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.