Fossil Onagraceae flower and insects with in situ or adhered pollen from the Eocene of Eckfeld, Germany

The Onagraceae or evening-primrose family has a fossil record composed mainly of dispersed pollen that has been discovered in Late Cretaceous to Holocene sediments around the globe. The pollen record suggests the family reached a cosmopolitan distribution during the Eocene. Currently, there is no reliable Onagraceae leaf record, and the meagre mesofossil record is composed of only a few fruits/seeds of Circaea and Ludwigia from the Oligocene to Pliocene of Eurasia. There is also a unique fossil Fuchsia flower that was described from the early Miocene of New Zealand, but other than that, there are no fossil Onagraceae flowers known to date. In addition, Onagraceae pollen has never been found adhering to fossil insects, and as such, there is no direct evidence of which insects visited Onagraceae flowers prior to modern times. Here we present an exceptional finding, an Onagraceae flower bud of Eocene age, from Eckfeld in Germany. Due to the flower’s bud stage the stamens were still packed with pollen. Nevertheless, the in situ pollen enabled us to assign the flower to the genus Ludwigia, based on a combination of unique morphological and ultrastructural traits observed with combined LM, SEM, and TEM, making it one of the earliest records of this genus. More importantly, we also discovered the same Ludwigia-type pollen adhering to the exterior of two different fossil beetles, a Buprestidae and Scarabaeidae, from the same locality. These provide the first-ever direct evidence for paleo-flower-insect visitation in Ludwigia and Onagraceae. Interestingly, we did not discover any Ludwigia-type pollen on the several Hymenoptera fossils investigated during this study, but Hymenoptera are the main flower visitors and pollinators of Ludwigia at present. These findings might suggest that beetles were the main flower visitors and potential pollinators of European Ludwigia during the Eocene and that there has been a shift in primary pollinators through the geological record.