Earliest fossil record of pollen-foraging megachiline bees

Le résumé fourni par la source

Megachilid bees are a diverse lineage of important pollinating insects. The high efficiency of megachilid pollination credits them as critical for ecosystem balance, angiosperm species survival, and sustaining agricultural productivity. Despite this importance, little is known about the origin and evolution of flower visitation or paleo-pollination by megachilid bees. This is partly due to the meagre fossil record of this group and the lack of methods to study paleo-pollination biology. To resolve part of this evolutionary history we examined pollen adhering to three fossil megachiline bees, Probombus nelorum Engel sp. nov. (Ctenoplectrellini), Tithonanthidium menatense Engel gen. et sp. nov. (Anthidiini?), and Megachilinae gen. et sp. indet. from the middle to late Paleocene (61.6–56 Ma) maar lake of Menat, France. Using stereomicroscopes equipped with UV- and blue-light fluorescence, dense pollen accumulations were detected in the metasomal scopae and subsamples were mechanically extracted from these pollen masses. Pollen morphology was investigated using combined light and scanning electron microscopy and the “single-grain method”. Each bee specimen preserved a dense scopal pollen load comprising five to seven pollen morphotypes representing at least 16 angiosperm taxa. These include cf. Saururaceae, Fabaceae, Rhamnaceae, Fagaceae ( Eotrigonobalanus sp.), Euphorbiaceae (cf. Acalyophoideae indet. and Rottlerinae indet.), Rutaceae ( Zanthoxylum sp.), Malvaceae (Tilioideae indet. 1 and indet. 2), Nyssaceae (cf. Mastixioideae indet.), Cornaceae ( Cornus sp.), Sapotaceae (indet. 1 and indet. 2), and three indeterminate morphotypes of eudicot pollen. Despite the diversity of pollen types associated with each bee, the quantitative composition of the extracted scopal pollen subsamples indicates that particular plant taxa dominated the pollen loads. The most abundant pollen type differ among the three bees and are represented by cf. Mastixioideae (Cornales), Sapotaceae (Ericales), or an undetermined eudicot. Extant relatives of the fossil parent plants producing the discovered pollen are presently confined to tropical and warm temperate climates, particularly in Southeast Asia. Many of the pollen represent plant taxa that have not been reported from Menat prior to this study and, therefore, our knowledge of Menat's Paleocene vegetation is widened, complementing the current dispersed palynoflora together with the leaf and fruit/seed record. Our results represent the first and oldest fossil megachiline bees preserved with adhering pollen. This links extant megachilid pollinators and their specialized foraging behaviour to extinct representatives dating back almost 60 million years.

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