Abstract
The Late Palaeozoic insect superorder Palaeodictyopterida exhibits a remarkable disparity of larval ecomorphotypes, enabling these animals to occupy diverse ecological niches. The widely accepted hypothesis presumed that their immature stages only occupied terrestrial habitats, although authors more than a century ago hypothesized they had specializations for amphibious or even aquatic life histories. Here, we show that different species had a disparity of semiaquatic or aquatic specializations in larvae and even the supposed retention of abdominal tracheal gills by some adults. While a majority of mature larvae in Palaeodictyoptera lack unambiguous lateral tracheal gills, some recently discovered early instars had terminal appendages with prominent lateral lamellae like in living damselflies, allowing support in locomotion along with respiratory function. These results demonstrate that some species of Palaeodictyopterida had aquatic or semiaquatic larvae during at least a brief period of their post-embryonic development. The retention of functional gills or gill sockets by adults indicates their amphibious lifestyle and habitats tightly connected with a water environment as is analogously known for some modern Ephemeroptera or Plecoptera. Our study refutes an entirely terrestrial lifestyle for all representatives of the early diverging pterygote group of Palaeodictyopterida, a greatly varied and diverse lineage which probably encompassed many different biologies and life histories.
Highlights
While the fossil record of hexapods extends to the Early Devonian, the first tangible evidence of aquatic insect specializations is documented from the Early Permian in some stem mayflies of Permoplectoptera and stoneflies (Plecoptera) [1,2,3]
The disparity of larval habitus was considerable among these lineages, from elongate slender body forms with a spined prothorax in some megasecopterans to robust onisciform larvae often bearing prominent prothoracic winglets in palaeodictyopterans
The smaller surface area of gill lamellae corresponds with limited oxygen uptake; this could have been compensated by higher levels of atmospheric oxygen during the Late Palaeozoic [32], more dissolved oxygen in the water
Summary
While the fossil record of hexapods extends to the Early Devonian, the first tangible evidence of aquatic insect specializations is documented from the Early Permian in some stem mayflies of Permoplectoptera and stoneflies (Plecoptera) [1,2,3]. The prior records of stem-group representatives of Ephemeroptera and Odonata with putatively aquatic larvae, are documented since the Late Carboniferous [4], the evidence is uncertain, perhaps with the exception of a meganeurid griffenfly larva Dragonympha srokai bearing lateral abdominal tracheal gills from the Late Carboniferous of the Mazon Creek ‘Konservat-Lagerstätte’ [5]. Based on their morphological specializations, most aquatic Permian immature and adult insects indicate lotic palaeoenvironmental conditions while the evidence from lentic habitats is lacking prior to the late Permian [6,7]. Wootton [6] considered that the increased role of insects in lake ecosystems, such as aquatic Heteroptera and Coleoptera, was due to the appearance of aquatic macrophytes in the early Mesozoic
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