fossils found in Morocco suggest the practice of forming orderly lines may date back 480 million years and could have had evolutionary advantage

Their study, published Thursday in the journal Scientific Reports, describes groups of blind trilobites—known as Ampyx—all facing in the same direction, apparently maintaining contact via their long rearward spines.  ...  Lucy McCobb, a paleontologist at the National Museum Wales who wasn't involved in the study, said that while similar 'conga lines' of fossilized Ampyx have been reported before, the researchers behind the study had built "a very strong case for the intentional lining up of the trilobites in response to some cue.

"These fossils give us a wonderfully vivid glimpse into the lives of these very ancient but clearly sophisticated creatures," she said.

The findings support the idea that collective behavior like forming lines emerged around the same time or shortly after animals first developed sophisticated nervous systems and sensory organs. He and fellow researchers said re-examining 520 million-year-old fossils of shrimp-like creatures found in China could offer evidence that such behavior began even earlier.

1Ampyx priscus in linear formation (Moroccan Lower Ordovician Fezouata Shale)
1Ampyx priscus in linear formation (Moroccan Lower Ordovician Fezouata Shale) - Credit: Jean Vannier, Laboratoire de Géologie de Lyon: Terre, Planètes, Environnement (CNRS / ENS de Lyon / Université Claude Bernard Lyon 1)

Collective behaviour in 480-million-year-old trilobite arthropods from Morocco

Vannier, Jean,  Muriel Vidal,  Robin Marchant,  Khadija El Hariri,  Khaoula Kouraiss,  Bernard Pittet,  Abderrazak El Albani,  Arnaud Mazurier and  Emmanuel Martin 
Scientific Reports volume 9, Article number: 14941 (2019) 

DOI: 10.1038/s41598-019-51012-3

Interactions and coordination between conspecific individuals have produced a remarkable variety of collective behaviours. This co-operation occurs in vertebrate and invertebrate animals and is well expressed in the group flight of birds, fish shoals and highly organized activities of social insects. How individuals interact and why they co-operate to constitute group-level patterns has been extensively studied in extant animals through a variety mechanistic, functional and theoretical approaches. Although collective and social behaviour evolved through natural selection over millions of years, its origin and early history has remained largely unknown. In-situ monospecific linear clusters of trilobite arthropods from the lower Ordovician (ca 480 Ma) of Morocco are interpreted here as resulting either from a collective behaviour triggered by hydrodynamic cues in which mechanical stimulation detected by motion and touch sensors may have played a major role, or from a possible seasonal reproduction behaviour leading to the migration of sexually mature conspecifics to spawning grounds, possibly driven by chemical attraction (e.g. pheromones). This study confirms that collective behaviour has a very ancient origin and probably developed throughout the Cambrian-Ordovician interval, at the same time as the first animal radiation events.