Comrade Trilobites Marched in Tandem 480 Million Years Ago

Arthropods from ants to lobsters migrate in orderly fashion but the emergence of the collective had been a mystery

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Linear clusters of trilobites (arthropods) on the surface of the rock slab, all oriented towards the same direction.
Linear clusters of trilobites (arthropods) on the surface of the rock slab, all oriented towards the same direction. Credit: Jean Vannier,AP

Hang your head, Russia. The emergence of collective behavior has been a longstanding enigma in evolution, and now it turns out that ancestral arthropods would march to a common drum, though why they did so remains a mystery. Fossil trilobites found in Morocco who died in an orderly queue, for whatever reason, indicate that the ancient animals were capable of group behavior 480 million years ago.

The power of the masses goes back a long way. Almost 100 million years earlier, sessile Ediacarans named Ernietta had apparently developed a knack for collective eating. These tiny multicellular animals turn out to have lived in groups around 560 million years ago and engaged in “gregarious suspension feeding,” catching primordial plankton. In other words, they lived in groups, died and were fossilized in groups. But these Ediacarans at least weren’t motile.

Fast forward to the lower Ordovician around 480 million years ago and we find trilobites, zippy primitive arthropods with a lot of legs, apparently engaging in a social behavior that led them to die and be fossilized in lines, as divulged in Scientific Reports

In other words, group behaviors found in motile modern arthropods existed almost half a billion years ago, explain Jean Vannier and colleagues.

The march of the lobster

Group behavior in today’s arthropods is widespread throughout that vast kingdom, which suggests very early origins. Migrations among the one million or so species of arthropod are the perfect example. Migratory caterpillars form and move in caravans, for example. Ants migrate in swarms and spiny lobsters lumber along the seabed in vast columns.

Spiny lobster migrationCredit: Youtube

Crabs are also famous for their migratory habits: The red crabs that emerge from the forests of Christmas Island, near Australia, and march en masse to the sea to breed is a stunning sight. Since the red crabs are a predictable lot – they spawn during the last quarter of the moon at the start of the wet season, this year the extravaganza is for November 22-24 or December 21-23, according to Parks Australia.

One Hundred Million Crabs | The Trials of Life Credit: Youtube/BBC Earth

Yet, the origin of collective behavior has remained largely unknown. Now Vannier and the team described linear clusters of Ampyx priscus, a trilobite from the lower Ordovician period, in Morocco.

In each cluster, the trilobites were lined up. They faced in the same direction, and maintained contact via their spines. Priscus is all of 16 to 22 millimeters long and sported a stout spine in front two elongated spines in the rear, presumably for protection from other motile animals of the Ordovician.

One must wonder if they weren’t somehow lined up by courtesy of the topography and ocean currents. The authors reply that based on the sheer scale of the patterns they observed, such consistent linearity and directionality is unlikely to be the result of passive transportation or accumulation by currents.

A trilobite, an extinct marine arthropod that once was common in the Earth's seas, is seen in this image released in Lund, Sweden on September 18, 2019. Credit: \ BIRGER SCHMITZ/ REUTERS

It isn’t that they suddenly all died while queueing. Something drastic must have happened to the unfortunate bugs, freezing this migratory (or in any case, motile) moment in time. On land, one can find moments of this kind thanks to volcanic eruptions killing the creature on the spot and burying it in its last format. Given the marine nature of these animals, the paleontologists suggest they were killed suddenly while travelling, possibly by a sudden wave of sediment during a storm.

In short, the authors suggest that Ampyx moved in groups just like modern arthropods, and maintained their single-row formation by physical contact of their elongated spines.

Trilobite rex and friends

Might the trilobites have been migrating? They might have. Or they might have decided to decamp en masse because of local inclemency, as spiny lobsters do. When storms disturb their environment, spiny lobsters line up and move away. Or the trilobites may have been migrating as part of their seasonal reproductive behavior. We do not have information on how trilobites courted.

In general trilobites were a stunning bunch. They all lived in the sea and first appeared in the Cambrian, over 540 million years ago. Ampyx was small, a mere centimeter or two in length, but the group included some monsters such as Isotelus rex, which could reach 75 centimeters long (yes, almost 2.5 feet).

Morocco in particular is famous for its array of fossil trilobites, including some gigantic ones. But mind that you don’t wind up buying fake ones in the souq, there is a big business in fake trilobites.

A Phacops rana crassituberculata trilobite, collected from the Silica Shale, near Sylvania Ohio, USA, from the Middle Devonian (Givetian).Credit: Dwergenpaartje

Back to the past: Based on the fossil record, which can be terribly misleading, it begs noting that trilobites achieved dominance of the marine domain until fading out after more than a quarter-billion years. They went extinct in the great Permian extinction event and do not, as far as we can tell, have any extant descendants.

Before they died out, many trilobites had huge compound eyes, which are sometimes fossilized to a startling degree.

Not so Ampyx priscus. It had no eyes. Given that it was sightless, the authors hypothesize that these rather small trilobites coordinated their movement using sensory stimulation via their spines, and possibly using chemical stimulation as well.

That in turn indicates that almost half a billion years ago, bugs had the neural complexity to develop collective behavior – albeit temporary in nature. Just like in Mother Russia.