Insect-plant mutualism long before flowering plants?

The site of Chekarda, in Russia, is a fossiliferous deposit dated to the Lower Permian, around 280 million years (Ma) ago. This locality is an important site compared to other deposits dated from the same period because it provided in fossil form, an important terrestrial fauna: 260 species of insects and many groups of plants. From this site, Russian and Polish researchers describe in a study in the journal Current Biology ten insects of the protomeropid group, which are among the oldest known holometabola, as these insects are called whose larvae are very different from adults. However, they have a particular morphological trait that distinguishes them from other insects known from the site…

A funny trunk

These insects have a proboscis. This trait, found in many animals, appeared relatively early since insects from the Carboniferous period (325 Ma) already possessed them, some of which were contemporaneous with those in this study, such as Hemiptera (an order of insects within of which cicadas are found). However, all these insects have a proboscis allowing them to pierce the tissues of plants, their seeds, and even the cuticle of other arthropods. This fauna from the end of the Paleozoic thus differs radically from the Chekarda protomeropids: their own mouthparts lack elements that allow them to pierce tissue. They could therefore only absorb fluids on the surface of a support. This discovery thus makes them the oldest insects to possess this type of mouth device. The oldest known to date were, for example, neuroptera (such as ants) dated to the mid-Jurassic (170 Ma), i.e. a difference of more than 100 Ma.

Photos of two fossils of proboscis insects that could only feed on fluids. They lived 280 Ma ago in present-day Russia. Credit Khramov et al. / Current Biology.

Among the first pollinators?

With this kind of trunk, “se feeding on sugary gymnosperm fluids is the only function imaginable“reminds to Science and Future Alexander Khramov, the lead author of the study. By feeding, the protomeropids could have contributed to the pollination, and therefore to the reproduction of plants, by bringing the pollens (the male gametes) to the ovum (the female gamete). This is the case of peltaspermales, plants that developed in an environment with an arid climate during the same period as the insects in the study. These plants have the particularity of having female reproductive organs, which remain little open to the external environment. Wind pollination therefore probably had only a limited effect on these plants. Like today, other factors most likely played a bigger role, such as flying insects.

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