The remarkable evolutionary success of the Holometabola is closely linked to the evolution of complete metamorphosis, which enabled larvae, pupae, and adults to exploit different ecological niches during ontogeny. One highly specialised adaptation of small, vermiform holometabolous larvae is leaf mining, an endophagous feeding strategy that allows exploitation of internal plant tissues while providing shelter, a stable microclimate and protection from predators. However, the evolutionary origin of leaf mining remains poorly understood because unequivocal fossil evidence has been exceedingly rare.

We provide the oldest evidence of Asteronomus leaf mines, together with the associated oviposition sites, from the Middle Pennsylvanian of the Piesberg near Osnabrück, northwestern Germany (latest Moscovian; Asturian, Westphalian D). We establish two new ichnotaxa for the components of this compound trace fossil: the ichnospecies Asteronomus piesbergensis M.Laaß, L.Luthardt, A.Leipner et R.Rößler isp. nov. for the mines, and the ichnogenus and ichnospecies Cavatovoidus nervicola M.Laaß, L.Luthardt, A.Leipner et R.Rößler igen. et isp. nov. for the associated oviposition scars. Additional records of Asteronomus isp. are reported from the latest Pennsylvanian of the Saale Basin, central Germany, and several early Permian localities in the Thuringian Forest, central Germany. All leaf mines are associated with pteridosperm foliage, indicating that specific seed plants (Medullosales and Peltaspermales) served as suitable host substrates for Paleozoic leaf-mining insects. Based on this observation, we hypothesise that the early evolution of leaf-mining behaviour was strongly linked to the macroevolution of their host plants, with the first occurrence of complex laminar megaphylls around the Devonian–Carboniferous transition and the early radiation of seed plants in the Mississippian as major benchmarks.

Our findings extend the fossil record of leaf mining back to the Middle Pennsylvanian (Moscovian), demonstrating that this specialised feeding strategy evolved much earlier than previously recognised. In addition, our study extends the stratigraphic range of the ichnogenus Asteronomus from the Middle Pennsylvanian to the Cisuralian, providing new insights into the early evolution and persistence of leaf-mining insects during the late Paleozoic.