The crown wasps, family Stephanidae, are generally believed to occupy a distinguished position as putative relicts of the earliest-diverging lineage of apocritan Hymenoptera (e.g., Sharkey et al., 2012; Mao et al., 2015). More recent analyses have cast some confusion over this hypothesis, with the family instead appearing closer to the Evanioidea or even Trigonalyoidea (Peters et al., 2017; Tang et al., 2019). From most analyses it is clear that the family extends well into the Cretaceous, with crown-group Stephanidae estimated to have appeared by at least the Early Cretaceous and a purported ghost-stem lineage extending into the Early Jurassic or even latest Triassic (Tang et al., 2019). At least parts of such a hypothesis are consistent with the number of mid-Cretaceous fossils representing a variety of crown wasps, including species of both the plesiomorphic subfamily Schlettereriinae as well as putative Stephaninae (Engel & Grimaldi, 2004; Engel et al., 2013; Engel & Huang, 2017; Li et al., 2017). Unfortunately, while such fossil occurrences are of considerable interest, the total available record of fossil crown wasps is poor, with most species documented from the Palaeogene (Engel, 2005; Engel & Ortega-Blanco, 2008), and hitherto only four species from the Late Cretaceous. Given the potentially long gap between the first divergence of the lineage and the appearance of the crown group (Tang et al., 2019), it is precisely for such a group that early diverging stem groups would be of considerable value in resolving relationships and documenting the appearance of apomorphies within the clade. Extensive study of Early Cretaceous and Jurassic deposits for stem-group Stephanidae is necessary in order to provide direct evidence into the early evolution of this critical family of the Euhymenoptera.
Two Early Cretaceous Fossils Document Transitional Stages in Alvarezsaurian Dinosaur Evolution
