How can you recognize a pollinating insect in the fossil record?

Pollination by insects is a decisive process for the survival and evolution of angiosperm plants (with flowers), and to a lesser extent, gymnosperms (without visible flower or fruit). There is increasing interest in studies on the origin of the relationship between insects and plants, especially in the current context of the progressive decline of pollinating insects on a global scale and its impact on food production. Pollinating insects can be recognized in the fossil record, although until now there was no protocol for their differentiation. Fossil pollinating insects have been found in both rock beds and amber beds, and it is precisely in rock beds that the first evidence of insect pollination of plants across the planet is being studied. But how can one determine which was a true pollinating insect in the past?

A study is now determining the criteria for differentiating an insect pollinator from a putative pollinator in the fossil record. This new research, which will facilitate the correct study of the origin and evolution of pollination by insects, has been led by doctoral student Constanza Peña-Kairath, first signatory of the work and member of the Faculty of Earth Sciences and the Research Institute of Biodiversity (IRBio) of the University of Barcelona (UB). Peña-Kairath is preparing the thesis under the supervision of Professor Xavier Delclòs, from IRBio, and researcher David Peris, from the Botanical Institute of Barcelona, ​​dependent on Barcelona City Council and the Spanish Higher Council for Scientific Research (CSIC). Delclòs and Peris are also co-authors of the work.

Other experts from IRBio, the Spanish Geological and Mining Institute (IGME), the American Museum of Natural History in New York (United States) and the University of Northampton (United Kingdom) have also participated in the study.

A specimen of the species Meioneurites spectabilis from the extinct family Kalligrammatidae (neuroptera). (Image: Xavier Delclòs, UB-IRBio. CC BY)

When gymnosperms dominated terrestrial ecosystems

Currently, angiosperms dominate most of the planet’s terrestrial ecosystems, but this has not always been the case: flowering plants appeared during the lower Cretaceous and diversified during the upper Cretaceous, about 100 million years ago, thus replacing the forests dominated by “gymnosperms” (conifers, ginkgos, cycas, etc.).

“Angiosperms are considered to have interacted with pollinating insects (a mutualistic relationship, with mutual benefits) since they appeared on the planet. Surely, its first pollinators were generalist insects (beetles, flies, etc.) that had previously already pollinated gymnosperms. In fact, different fossils are known in Cretaceous amber in which pollinating agents most likely already existed”, details Constanza Peña-Kairath, member of the Department of Earth and Ocean Dynamics at the UB.

How to classify an insect as a pollinator?

Studying, through the fossil record, a process as complex as insect pollination is a challenge in paleontology. In order to identify a pollinating species that inhabited ecosystems in the past, it is not possible to carry out the analyzes that are currently applied to organisms found in the natural environment (for example, the analysis of the increase in the formation of fruits in certain plants if they are visited by certain insects).

“For this reason, it is necessary to define when a fossil insect can be considered a pollinating agent and thus establish a whole set of key characteristics that can also be observed in a general way in fossils,” explains Peña-Kairath.

The study has identified 193 families of insects from ten different orders that are considered to be pollinators of angiosperms and gymnosperms. The study authors have also established when they appear in the fossil record and have developed a classification of fossil insects that have so far been described as pollinators.

By combining these scientific data, the team has developed a key to be able to differentiate fossil insects into two categories —pollinator and probable pollinator— and thus rule out those that do not present sufficient evidence of this type of mutualism with plants. Thus, in order to classify a fossil insect as a pollinator, it is necessary for the arthropod to have pollen grains attached to its body and to belong to a group of current insects considered pollinators, among other characteristics.

From the analysis of the entire fossil record, it is evident that all current orders of insects with some pollinating species already existed before the appearance of angiosperms in the lower Cretaceous. There are even examples of groups of insects that were pollinators during the Cretaceous —such as mecoptera or scorpion flies—, but that currently no longer have pollinating species.

The oldest fossil record of pollination

The conclusions of the study suggest that the oldest record of the mutualistic relationship that insect pollination supposes is related to an extinct group of neuropteran insects and originated, at least, in the Late Jurassic —about 163 million years ago— much before the appearance of the first flowering plants.

«This information is extremely relevant because it reveals that insects have had a close relationship with “gymnosperms” since very ancient times. Therefore, it is not surprising that some of these plants currently continue with this beneficial relationship”, conclude the researchers Xavier Delclòs and David Peris.

The study is titled “Insect pollination in deep time.” And it has been published in the academic journal Trends in Ecology & Evolution. (Source: UB)