A New Cretaceous Family Illuminates the Early Evolutionary History of True Sawflies (hymenoptera: Tenthredinoidea)

A new family, †Eotenthredinidae, including two new species, †Eotenthredo sinensis gen. et sp. nov. and †Eotenthredo magna sp. nov., is described, based on nine adult specimens from the Early Cretaceous Yixian Formation, ~125 Ma. As the oldest known crown tenthredinoids to date, †Eotenthredinidae exhibits a mosaic of plesiomorphic and derived traits, serving as a crucial transitional taxon linking stem and crown tenthredinoids. We conduct a morphology-based phylogenetic analysis of Tenthredinoidea incorporating the most extensive sampling of Mesozoic fossils to date. Our results support the monophyly of Tenthredinoidea, recover the †'Xyelotomidae' as a paraphyletic grade and place †Eotenthredinidae as the sister group to core tenthredinoids. Morphometric analyses of the mesothorax further support the intermediate position of †Eotenthredinidae. Combining lineage-through-time analyses of plants and statistical analysis of tenthredinoid fossils, we emphasize the deep-time evolutionary association between tenthredinoids and their host plants. This study provides critical paleontological evidence for clarifying the early evolutionary history of Tenthredinoidea and deepens our understanding of plant-insect co-evolution and ecosystem restructuring during the Cretaceous Terrestrial Revolution from the insect perspective.