Urticaceae leaves with stinging trichomes were already present in latest early Eocene Okanogan Highlands, British Columbia, Canada

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.