Oldest insect resource pulses revealed by fos

image: Three fossil surface pieces with mayfly swarms from the Shiti Formation in southern China. All have the same scale and orientation.
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Resource pulses, that is, occasional episodes of short-lived resource gluts, represent a fundamental mechanism by which energy, nutrients, and biomass are transported through ecotones. They are widespread in existing ecosystems; however, little is known about their deep time record.

Recently, ZHANG Qianqi, a doctoral student at the Nanjing Institute of Geology and Paleontology of the Chinese Academy of Sciences (NIGPAS), under the supervision of Profs. WANG Bo and ZHANG Haichun, and collaborators reported such a resource pulse – the oldest known mayfly swarm – at a recently discovered fossil locality in the Xiwan Basin of Hezhou City, Guangxi Zhuang Autonomous Region. , in southern China.

The results were published in Geology and Historical biology.

Mesozoic research in the Xiwan Basin has a long history, with reports of numerous insect fossils from the Lower Jurassic Shiti Formation. However, the newly discovered fossil locality in the basin features an abundance of new insects as well as plants and shark egg capsules.

“In view of the abundance of various insects and plants, we propose that the fossil assemblage of the lower unit of the Shiti Formation be named Xiwan Biota, which is among the richest Jurassic biota in China,” said ZHANG Qianqi.

In a fossil layer in the Xiwan Biota, researchers found hundreds of mayflies. These mayflies have been classified as a new taxon, Jurassephemera zhangi Zhang et al. (2022) and assigned to the extinct family Sharephemeridae, a stem group of mayflies in terms of taxonomic position. This is the first discovery of this family of mayflies in China, and it is the best-preserved fossil of the family.

“We measured the orientations of 381 mayflies and found that these mayflies show no obvious directionality, although the diagram in pink suggests a south-westerly trend which may reflect the effect of a slight movement of water from the bottom after the accumulation of carcasses,” said ZHANG Qianqi.

Additionally, all mayflies are complete with body, appendages, and wings attached, which when taken with the above indicates that the mayflies were not carried a significant distance in water. after death and were buried in a low energy preservation environment. .

Extant mayflies spend most of their lives in the aquatic environment as nymphs, and adults usually live 1–2 hours to a few days. During their short adult phase, the males form dense aggregations and the females must find mating partners while flying into and through large swarms, where they copulate and eventually find a suitable place to lay their eggs.

Mating swarm behavior was previously known only in crown mayflies; however, the finding of this study reveals that such complex behaviors were already well established in stem group mayflies in the early Jurassic, representing the first evidence for mating swarm behavior in insects.

Elemental energy spectroscopy and Raman component analysis showed that the Xiwan Biota fossils are mainly covered with iron oxides and clay mineral residues, but the surface layer composition of the fossils was still dominated by carbon compounds.

Aquatic insects play an important role in aquatic food webs, acting as consumers of aquatic plants and animals. In return, they are eaten by fish and other predators. But aquatic insects can also be important parts of terrestrial food webs when they emerge from the water as adults and fly to disperse and find mates. In addition to providing food wealth to predators, emerging insects can also have a fertilizing effect on plant communities near lakes and streams as they die and decompose.

This study shows that some Jurassic mayflies emerged all at once in large swarms, representing a “pulse” of insects moving from water to land. This impulse likely resulted in massive ecosystem fluxes into riparian habitats with impacts on basal ecology and biogeochemical cycling. Such a mechanism may play an important role in transporting nutrients from aquatic ecosystems to surrounding terrestrial ecosystems, while this link between aquatic and terrestrial ecosystems may be a key novelty in Mesozoic lake ecosystems.

“Our finding highlights the underappreciated ecological importance of insects in short-lived food bonanzas and mass mortalities in deep-time lake ecosystems,” ZHANG Qianqi said.

This research was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences and the National Natural Science Foundation of China.

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