The thorax of E ubleptus illuminates evolutionary history of winged insects.

The hexapod thorax represents a specialized tagma for body locomotion, and its formation was one of the crucial steps in arthropod evolutionary history. In insects, the thoracic morphology was crucial for the acquisition of wings, which was closely connected with the reinforcement of the lateral thoracic wall (pleuron). Here, we uncover the structure of the pleuron for the first time in a Carboniferous palaeodictyopteran, Eubleptus danielsi from the Mazon Creek Lagerstätte (Illinois, USA). We reconstructed a 3D model of E. danielsi and compared flight-related traits with a broad range of extant insect taxa. The thoracic pleuron is strikingly different across the main pterygote lineages. Whereas the pleura of Odonata and Ephemeroptera exhibit specialization with many autapomorphic characters, the arrangement observed in E. danielsi actually resembles the pattern seen in Neoptera. It is plausible that this represents a pattern close to that of the protopterygote, the hypothetical common ancestor of Pterygota.