Paleontologists have found an exceptionally preserved short-winged flower beetle and associated pollen aggregations and coprolites in a piece of mid-Cretaceous amber originating from northern Myanmar. The discovery provides the direct evidence of pollen-feeding in a Cretaceous beetle and confirms that diverse beetle lineages visited early angiosperms (flowering plants) in the Cretaceous period.
Ecological reconstruction of Pelretes vivificus in the Burmese amber forest; the flowers in the reconstruction are based on Lijinganthus revoluta. Image credit: J. Sun.
Beetles are often cited as likely candidates for the earliest pollinators of angiosperms due to their long evolutionary history.
Ii has been suggested that early associations between beetles and angiosperms in the Cretaceous played a key role in the diversification of both groups.
Until now, pollination in beetles has been determined only on the basis of amber inclusions being preserved alongside pollen grains, possessing morphological features interpreted as possibly facilitating pollination, and having living relatives that are known to feed on pollen.
The newly-identified pollen-feeding beetle, named Pelretes vivificus, lived 98.2 million year ago in what is now Myanmar.
Its closest relatives are short-winged flower beetles (family Kateretidae) that today occur in Australia, visiting a diverse range of flowers and feeding on their pollen.
“Pelretes vivificus is associated with clusters of pollen grains, suggesting that short-winged flower beetles visited angiosperms in the Cretaceous,” said Professor Chenyang Cai, a paleontologist at the Nanjing Institute of Geology and Palaeontology.
“Some aspects of the beetle’s anatomy, such as its hairy abdomen, are also adaptations associated with pollination.”
The piece of amber examined by the team came from a mine in the Hukawng Valley, Kachin State, northern Myanmar.
“Besides the unparalleled abundance of fossil insects, the amber dates back to the mid-Cretaceous, right when angiosperms were taking off,” said Dr. Erik Tihelka, an entomologist and paleontologist at the University of Bristol.
Photomicrographs of Pelretes vivificus from mid-Cretaceous Burmese amber: (a) habitus, dorsal view, with inset highlighting a Tricolpopollenites pollen grain; (b) head of Pelretes vivificus, dorsal view; (c) habitus, ventral view; (d) head of Pelretes vivificus, ventral view (area indicated in c); (e) protarsus of Pelretes vivificus; (f) metatarsus of Pelretes vivificus; (g) abdominal apex of Pelretes vivificus, dorsal view (area indicated in a), with arrowheads highlighting pollen grains. Abbreviations: a1–11 – antennomeres 1–11; abd – abdomen; el – elytra; ey – eye; he – head; ma – mandibles; mp4 – maxillary palpomere 4; mtt1–5 – metatarsomeres 1–5; pg – pollen grain; mtv – metaventrite; pr – pronotum; ps – prosternum; pt2, 4 and 5 – protarsomeres 2, 4 and 5; se – sensory cell. The images in a, b, e and f were obtained under normal reflected light; the others were obtained under confocal laser scanning microscopy. Scale bars – 200 μm in a and c; 100 μm in b, d and g; 50 μm in e and f. Image credit: Tihelka et al., doi: 10.1038/s41477-021-00893-2.
While Pelretes vivificus is not the first pollinating beetle to be described from Cretaceous amber, this unique specimen preserves a bizarre clue about its diet.
The fossil is associated with beetle coprolites that provide a very unusual but important insight into the diet of short-winged flower beetles in the Cretaceous.
The coprolites are completely composed of pollen, the same type that is found in clusters surrounding the beetle and attached to its body, which suggest that Pelretes vivificus visited angiosperms to feed on their pollen.
This finding provides a direct link between early flowering plants in the Cretaceous and their insect visitors.
It shows that these insect fossils were not just incidentally co-preserved with pollen, but that there was a genuine biological association between the two.
“The pollen associated with the beetle can be assigned to the fossil genus Tricolpopollenites,” said Dr. Liqin Li, a fossil pollen specialist at the Nanjing Institute of Geology and Palaeontology.
“This shows that pollinators took advantage of early angiosperms soon after their initial diversification and visited a diverse range of groups by the mid-Cretaceous,” Professor Cai added.
The team’s paper was published in the journal Nature Plants.
E. Tihelka et al. Angiosperm pollinivory in a Cretaceous beetle. Nat. Plants, published online April 12, 2021; doi: 10.1038/s41477-021-00893-2