Angiosperms, also known as flowering plants, originated in the Mesozoic and are the most diverse group of land plants. On July 22nd, 1879, Darwin expressed his puzzle about the early origin and fast radiation of angiosperms in his letter to the botanist Sir Joseph Dalton Hooker. He thought, the evolution and development of every taxon should abide by the fundamental principle of going from simplicity to complexity. The fossil record of flowering plants that Darwin had to rely upon was sparse and the paleobotanists at that time were ignorant of the evolutionary steps they went through. They made the mistake of relating nearly all fossil flowering plants to living genera. Thus Darwin, who was looking for evolutionary stages but found none, called their origin an “abominable mystery”. With recent research this is no longer such a mystery.
Since Darwin, numerous scholars thought insect pollination should be a key contributor to the Cretaceous radiation of angiosperms. However, direct evidence of insect pollination of Cretaceous flowering plants is lacking. Previous evidence of insect pollination of flowering plants dates to the Middle Eocene, around 48-45 million years ago. Recently, we report the discovery of a tumbling flower beetle preserved in mid-Cretaceous Burmese amber that bears evidence of insect pollination of flowering plants. The finding deepens the history of insect pollination of flowering plants by around 50 million years and suggests the existence of such mutualism at least as far back as 99 million years ago.
Figure 1 Cretaceous tumbling flower beetle and tricolpate pollen grains.
Encased in the fragment of amber, the beetle, exhibited a suite of evidence suggesting its role as a pollinator of flowering plants: a curved, compressed, and wedge-shaped body with a declined head that likely facilitated feeding inside flowers, well-developed hind legs to move between flowers, and fine hairs on the thorax and abdomen whose height and spacing are apt for holding and transporting pollen. Its mouthparts are important characters but are partly blocked by a thrip. So we use the X-ray microcomputed tomography (micro-CT) to reconstruct its mouthparts that are seemingly tailored for collecting and likely transporting pollen.
Figure 2 Microtomographic reconstruction of the beetle.
Small angiosperm pollen grains in amber, especially those buried under insect body hairs, are often not visible under optical microscopy and, thus, could be easily overlooked. In this study, we utilize the confocal laser scanning microscopy (CLSM) to detect the pollen grains between body hairs, based on the fluorescence contrast of the brighter pollen and the surrounding dark insect cuticle. We find that the beetle is covered by abundant tricolpate pollen grains that are mainly distributed on the thorax and abdomen. These tricolpate pollen grains belong to the eudicot group which comprises ∼75% of extant angiosperm species. The pollen grains' reticulate surfaces and signs of pollen clumping together suggest beetle-aided pollination.
Figure 3 Ecological reconstruction of the Cretaceous tumbling flower beetle. These tumbling flower beetles are feeding on eudicot flowers. The color of the beetles and flowers is artistic only.
Finally, our study provide multiple lines of evidence, including pollen-feeding mouthparts, pollen-carrying hairs on the body, and zoophilous pollination attributes of the tricolpate pollen, to strongly support a specialized beetle-angiosperm pollination mode. Our results support the hypothesis that specialized insect pollination modes were present in eudicots 99 million years ago. With the description of additional well-preserved fossils, our knowledge of Cretaceous insect–angiosperm mutualisms will greatly increase during the near future.