Abstract
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.
Highlights
Interactions and coordination between conspecific individuals have produced a remarkable variety of collective behaviours
Linear and unidirectional fossil clusters of conspecific trilobite arthropods occur in the Palaeozoic, which have been assumed to result from feeding, reproduction, moulting or sheltering behaviours[14,15,16,17,18,19,20]
Ampyx priscus Thoral, 193531 is a raphiophorid trilobite characterized by a stout glabellar spine and a pair of very long librigenal spines projecting posteriorly (Supplementary Text), which occurs at various horizons through the Fezouata Shale (Supplementary Fig. 1a), as isolated individuals or in linear clusters
Summary
Interactions and coordination between conspecific individuals have produced a remarkable variety of collective behaviours. 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. This group behaviour may have been a response to environmental stress due to periodic storms shown by sedimentological evidence, or was associated with reproduction. This record of linear clustering in early euarthropods suggests that intraspecific group-level patterns comparable to those of modern animals already existed 480 million years ago in the early stages of the Great Ordovician Biodiversification Event[30]
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