Ancient arthropod collective

The American sociologist and economist Franklin H Giddings is credited with coining the term “collective behaviour” in the early 20th century, but new research suggests group behaviours likely had considerably older origins.

Collective and social behaviour is known to have evolved through natural selection over millions of years.

Modern arthropods provide numerous examples – think of the migratory chains of caterpillars, ants or spiny lobsters. But when did such behaviours originate?

Fossils of ancient arthropods discovered in Morocco may hold clues.

A research team led by palaeontologist Jean Vannier, of the French National Centre for Scientific Research, believe the fossils indicate a collective behaviour either in response to environmental cues or as part of seasonal reproductive migration.

The team’s findings, published in the journal Scientific Reports, suggest that group behaviours comparable to those of modern animals existed as long as 480 million years ago.

Vannier and his colleagues described several clusters of Ampyx priscus, a trilobite arthropod from the lower Ordovician period. A. priscus individuals were 16–22 millimetres long. They had a stout spine at the front of their bodies and a pair of very long spines at the back.

In each of the trilobite fossil clusters studied, individuals were arranged in an orderly line with the front of their bodies facing in the same direction. They maintained close contact through their spines.

The researchers suggest that, given the scale of the patterns seen, the consistency of this arrangement – in lines and facing the same way – is unlikely to be the result of a passive environmental action, such as the movement of water and currents.

They say it’s more likely that A. priscus were killed suddenly while travelling, for example by being buried rapidly by sediment during a storm.

The study suggests the trilobites probably migrated in groups and used physical contact by their spines to maintain a single-row formation as they moved along the sea floor. This may have been a stress response to environmental disturbance, such as storms, which they detected by motion and touch sensors, prompting them to migrate to safer waters. A similar behaviour is seen in modern spiny lobsters.

Alternatively, researchers say, the linear arrangement may have been prompted by seasonal reproductive behaviour – the migration of sexually mature individuals to spawning grounds. A. priscus was blind, and the study authors hypothesise that the trilobites may have coordinated using sensory stimulation via spines and chemicals.

The study shows that a 480-million-year-old arthropod may have used its neural complexity to develop a temporary collective behaviour. This is likely to have given it an evolutionary advantage, allowing it to survive environmental stress and improve its reproductive success.