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Article Dans Une Revue Geological Magazine Année : 2021

Large trilobites in a stress-free Early Ordovician environment

Farid Saleh
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Muriel Vidal
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Lukáš Laibl
Pierre Sansjofre
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Pierre Guériau
Francesc Perez Peris
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Lorenzo Lustri
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Victoire Lucas
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Bernard Pittet
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Khadija El Hariri
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Allison Daley
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Résumé

Understanding variations in body-size is essential for deciphering the response of an organism to its surrounding environmental conditions and its ecological adaptations. In modern environments, large marine animals are mostly found in cold waters. However, numerous parameters can influence body size variations other than temperatures, such as oxygenation, nutrient availability, predation, or physical disturbances by storms. Here, we investigate trilobite size variations in the Lower Ordovician Fezouata Shale deposited in a cold water environment. Trilobite assemblages dominated by small-to normal-sized specimens that are few cm in length are found in proximal and intermediate settings, while those comprising larger taxa more than 20cm in length are found in the most distal environment of the Fezouata Shale. Drill core material from distal settings shows that sedimentary rocks hosting large trilobites preserved in-situ are extensively bioturbated with a high diversity of trace fossils, indicating that oxygen and nutrients were available in this environment. In intermediate and shallow settings, bioturbation is less extensive and shallower in depth. The rarity of storm events (minimal physical disturbance) and the lack of predators in deep environments in comparison to shallower settings would have also helped trilobites attain larger body sizes. This highly resolved spatial study investigating the effects of numerous biotic and abiotic parameters on body size has wider implications for the understanding of size fluctuations over geological time.
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Dates et versions

hal-03005087 , version 1 (13-11-2020)

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Farid Saleh, Muriel Vidal, Lukáš Laibl, Pierre Sansjofre, Pierre Guériau, et al.. Large trilobites in a stress-free Early Ordovician environment. Geological Magazine, 2021, 158, pp.261-270. ⟨10.1017/S0016756820000448⟩. ⟨hal-03005087⟩
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