An international team of researchers has found the oldest evidence of sexual reproduction in a flowering plant. Researchers from Oregon State University (OSU) and Germany said that the evidence was preserved in a 100-million-year old piece of amber.

The amber, discovered in the Hukawng Valley mines of Myanmar, consists of a cluster of 18 tiny flowers from the Cretaceous period, with one of the flowers in the process of producing new seeds. The sexual reproduction observed in the amber is similar to what is found in present-day flowering plants or 'angiosperms.'

The researchers said that the cluster of flowers from the now extinct plant was found in a 'remarkable' and complete condition. They belonged to plants that were 'quite small.' The pollen of the flowers were found 'sticky' that suggest they were carried by a pollinating insect. 

"In Cretaceous flowers we've never before seen a fossil that shows the pollen tube actually entering the stigma," George Poinar, Jr., a professor emeritus in the Department of Integrative Biology at the OSU College of Science, said in a press release. "This is the beauty of amber fossils. They are preserved so rapidly after entering the resin that structures such as pollen grains and tubes can be detected with a microscope."

During the Cretaceous period, the plant life was mostly comprised of conifers, ferns, mosses, and cycads. In the same period, different kinds of mammals, birds and flowering plants were beginning to emerge. The period was however dominated by dinosaurs.

"The evolution of flowering plants caused an enormous change in the biodiversity of life on Earth, especially in the tropics and subtropics," Poinar said.

"New associations between these small flowering plants and various types of insects and other animal life resulted in the successful distribution and evolution of these plants through most of the world today. It's interesting that the mechanisms for reproduction that are still with us today had already been established some 100 million years ago."

The findings have been published in the Journal of the Botanical Institute of Texas.