Quantitative morphological analysis reveals a greater morphological diversity in Cretaceous Helotidae (Coleoptera, Nitiduloidea), with description of a new genus and species from mid-Cretaceous Kachin amber

The family Helotidae is a small group of cucujiform beetles associated with tree sap, rotten fruits or flowers. In this study, a new genus and species of early Helotidae, Mysteriohelota metallicus gen. et sp. nov ., is described from mid-Cretaceous amber from northern Myanmar (Cenomanian, ca. 99 Ma) based on comparative morphology. Its familial placement is further confirmed through constrained morphology-based phylogenetic analyses under maximum parsimony. To test the previously hypothesised higher morphological diversity of Cretaceous Helotidae and to investigate their evolutionary patterns, diagnostic characters of helotid genera were quantified and analysed using two metrics: weighted mean pairwise dissimilarity (WMPD) and the sum of variance (SOV). Rarefaction-based sample-size standardisation was applied to compare the estimates of morphological disparity between extant and Cretaceous Helotidae. The results reveal that the Cretaceous helotid beetles exhibit significantly greater morphological disparity than their extant descendants, suggesting that this small cucujiform family was much more diverse during its early stage of evolution but underwent a loss of taxonomic diversity across time. The present study facilitates the understanding of the palaeobiodiversity and macroevolutionary trajectory of Helotidae.