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.

中文翻译：

---

无压力的早奥陶世环境中的大型三叶虫

了解身体大小的变化对于破译生物体对其周围环境条件及其生态适应的反应至关重要。在现代环境中，大型海洋动物大多生活在寒冷的水域。然而，许多参数会影响除温度以外的体型变化，例如氧合、营养供应、捕食或风暴造成的物理干扰。在这里，我们研究了在冷水环境中沉积的下奥陶统 Fezouata 页岩的三叶虫尺寸变化。在近端和中间环境中发现了以长度为几厘米的小到正常大小的标本为主的三叶虫组合，而在 Fezouata 页岩的最远端环境中发现了包含长度超过 20 厘米的较大类群的三叶虫组合。原位被广泛地生物扰动，具有高度多样性的微量化石，表明在这种环境中可以获得氧气和营养物质。在中间和浅层环境中，生物扰动的范围较小，深度较浅。与较浅的环境相比，风暴事件的罕见性（最小的物理干扰）以及在深层环境中缺乏捕食者也有助于三叶虫获得更大的体型。这项高分辨率的空间研究调查了许多生物和非生物参数对身体大小的影响，对于理解地质时间的大小波动具有更广泛的意义。