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

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.

Large trilobites in a stress-free Early Ordovician environment

“…It is likely that trilobites, similarly to modern vagile arthropods, were able to adapt against physical instabilities and were little affected by storm turbulences. Some of them even showed collective behaviour by migrating during storm seasons, in a similar way to extant spiny lobsters (Vannier et al 2019).…”

Synchronized moulting behaviour in trilobites from the Cambrian Series 2 of South China

“…One of these adaptations is the synchronized moulting seen in modern social and subsocial groups of arthropods such as spiders, shrimps, prawns, crabs and springtails 5 , 8 – 11 and its presence is being increasingly observed in the fossil record (e.g. trilobites, eurypterids, megacheirans and other crusaceomorphs, see 12 – 18 ).…”

“…Synchronized moulting behaviour in trilobites has been previously suggested for assemblages of carcasses and moults of Balcoracania dailyi Pocock, 1970 19 , Homotelus bromidensis (Esker, 1964) 20 (see 14 , 16 ) and the linear clusters of Ampyx priscus Thoral, 1935 21 in Vannier et al . 18 . Like in other groups of arthropods, synchronized moulting in trilobites may also be due to a variety of abiotic factors, such as temperature, tides or photoperiod and lunar cycles, or due to biotic factors such as predators, age, reproductive status or feeding (see 8 , 17 , 22 ).…”

Coupled exuviae of the OrdovicianOvalocephalus(Pliomeridae, Trilobita) in South China and its behavioral implications

“…Preservation of two Ovalocephalus tetrasulcatus exoskeletons preserved together is similar to the preservation of an arthropod in the middle of the act of molting (García-Bellido & Collins, 2004), but the latter case includes one corpse and one exuvia, whereas all these specimens from Jingshan are exuviae, without corpses; thus, the possibility that the Ovalocephalus tetrasulcatus individuals were buried when molting can be excluded. Several cases of queuing of trilobites have been reported in the past, but most of these cases involve corpse fossils, and the number of trilobites is typically more than three; thus, they are regarded as representing unexpected burial during the migration of trilobites (Radwański, Kin & Radwańska, 2009;Błazejowski et al, 2016;Vannier et al, 2019). Chatterton & Fortey (2008) reported trilobites aligned for molting, but they were preserved in burrows.…”