Abstract: Recent phylogenomic studies have confirmed that Scirtidae is one of the earliest-diverging groups of polyphagan beetles. Cretaceous fossils and genome-scale data have shown promise in elucidating the evolutionary history of Scirtidae. However, knowledge about the Mesozoic diversity of scirtids remains limited, and a recent phylogenomic study of Australasian Scirtinae failed to consider among-site compositional heterogeneity. In this study, we present a refined phylogeny of Scirtinae by analyzing ultraconserved element data under the better-fitting site-heterogeneous CAT-GTR+G4 model. A new scirtine fossil, Serracyphon philipsi gen. et sp. nov., is reported from mid-Cretaceous Kachin amber. This fossil is characterized by serrate antennae, uncarinated antennomere 1, absence of subocular carinae, and absence of a buttonhole on subgenal ridges. The placement of Serracyphon is evaluated within our updated phylogenomic framework for scirtine evolution. Additionally, we critically reevaluate the taxonomy of the “Scirtes” fossils previously described from the Eocene of the Isle of Wight.

Key words: fossil, Kachin amber, phylogenomics, Scirtinae.