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Large trilobites in a stress-free Early Ordovician environment

Published online by Cambridge University Press:  18 May 2020

Farid Saleh*
Université de Lyon, Université Claude Bernard Lyon1, École Normale Supérieure de Lyon, CNRS, UMR5276, LGL-TPE, Villeurbanne, France
Muriel Vidal
Université de Brest, CNRS, IUEM Institut Universitaire Européen de la Mer, UMR 6538 Laboratoire Géosciences Océan, Place Nicolas Copernic, 29280Plouzané, France
Lukáš Laibl
Institute of Earth Sciences, University of Lausanne, Géopolis, CH-1015Lausanne, Switzerland Czech Academy of Sciences, Institute of Geology, Rozvojová 269, 165 00 Prague 6, Czech Republic Institute of Geology and Palaeontology, Faculty of Science, Charles University, Albertov 6, Prague, 12843, Czech Republic
Pierre Sansjofre
MNHN, Sorbonne Université, CNRS UMR 7590, Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, Paris, France
Pierre Gueriau
Institute of Earth Sciences, University of Lausanne, Géopolis, CH-1015Lausanne, Switzerland
Francesc Pérez-Peris
Institute of Earth Sciences, University of Lausanne, Géopolis, CH-1015Lausanne, Switzerland
Lorenzo Lustri
Institute of Earth Sciences, University of Lausanne, Géopolis, CH-1015Lausanne, Switzerland
Victoire Lucas
Université de Lyon, Université Claude Bernard Lyon1, École Normale Supérieure de Lyon, CNRS, UMR5276, LGL-TPE, Villeurbanne, France
Bertrand Lefebvre
Université de Lyon, Université Claude Bernard Lyon1, École Normale Supérieure de Lyon, CNRS, UMR5276, LGL-TPE, Villeurbanne, France
Bernard Pittet
Université de Lyon, Université Claude Bernard Lyon1, École Normale Supérieure de Lyon, CNRS, UMR5276, LGL-TPE, Villeurbanne, France
Khadija El Hariri
Département des Sciences de la Terre, Faculté des Sciences et Techniques, Université Cadi-Ayyad, BP 549, 40000Marrakesh, Morocco
Allison C. Daley
Institute of Earth Sciences, University of Lausanne, Géopolis, CH-1015Lausanne, Switzerland
Author for Correspondence: Farid Saleh, Email:


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 a few centimetres in length are found in proximal and intermediate settings, while those comprising larger taxa more than 20 cm 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 also have 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.

Original Article
© The Author(s), 2020. Published by Cambridge University Press

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