Evolution of filter‐feeding in aquatic insects dates back to the Middle Triassic: new evidence from stem‐group mayflies (Insecta, Ephemerida) from Grès à Voltzia, Vosges, France

Abstract

In the aquatic insects, mayfly nymphs have developed a remarkable diversity of life strategies and feeding habits. However, the early evolution of these strategies and the role of mayfly nymphs in freshwater palaeoecosystems remain largely unknown. We investigated fossil mayfly nymphs from the Gr es a Voltzia Formation from the Middle Triassic, France, which represents the oldest known outcrop yielding a diverse mayfly fauna. We focused on three species, namely Mesoplectopteron longipes, Vogesonympha ludovici and Triassoephemera punctata, and investigated both original type material and numerous previously unstudied specimens. As a result, we provide a new interpretation of their morphology, life histories, and systematics: the nymphs of M. longipes, V. ludovici and T. punctata were aquatic and at least the former two species undoubtedly lived in flowing water as passive filter-feeders. This finding represents the so far earliest occurrence of this feeding strategy in aquatic insects. The most obvious morphological adaptations to this lifestyle are long filtering setae arranged in rows on forelegs, analogous to some modern mayfly nymphs. We place the three investigated species M. longipes, V. ludovici and T. punctata in a newly established, unranked higher taxon, Sinebranchia, representing the sister group to Heptabranchia (Coxoplectoptera + Ephemeroptera). Sinebranchia share a single-segmented tarsus synapomorphic with Heptabranchia, but retain the plesiomorphic condition of double tarsal claws. Sinebranchia are also characterized by their autapomorphic lack of tracheal gills, probably connected to their life in flowing, welloxygenated water.

Early detritivory and sedimentivory in insects based on in situ gut contents from Triassic aquatic nymphs

“…Differences in the nature of the pool sediments was not likely to be the cause of the absence of gut content in the mayfly nymphs, considering that the insects in Grès à Voltzia have also been found in very fine‐grained laminated clay (Briggs & Gall 1990) and are mostly articulated, indicating limited biostratinomic transport, similar to the case of Pedra Alta. Two species from Grès à Voltzia, Mesoplectopteron longipes Handlirsch, 1918 and Vogesonympha ludovici Sinitshenkova & Papier in Sinitshenkova et al ., 2005, were recently regarded as passive filter feeders by Sroka & Staniczek (2022), based on their double row of pronounced setae along the prothoracic leg and their dense mouthpart setation. Details enabling the determination of feeding habits with confidence have not been described from other Grès à Voltzia mayfly nymphs, but they probably were detritus collectors or algal feeders, while burrowing forms could filter detritus (Sinitshenkova et al .…”

“…2005; Lukashevich et al . 2010; Sroka & Staniczek 2022), a detailed analysis of the insect preservation is yet to be carried out.…”

Fluctuation in the diversity of mayflies (Insecta, Ephemerida) as documented in the fossil record

“…We restricted our analysis to Ephemerida 51 , 52 , containing the order Ephemeroptera and its close stem-group (Coxoplectoptera, Sinebranchia, and Permoplectoptera). We excluded Carboniferous members of the stem-group (Syntonopteroidea) due to their unclear phylogenetic affinities and the uncertain presence of aquatic larvae in this group.…”