This recent article discusses a paper by Vannier, J., Vidal, M., Marchant, R. et al. Collective behaviour in 480-million-year-old trilobite arthropods from Morocco. Sci Rep 9, 14941 (2019) doi:10.1038/s41598-019-51012-3 .
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In relation to some of the questions the authors raise, it should be said that we have discussed some of the evolutionary advantages of such behavior in Trenchard, Brett, Perc. 2017. "Trilobite 'pelotons': possibly hydrodynamic drag effects between leading and trilobites in trilobite queues." Palaentology 60(4). The behavior, we suggest, may derive from a "drafting" effect by which following individuals save energy. This allows groups to migrate farther and faster than isolated individuals. It also gives us a clue to the size range observed among trilobite species.
https://onlinelibrary.wiley.com/doi/abs/10.1111/pala.123

Ancient 'Cockroaches of the Sea' Fossilized While Playing 'Follow the Leader'
Trilobites fossilized in Morocco seem to have been buried by a storm as they marched in an orderly queue.

In relation to some of the questions the authors raise, it should be said that we have discussed some of the evolutionary advantages of such behavior in Trenchard, Brett, Perc. 2017. "Trilobite 'pelotons': possibly hydrodynamic drag effects between leading and trilobites in trilobite queues." Palaentology 60(4). The behavior, we suggest, may derive from a "drafting" effect by which following individuals save energy. This allows groups to migrate farther and faster than isolated individuals. It also gives us a clue to the size range observed among trilobite species.
https://onlinelibrary.wiley.com/doi/abs/10.1111/pala.123