Ecomorphological diversification of the Late Palaeozoic Palaeodictyopterida reveals different larval strategies and amphibious lifestyle in adults

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.

Fossil dragonfly-type larva with lateral abdominal protrusions and implications on the early evolution of Pterygota

“… (C) SNSB-BSPG 2021 XII 4 described in this paper, presumably a later stage of A. filicauda . (D) Adult representative of Corydaloides scudderi (Palaeodictyopteroidea), wings omitted (based on Prokop et al., 2019 ). (E) Adult representative of Branchioperla ianstewarti (Plecoptera), wings omitted (based on Sroka and Staniczek 2020 ).…”

“… (D) Adult representative of Corydaloides scudderi (Palaeodictyopteroidea), wings omitted (based on Prokop et al., 2019 ). …”

“…Many immatures of Palaedictyopteroidea possessed prominent lateral protrusions on the abdominal segments ( Kukalová-Peck 1978 ; Garwood et al., 2012 ; Haug et al., 2016 ; Kiesmüller et al., 2019 ; Prokop et al. 2016 , 2019 ). Yet, we largely lack a clear indication of these structures bearing/covering gills, or functioning in this way ( Prokop et al., 2016 ) and in most cases, the protrusions appear to be simple lateral extensions of the tergites.…”

“…Yet, we largely lack a clear indication of these structures bearing/covering gills, or functioning in this way ( Prokop et al., 2016 ) and in most cases, the protrusions appear to be simple lateral extensions of the tergites. Only one adult specimen preserves structures laterally on the abdomen that are reminiscent of gills ( Prokop et al., 2019 , their Figure 4; Figures 2 D and 2G). This was interpreted as retention of larval features into the adult phase, as known in quite a number of extant species ( Prokop et al., 2019 , their Figure 5).…”

“…Grimaldi and Engel 2005 ; Prokop et al., 2016 ). Yet, newer findings indicate possible aquatic immatures also in this lineage ( Prokop et al., 2019 ).…”