The beetle family Mysteriomorphidae (Coleoptera: Elateroidea) was recently described based on a single species from the mid-Cretaceous Burmese amber. Only six specimens of an unknown sex were reported in previous studies. Here, for the first time, we examine female specimens of Mysteriomorphus Alekseev and Ellenberger, 2019 for which we describe the morphology of the ovipositor and report striking differences between them in several diagnostic characters. Additionally, we synonymize the recently described genus Trapezioceps Qu, Jarzembowski and Luo, 2023, syn. nov. with Mysteriomorphus. The monospecific Trapezioceps, also known only from Burmese amber, was erroneously classified in Cantharidae. We tentatively keep the species originally described in Trapezioceps valid, hence Mysteriomorphidae currently contain two described species: M. pelevini Alekseev and Ellenberger, 2019 and M. longelytrus (Qu, Jarzembowski and Luo, 2023), comb. nov. Based on the new findings, we discuss the morphology, variability, and diversity of Mysteriomorphus.(c) 2023 Elsevier Ltd. All rights reserved.