A Cretaceous Chafer Beetle (Coleoptera: Scarabaeidae) with Exaggerated Hind Legs—Insight from Comparative Functional Morphology into a Possible Spring Movement

SIMPLE SUMMARY: Exaggerated morphological structures are fascinating for evolutionary biologists and the public, and scarab beetles in particular are famous for their diverse exaggerated characters. Here, we report a new genus and species of Mesozoic scarab beetle with unusually robust and structured hind legs and fine color marking patterns on the dorsal and ventral surfaces of the body. Based on morphological characters, we performed phylogenetic and morphometric analyses. The results support the placement of this new taxon in the pleurostict lineage of scarab beetles, consequently representing one of its earliest records. We hypothesize that the exaggerated leg structures supported springing movements and fighting. Furthermore, the unusual marking patterns of this fossil suggest that the new taxon exhibited diurnal foraging behavior, potentially visiting leaves or flowers of Lower Cretaceous plants. This study provides new insights into the exaggerated structures of Mesozoic insects and the timing of the evolution of this diverse beetle family. ABSTRACT: The phenomenon of exaggerated morphological structures has fascinated people for centuries. Beetles of the family Scarabaeidae show many very diverse exaggerated characters, for example, a variety of horns, enlarged mandibles or elongated antennal lamellae. Here, we report a new Mesozoic scarab, Antiqusolidus maculatus gen. et sp. n. from the Lower Cretaceous Yixian Formation (~125 Ma), which has unusually robust and structured hind legs with greatly enlarged spurs and a unique elongated apical process. Based on simulations and finite element analyses, the function of these structures is hypothesized to support springing to aid movement and fighting. Based on available morphological characters, we performed phylogenetic analyses (maximum parsimony) of the main subfamilies and families of Scarabaeoidea. The results support the placement of Antiqusolidus gen. n. as a sister group of Rutelinae within the phytophagous lineage of pleurostict Scarabaeidae. Furthermore, the unusual delicate color marking patterns in the fossil specimens suggest that the new species might have been diurnal and potentially visited the leaves or flowers of Early Cretaceous plants. This morphological and functional study on this extinct scarab species provides new sights into exaggerated structures in Mesozoic insects.