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Flowers have been blooming on Earth for 123 million years, pollen grains reveal

They are very tiny, but they are a key source of information when it comes to Earth's evolutionary history: pollen grains are usually no larger than 20 micrometers, or 0.02 millimeters. Using these tiny particles, a research team at Leibniz University Hannover (LUH) and the University of Bonn has managed to pinpoint the earliest emergence of flowering plants, so-called angiosperms, much more precisely.

While it has so far been assumed that eudicotyledonous flowering plants, the eudicots, first appeared around 121 million years ago, the researchers have now been able to use new findings to prove that flowering plants already existed at least 2 million years earlier.

According to existing research, the very first land plants emerged in the Ordovician geological period, roughly 485 to 444 million years ago. At first there were mosses, then ferns, ginkgos and conifers. Flowering plants—now the most diverse group of land plants—did not appear until more than 300 million years later.

Researchers have now identified the oldest pollen produced by eudicot flowering plants in sedimentary successions from Portugal. Together with their team, Prof. Dr. Ulrich Heimhofer of the LUH Institute of Earth System Sciences and Dr. Julia Gravendyck of the Bonn Institute of Organismic Biology at the University of Bonn identified fossilized angiosperm pollen from coastal marine sediments deposited within the Lusitanian Basin in Portugal. They dated these deposits to approximately 123 million years ago.

How the flowering plants developed, and from which plants, remains unclear. What is considered fact, however, is that angiosperms had a lasting impact on the development of life on our planet. They significantly enriched the diversity of species on Earth.

"The emergence of flowering plants altered the biological diversity considerably," said Professor Heimhofer. "But exactly where and when this development began has been an enigma that Darwin already called an 'abominable mystery,'" said Dr. Gravendyck. It is still unclear what influence plate tectonic processes and large-scale climatic changes had as potential drivers of angiosperm development.

The pollen grains which were examined originate from sedimentary layers deposited in a shallow ocean more than 100 million years ago. Rivers carrying plant material as well as pollen grains flowed into this body of water. Sequences of sedimentary rock are usually dated using fossils.

In order to detect the specific pollen in the sediment samples, the research team first used the rare angiosperm pollen's strong fluorescence signal. With the help of high-resolution laser scanning microscopy, four individual microfossils were identified as tricolpate pollen grains.

Tricolpate refers to the morphology of the grains, which exhibit three small furrows on their external wall. Today, approximately 72% of the living angiosperm species produce tricolpate pollen. Based on their characteristic tricolpate configuration, the grains were classified as originating from flowering plants.

Fossilized seashells from the same sediment layer were also examined using strontium isotope analysis. The shells, which are composed of calcium carbonate, store the chemical signature of the seawater at the time they were formed. In consequence, the strontium isotope signature of the surrounding seawater is also archived in the shell.

This can then be compared with existing reference curves to date the formation of the seashells. The ages determined for the shallow marine sediments in this way are far more precise than conventional age dating using fossils.

By combining the strontium isotope dating of fossilized seashells with biostratigraphic information, the researchers have not only shifted back the earliest known appearance of tricolpate pollen by approximately two million years, but have also provided the most precise and reliable evidence of the first appearance of eudicotyledonous flowering plants, so-called eudicots.

The paleogeographic position of the Lusitanian Basin suggests that early forms of angiosperms, which are assumed to have developed in the tropics, were possibly more common in the mid-latitudes than previously thought.

From the perspective of the research team, the new approach could serve as a blueprint for the improved dating of fossilized plant remains and enable a better understanding of the origins and diversification of angiosperms.

The work is in the Proceedings of the National Academy of Sciences.