Early evolution of the stonefly lineage (Insecta, Plecoptera) revealed by a new diversity of Carboniferous stem-group species

Stoneflies represent an ancient lineage of polyneopteran insects, with stem-group representatives appearing as early as the Carboniferous (Pennsylvanian). However, their findings in the Carboniferous are extremely rare, making knowledge of the early phase of their evolution obscure. We report several newly discovered Pennsylvanian stem-stoneflies, described herein as two new genera and species within the newly established family Carbonoperlidae fam. nov. In light of this new material, we discuss the distinction between the stonefly stem and crown groups and offer insights into the diversification process of the stonefly crown group, possibly linked to environmental changes during the Late Palaeozoic. We restrict the ocurrence of the genuine stonefly stem group to the Carboniferous, being limited to the families Gulouidae and Carbonoperlidae. We propose to use the ordinal name Plecoptera for the restricted number of taxa, exhibiting a specialized ra-rp cross-vein, as present in the crown-group. For the stonefly total group, we designate a new unranked taxon, Plecopterida. The palaeolatitudes of Carboniferous stem-group occurrences were always situated in the narrow equatorial belt, whereas the Permian taxa were uniformly distributed at higher palaeolatitudes, a pattern generally followed by the distribution of stoneflies today. This process began with the progressive aridization of the equatorial belt during the Permian, likely favouring the taxa adapted to higher-latitude environmental conditions.