A new megaspilid wasp from Eocene Baltic amber (Hymenoptera: Ceraphronoidea), with notes on two non-ceraphronoid families: Radiophronidae and Stigmaphronidae

Ceraphronoids are some of the most commonly collected hymenopterans, yet they remain rare in the fossil record. Conostigmus talamasi Mikó and Trietsch, sp. nov. from Baltic amber represents an intermediate form between the type genus, Megaspilus, and one of the most species-rich megaspilid genera, Conostigmus. We describe the new species using 3D data collected with synchrotron-based micro-CT equipment. This non-invasive technique allows for quick data collection in unusually high resolution, revealing morphological traits that are otherwise obscured by the amber. In describing this new species, we revise the diagnostic characters for Ceraphronoidea and discuss possible reasons why minute wasps with a pterostigma are often misidentified as ceraphronoids. Based on the lack of ceraphronoid characteristics, we remove Dendrocerus dubitatus Brues, 1937, Stigmaphronidae, and Radiophronidae from Ceraphronoidea and consider them as incertae sedis. We also provide some guidance for their future classification.

A new megaspilid wasp from Eocene Baltic INTRODUCTION 1. Compact mesosoma In Ceraphronoidea, the pronotum, mesopectus, metapectus, first abdominal 42 tergum, and the metanotum comprise a single, compact sclerite (Fig. 1A). This modification is only found 43 in some wingless hymenopterans, as the presence of the conjunctivae that allows mobility between the 44 above-mentioned mesosomal regions is less important in flightless hymenopterans (Reid, 1941   unknown. A single vein on the anterior wing margin is present in numerous other hymenopterans (e.g., 64 aphidiine braconids) but the triangular elements are seemingly specific to Ceraphronoidea (IM, personal 65 observation). In many Chrysidoidea the two wing veins on the anterior wing margin (costal and subcostal 66 veins) are adjacent and may superficially look "fused", but they are always separated by a faint line (Olmi,67 1994; Richards, 1939).

69
The two type specimens of the present paper are embedded in an approximately 33-55 million- as Megaspilus, a genus that has never been recorded from any fossil deposit. However, after looking at 77 the high resolution 3D data (see Supplementary 3D PDF of synchrotron-based micro-CT), we were able 78 to observe otherwise-obscured traits and determine that the new species actually belongs in Conostigmus, 79 as part of a species group that shares some key features with Megaspilus. 80 While small body size, reduced wing venation, and the presence of a pterostigma are often used to 81 determine specimens as Ceraphronoidea, these characters occur in almost all hymenopteran superfamilies.

82
Despite the above-listed clear and obvious ceraphronoid synapomorphies, it is common to find smaller 83 Ichneumonoidea and Aculeata, especially Bethylidae and Dryinidae, misidentified as Ceraphronoidea 84 in collections. Perhaps the most commonly misidentified are small specimens of the dryinid genus 85 Aphelopus (Fig. 2C). One factor that may contribute to these misidentifications is the use of low-power 86 microscopes for sorting and identification of specimens. Even the most distinct external traits specific to 87 ceraphronoids are obscure with lower magnification and inadequate lighting. In such poor conditions,    Manuscript to be reviewed that have a distinct pterostigma and, in some cases, two protibial spurs. Besides these traits, however, these two families do not share any other characteristics with Ceraphronoidea. Here, we remove them from the 97 superfamily, leaving them incertae sedis, and provide some guidance for their future classification.

99
The two specimens for the present study were obtained from the private collection of Baltic amber

155
The online version of this work is archived and available from the following digital repositories: PeerJ, 156 PubMed Central and CLOCKSS.

159
The new species belongs in Conostigmus based on the presence of a distinct sternaulus (ste: Fig. 5 A),  The anteromedian projection of the metanoto-propodeo-metapecto-mesopectal complex in Conostigmus 171 talamasi is distinct, but shorter than that of Megaspilus species.       Metasoma: S1 length vs. shortest width: S1 wider than long. Transverse carina of petiole count: other male megaspilids where the first flagellomere is either shorter or slightly longer than the scape 285 (Mikó et al., 2016;Dessart, 1972Dessart, , 1981Dessart, , 1974Dessart, , 1995Dessart, , 1997Dessart, , 1999Dessart, , 2001. from or anterior to the midpoint of the pterostigma (Fig. 1D, Masner 1993). A single fore wing vein can 297 be found in numerous hymenopteran families, but unlike in these fossils, it is always well separated from 298 the anterior margin.

299
The remaining fossil hymenopterans currently classified as ceraphronoids (Supplementary Figure   300 1) lack key ceraphronoid characteristics listed in the introduction. Although they do each possess a 301 pterostigma, the remaining wing venation traits are inconsistent with the superfamily. These taxa might 302 not even be closely related to Ceraphronoidea.