24 million years of pollination interaction between European linden flowers and bumble bees

Pollination is the most common insect-plant mutualism, binding them in a co-evolutionary framework. Historic evidence of this interaction can be partly inferred from time-calibrated molecular phylogenies of plant and insect lineages or directly from fossils. Fossils providing such evidence are sparse and only a few fossiliferous localities offer adequate preservation of both flowers and insects. We screened fossil flowers and bees from the Late Oligocene (Chattian) of Enspel, Germany, using white and fluorescent light, followed by palynological sampling and detailed investigation. Flowers are identified via pollen and floral morphology in comparison with modern taxa. The bumble bees are described and placed into a morphological framework with extant congeners. The pollination biology of extant Tilia is summarized and complemented by field observations. We report the new fossil species Tilia magnasepala C. Geier et Schönenb. sp. nov. (Tilioideae, Malvaceae), Bombus (Kronobombus) messegus Engel et Wappler, sp. nov., and Bombus (Timebombus) palaeocrater Engel et Wappler, sp. nov. (Apidae: Bombini). The presence of the same Tilia pollen in situ in flowers and adhering to the exterior of the bumble bees provides direct evidence for their interaction and the role of Bombus as a pollinator for Tilia by at least the Late Oligocene and persisting to the present.