In a paper published in the journal Scientific Reports, University of Lyon’s Dr. Jean Vannier and colleagues described several fossilized clusters of Ampyx priscus, a species of trilobite that lived 480 million years ago (Ordovician period); the paleontologists interpreted the fossils as a collective behavior related to seasonal reproduction or triggered by environmental cues.

Ampyx priscus were between 16 and 22 mm long and had a stout spine at the front of their bodies and a pair of very long spines at the back.

In each cluster of trilobites examined by Dr. Vannier and co-authors, individuals were arranged in a line, with the front of their bodies facing in the same direction, maintaining contact via their spines.

“Given the scale of the patterns seen, this consistent linearity and directionality is unlikely to be the result of passive transportation or accumulation by currents,” the researchers said.

“Instead, it is more likely that Ampyx priscus were killed suddenly while traveling, for example by being buried rapidly by sediment during a storm.”

The study authors suggest that Ampyx priscus were probably migrating in groups and used their long spines to maintain a single-row formation by physical contacts.

“This may have been a stress response to disturbance of their environment by storms, detected by motion and touch sensors, which motivated Ampyx priscus to migrate to quieter and deeper waters,” they said.

“A comparable behavior is seen in present-day spiny lobsters.”

“Alternatively, the pattern may have been the result of a seasonal reproductive behavior involving the migration of sexually mature individuals to spawning grounds.”

“Knowing that Ampyx priscus were blind, they may have coordinated using sensory stimulation via spines and chemicals,” the scientists said.

“The discovery shows that a 480-million-year-old arthropod may have used its neural complexity to develop a temporary collective behavior.”

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Jean Vannier et al. 2019. Collective behaviour in 480-million-year-old trilobite arthropods from Morocco. Scientific Reports 9, article number: 14941; doi: 10.1038/s41598-019-51012-3