The grains recovered from drilling cores are the oldest known fossils of the direct ancestors of flowering plants, the researchers report in the journal Frontiers of Plant Science.
Flowering plants evolved from extinct earlier plants related to conifers, ginkgos, cycads, and seed ferns, and that was thought to have occurred around 140 million years ago during the Cretaceous period, the researchers said.
But the newly discovered fossilized pollen grains from flowering plants have been shown to be around 250 million years old, they said, putting them firmly in the Triassic era.
There were found in drilling cores from northern Switzerland, which in the Triassic period was in the subtropics.
"We believe that even highly cautious scientists will now be convinced that flowering plants evolved long before the Cretaceous," researcher Peter Hochuli of the University of Zurich said.