Marine deposits of earliest Eocene age in northern Jutland, Denmark, are renowned for yielding diverse teleost assemblages that have proved central for enhancing our understanding of the early evolution of many extant actinopterygian clades. In this study, we investigate diminutive larval fish fossils from the Stolleklint Clay, Ølst Formation, that retain multiple soft-tissue features preserved as distinct dark-coloured stains. To examine the elemental and molecular composition of these soft parts, we employed a combination of time-of-flight secondary ion mass spectrometry (ToF-SIMS), scanning electron microscopy (SEM) and energy-dispersive x-ray spectroscopy (EDS). Our analyses revealed that the preserved structures contain chemically identifiable eumelanin intimately associated with densely aggregated microbodies that are morphologically consistent with melanosome organelles. Thus, we conclude that the carbonaceous structures represent traces of originally melanized body parts, including the eyes and peritoneum. Comparable pigmentation patterns are seen in many extant teleost larvae that use semi-transparency as a means of camouflage in pelagic environments, to suggest a similar visual appearance of the Stolleklint Clay fish fossils. This in turn suggests that adaptations for concealment and UV-protection had already evolved by the beginning of the Eocene, notably during a time interval characterized by an extreme greenhouse climate, when the global fish fauna become increasingly modern in composition.

中文翻译：

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远洋领域的隐匿症：来自丹麦始新世 Stolleklint Clay 的保存异常完好的化石鱼幼虫的证据

丹麦日德兰半岛北部最早始新世的海洋沉积物以产生多种硬骨鱼组合而闻名，这些组合已被证明是增强我们对许多现存放线鱼类进化枝早期进化的理解的核心。在这项研究中，我们调查了来自 Stolleklint Clay, Ølst Formation 的小型鱼幼鱼化石，这些化石保留了多种软组织特征，保留为明显的深色污渍。为了检查这些软部件的元素和分子组成，我们结合使用了飞行时间二次离子质谱 (ToF-SIMS)、扫描电子显微镜 (SEM) 和能量色散 X 射线光谱 (EDS)。我们的分析表明，保存的结构包含化学上可识别的真黑色素，与密集聚集的微体密切相关，这些微体在形态上与黑素体细胞器一致。因此，我们得出结论，碳质结构代表了最初被黑化的身体部位的痕迹，包括眼睛和腹膜。在许多现存的硬骨鱼幼虫中可以看到类似的色素沉着模式，这些幼虫在远洋环境中使用半透明作为伪装手段，以表明与 Stolleklint 粘土鱼化石相似的视觉外观。这反过来表明，在始新世开始时，隐蔽和紫外线防护的适应性已经进化，特别是在以极端温室气候为特征的时间间隔内，当时全球鱼类群的组成变得越来越现代。