POLLINATION OF CRETACEOUS FLOWERS

During the Jurassic–Cretaceous period, vegetation types on the earth were shifting significantly — angiosperms (flowering plants) rapidly replaced the dominance of gymnosperms, an event which Charles Darwin called the “Abominable Mystery”. Hypothetically, insect pollination is responsible for the majority of the world’s flowering plant diversity and is key to the Cretaceous radiation of angiosperms. Although both insects and angiosperms were common by the mid–Cretaceous, direct fossil evidence of insect pollination was lacking. Recently, we reported a specialized beetle–angiosperm pollination mode from mid–Cretaceous Burmese amber (99 Ma) in which a tumbling flower beetle (Mordellidae), Angimordella burmitina Bao 2019, has many tricolpate pollen grains attached. A. burmitina exhibits several specialized body structures for flower–visiting behavior, including its body shape and pollen–feeding mouthparts revealed by X–ray microcomputed tomography. The tricolpate pollen in the amber belongs to the eudicots that comprise the majority of extant angiosperm species. These pollen grains exhibit zoophilous pollination attributes including their ornamentation, size, and clumping characteristics. Tricolpate pollen grains attached to the beetle’s hairs are revealed by confocal laser scanning microscopy, which is a powerful tool for investigating pollen in amber. The findings provide the earliest direct evidence of insect pollination of Cretaceous angiosperms, extending the range of insect–angiosperm pollination association by at least 50 million years. The results support the hypothesis that specialized insect pollination modes were present in eudicots at least 99 million years ago.