An international research team led by the Department of Botany and Biodiversity Research at the University of Vienna has made an extraordinary discovery: fossilised lime blossoms and fossilised bumble bees were found in 24-million-year-old sediments at the Enspel Fossil-Lagerstätte (Rhineland-Palatinate, Germany) – along with evidence of their interaction in the form of preserved pollen grains. These findings show that bumble bees were already among the most important pollinators of linden (or lime/basswood) trees millions of years ago – just as they are today. In view of the current global decline in insect populations and of pollinators such as wild bees, it is becoming increasingly important to understand the origins and evolution of flower pollination. The results of the study are published in the journal New Phytologist.

"We examined hundreds, even thousands, of fossil flowers and insects for pollen in the hope of gaining insight into the evolution of flowers, flower visitors and pollination," explains Friðgeir Grímsson, project leader and last author from the Division of Structural and Functional Botany at the University of Vienna.

Old fossils – new methods

These microscopic pollen grains were made visible using UV and blue light and then extracted either individually or in clumps from flowers or the insects' hair using a very small, fine needle in a minimally invasive procedure. After careful cleaning of the pollen grains, they were analysed using high-resolution light and electron microscopy. The result of the pollen study: some of the flowers examined came from the linden tree, and numerous bumblebees had visited linden flowers before they drowned in a former volcanic crater lake and fossilized.

Three new species described

"The newly discovered linden flowers were named Tilia magnasepala, meaning “linden tree with large sepals", explains Christian Geier, lead author and doctoral student at the Division of Structural and Functional Botany at the University of Vienna. Two new bumble bee species were also named: Bombus (Kronobombus) messegus and Bombus (Timebombus) paleocrater. Their names refer to their old age, their morphological characteristics and the location where they were found. The fossils were found during excavations in the lake sediments of the former volcanic lake near Enspel in Rhineland-Palatinate, Germany.

Fossil flowers and their pollinators described for the first time from the same sediments

The linden flowers described are the first of their kind from the European continent to be recorded and described according to palynological criteria – i.e. the science of pollen. The fossil bumble bees are among the oldest representatives of their genus; only one species from Colorado (USA) is older. "This is the first time worldwide that a fossil flower and its pollinating bees have been described from the same sediments and directly linked to each other using pollen. This type of research has great potential to shed light on our knowledge of pollinators of the past," Geier summarises.

Learning from the past for the future

The fossil record provides insights into dynamic developments in the past: changes due to climate change, species extinction and evolutionary adaptations.

The analysis of fossil animal and plant groups allows conclusions to be drawn about their behaviour and their response to environmental changes. "In our study, we were able to identify a certain degree of flower constancy in the bumble bees we studied. This means that they only visit one type of plant during a single flight," explains Geier. "Findings like these are important for better understanding the delicate interplay and resilience of today's ecosystems."

The study was conducted under the lead of the University of Vienna in collaboration with experts from Germany (Senckenberg Research Institute and Natural History Museum and Hessisches Landesmuseum Darmstadt) and the USA (American Museum of Natural History) and was carried out as part of the FWF project "Flower Power: can in-situ pollen link fossil plants to floral visitors? (FWF-Projekt P 34303-B)".