Mysterious Event Buried Orderly Arthropods In A Perfect Line 480 Million Years Ago

Ampyx priscus in linear formation located at the Moroccan Lower Ordovician Fezouata Shale. Jean Vannier

A group of orderly ancient arthropods found preserved for eternity in a neat line is changing what we know about animal behavior, according to a new analysis of 480-million-year-old fossils discovered in Morocco.

Scientists know quite a bit about the grouping behavior of modern arthropods. Take the migratory pine processionary caterpillars, for example, who use pheromone trails to travel head-to-tail in “large groups over long distances” to find pupation sites. In the Pacific Ocean, spiny lobsters perform mass single-file migrations across the open ocean when shallow waters get rough to reach spawning grounds. In both cases, these arthropods remain in contact with each other by literally touching their comrades with sensors and communicate using chemical cues.

But the history of the behavior in ancient species has remained unclear. Collective and social behavior evolved through natural selection millions of years ago, but its origin is poorly understood. Now, linear clusters of Ampyx priscus, a trilobite arthropod found in the oceans 480 million years ago, is helping the research community understand how ancient animals formed groups.

Scientists examined a line of arthropods between 16 and 22 millimeters long whose front bodies all faced the same direction and maintained contact with spines. When compared with the behavior of spiny lobsters, the researchers found that arthropods exhibited the same type of behavior that could not be the result of “passive transportation” but instead represents a collected, coordinated behavior.

Ampyx priscus in linear formation located at the Moroccan Lower Ordovician Fezouata Shale. Jean Vannier

Ampyx priscus was probably migrating in groups and used its long projecting spines to maintain a single-row formation by physical contacts possibly associated with mechano-receptors and/or chemical communication,” wrote the study authors in Scientific Reports. “This group behavior may have been a response to environmental stress due to periodic storms shown by sedimentological evidence, or was associated with reproduction.”

Similar clusters have been found around the world, from Canada to Poland to Portugal, but often lack information on how the animals ultimately wound up in their final tombs. In the case of the Moroccan Ampyx priscus, the researchers believe they were likely killed suddenly while they were traveling, perhaps after being buried by sediment during a storm.

“The amount of sediment deposited during a storm event was probably sufficient to entomb trilobites and other epibenthic animals in situ but not powerful enough to take them away,” note the authors.

Because Ampyx priscus were blind, they would have communicated using sensory stimulation with their spins and through the emission of chemicals like pheromones. If this is indeed a coordinated effort, whether with the intention of finding breeding grounds or to escape extreme environmental conditions, the authors say it improves our understanding of how animals evolved collective group behavior.

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