Scientists have discovered a 100-million-year-old chunk of amber that captures what they say is the oldest evidence of sexual reproduction in a flowering plant.

The now-extinct but well-preserved flower, named Micropetasos burmensis, gives researchers a peek at some of the biodiversity that began covering the Earth around the Cretaceous Period, which lasted about 79 million years, from about 145 million years ago to 66 million years ago.

The scene that's frozen in the fossilized tree resin shows a cluster of 18 tiny flowers, with one of them in the process of making some new seeds during a period when dinosaurs still roamed the planet.

Microscopic images reveal pollen tubes growing out of two grains of pollen and penetrating the flower's stigma.

The process of the male pollen tube entering the female reproductive stigma is the same sexual reproductive process used by modern flowering plants, which are known as angiosperms, explained George Poinar Jr., a professor emeritus in the Department of Integrative Biology at Oregon State University, in a press release.

"In Cretaceous flowers we've never before seen a fossil that shows the pollen tube actually entering the stigma," Poinar said in the 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."

The newly described fossils were discovered in mines in Burma, also known as Myanmar.

The research from Oregon State University and Germany was published in the Journal of the Botanical Institute of Texas.