form was simply preserved by sediment.
I do have doubts about the theory of procession, and think it more likely that, with changing tides, they all were moving independently towards a goal along a smooth channel-way.
Why the trilobites crossed the stream? (There were no road nor bridge then.)
Trilobites were exclusively aquatic organisms. Their delicate, flexible gill structures placed between their rigid thorax plates and their stiff “walking legs” would not have survived the exposure.
I think they were strictly normal-salinity marine : I can’t think of any reports of them from brackish or even fresh water.
All very interesting. I also expect they were following each other with tactile cues, like the lobsters, since they have fairly long antennae.
As a cave ecologist the statement “(Any “communication” must have been tactile as these trilobites were blind.)” caught my eye and got me thinking. Were these trilobites blind or eyeless? Often cave organisms in my neck o’ the woods are colloquially referred to as blind (e.g., the so-called blind cavefish) when, in fact, they are eyeless.
Nope. You’re all wrong. It’s obviously an Ordovician conga line. Those bites were partying like it was 4.8e8 BCE.
Nah. Conga wasn’t invented until way later.
Yeah not until just before the Silurian.
Sounds like a drink with an little umbrella.
Yep. The Conga for sure.
It may surprise you to know that I imagined the same thing!
My first thought was they were headed to spawning grounds. Potentially it could also be towards large gatherings of prey that are spawning. We see both of those in extant species.
But, any of the above would likely result in gathering lines, not a single line.
Fascinating.
Hermit crabs form a similar line and even do it according to size (biggest in front)!
They do it because being without a shell is dangerous, so it’s an adaptive feature if you line up behind a slightly bigger hermit crab when it looks like he/she is going to change their shell, because that way you can take it quick. And if every crab has that instinct…you get the ‘conga line’ when a bunch are ready to change shells.
I don’t think the same logic would apply to trilobites…as far as I know, their bony parts are skeletal rather than a separate shell. But, it just goes to show that nature can come up with these weird patterns of behavior for very individualistic, ‘selfish gene’ type reasons. Evolution rocks. 🙂
Triobites grow by splitting their external skeleton along suture lines, exiting from the old shell, then “pumping up” their bodies before their new shell hardens.
Many fossils in collections are actually the ejected shells from a moulting. It’s a good trick for squeezing an extra point out of a student in a practical exam – do they notice that the trilobite is actually an exuvium?
Great fossil, nice discussion. I doubt that “marching in a line reduces drag” unless they move much faster than seems at all likely.
I’m not going to take the time to work it out but my physics intuition tells me that moving in a line through the ocean would definitely reduce the drag experienced by most of the trilobites in the line. The water surrounding them would move at some speed in their general direction and the resistance they meet reduced accordingly. I’m not suggesting that’s why they do it though. Who knows?
Could they all roll up though? These seem a little short to be able to do that…
I just discovered that this is not the first time trilobites on parade were found. Polish paleonthologists 2010 found in Polish mountains slightly younger fossilized trilobites (350 million years old) marching one after another. I can’t find any proper information about it in English and I have just a link to an article in Polish from popular media: https://www.tvp.info/1585856/po-gorach-swietokrzyskich-chodzily-trylobity
Unfortunately, the video at that site was under licensing restrictions.
I was trying to find more articles with pictures or videos. There are quite a lot of articles but without any illustrations and, of course, all are in Polish.
A picture, as they say, is worth a thousand words, but, we can rest assured you reported faithfully and true. 😎
Yes, I did. There are many more details in the article and in other reports about this find. Maybe a Google translation can help a bit?