Mantispidae (mantid lacewings) are a family of Neuroptera with ample fossil records extended back to the Early Jurassic. Mantid lacewings are characterized by their elongated prothoraces and raptorial forelegs. The family are divided into six subfamilies, i.e., Mesomantispinae, Doratomantispinae, Symphrasinae, Drepanicinae, Calomantispinae, and Mantispinae. Among them, Mesomantispinae and Doratomantispinae are extinct and only found in the Mesozoic, accounting for the majority of fossil mantispid genera and species, while the other four subfamilies have extant representatives, totally including more than 400 species. The extant mantid lacewings are distributed worldwide except the Antarctic. The fossil mantid lacewings have been found in multiple strata from the Jurassic to the Neogene. The oldest mantispid, Liassochrysa stigmatica Ansorge and Schluter, 1990, was collected from the Lower Jurassic in Germany, but only preserved with one forewing. Clavifemora rotundata Jepson et al., 2013, from the Middle Jurassic of China, is the earliest mantispid preserved with raptorial forelegs, which possessed stout forecoxae and forefemora, rows of short spines on the ventral of forefemora. Most genera from the Jurassic to the Early Cretaceous were assigned to Mesomantispinae. In the Late Cretaceous, 24 species of 11 genera have been descirbed from the Burmese ambers, many of which belong to the Doratomantispinae. In the Cenozoic, most fossil mantid lacewings were assigned to the extant subfamilies and genera. Mesomantispinae, as the earliest subfamily, have been found in strata from the Middle Jurassic to the Early Cretaceous in China, Russia and Kazakhstan. The raptorial forelegs of this subfamily presented the primitive stage of their evolutionary history, distinguished with elongated and stout forecoxae, stout foremora with developed spines. But the major long femoral spine, which is commonly present in the extant mantid lacewings, was absent in the Mesomantispinae. The subfamily are also distinct in their forewing venation, i.e., recurrent humeral veinlet present, the basal costal area broad, costal crossveins mostly forked, Sc and R1 fused distally, CuA pectinately forked. In this study, we describe one new genus and new species of Mesomantispinae, Yatsenia suni gen. et sp. nov., from the Lower Cretaceous Yixian Formation at Huangbanjigou village, Liaoning Province, China. The new genus and species exhibits distinct forewing venation and wing markings, including Sc and R1 fused near forewing midway, CuP dichotomously forked, 1A pectinately forked, which is distinguished from the previously reported genera of the subfamily. The new finding indicates the morphologic disparity and species diveristy of the Cretaceous mantid lacewings. Eight genera and ten species from the Mesozoic have been described and assigned to Mesomantispinae up to now, including the new genus and species, which represented the first evolutionary radiation of the family. According to the evolutionary rates based on phylogenetic tree, the family was in a phase of rapid changing of integrated morphology, foreleg structure and forewing venation during this geochronological interval. Meanwhile, the lineage number increased quickly, which indicates the synchronous accumulation of morphologic disparity and species diversity in Mantispidae during their early evolution history. This is plausibly relevant to the preceding mass extinction event at the end of the Triassic, which resulted in abundant vacant ecological niches. During the Jurassic, as the terrestrial ecosystem was recovering, it facilitated the rapid evolution of the mantid lacewings. The foreleg morphology of Mesomantispinae were testified as the primary state of their foreleg evolution. Functional morphology analyses revealed that their predatory functionalities were among the average levels of all mantid lacewings. Henceforth, the raptorial forelegs diversified into multiple specialized morphologies with their functional properties strengthened in various ways.
