PREMISE Paleontologists use tooth form to assess diets of fossil mammals. Plants would also be expected to adapt their morphology to respond to herbivory. Fossil nettle leaves with definitive stinging trichomes (tribe Urticeae, family Urticaceae) are described from the early Eocene upland lacustrine floras of the Okanogan Highlands, British Columbia, Canada. This is the first report of stinging trichomes in the fossil record. Their occurrence in western North America at a time of major large herbivorous mammal radiation suggests they acted, as they do today, as a deterrent for mammal herbivory. METHODS Fossil leaf compressions and extant leaves were photographed with standard methods. Focus-shift stacking was used to layer photos of the fossil leaves. RESULTS Urticaceous fossil leaves from the Okanogan Highlands greatly resemble their modern relatives in leaf morphology and particularly in both stinging and nonstinging trichomes. Nettles are common components of the flora of the Volcanoes National Park in Rwanda. This region is used as a modern analogue for the Okanogan Highlands, based on comparable elevation, equable conditions that host both similar floras and large folivores. CONCLUSIONS Nettles in tribe Urticeae (Urticaceae) producing leaves with stinging and nonstinging trichomes were already present in the early Eocene of western North America at a pivotal time during the early radiation of modern mammalian herbivore groups. They offer tantalizing evidence of a selective response that plants may have developed to protect themselves from the evolving mammalian herbivores of that time.

中文翻译：

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带有刺毛状体的荨麻科叶子已经存在于加拿大不列颠哥伦比亚省早始新世奥卡诺根高地

前提古生物学家使用牙齿形状来评估化石哺乳动物的饮食。植物也有望调整其形态以应对食草动物。从加拿大不列颠哥伦比亚省奥卡诺根高地的早始新世高地湖泊植物群中描述了具有明确刺状毛状体（荨麻科，荨麻科）的荨麻叶化石。这是化石记录中关于刺毛状体的第一份报告。它们在主要大型食草哺乳动物辐射时期出现在北美西部，表明它们像今天一样对哺乳动物食草动物起到了威慑作用。方法 用标准方法拍摄化石叶压缩和现存叶。焦点偏移堆叠用于对化石叶子的照片进行分层。结果奥卡诺根高地的荨麻化石叶子在叶子形态上非常类似于它们的现代近亲，特别是在有刺和无刺毛的毛状体中。荨麻是卢旺达火山国家公园植物群的常见组成部分。该地区被用作奥卡诺根高地的现代类似物，基于可比的海拔、拥有相似植物群和大型食叶动物的平等条件。结论 在北美洲西部始新世早期，现代哺乳动物食草动物群早期辐射的关键时期，荨麻科（荨麻科）中的荨麻产生带刺和无刺毛的叶子。它们提供了诱人的证据，证明植物可能已经发展出一种选择性反应，以保护自己免受当时不断进化的哺乳动物食草动物的伤害。