A new Darwin wasp species, Lithoserix oublierus sp. nov. is described and illustrated from the early Oligocene limestone formation Calcaires de Campagne-Calavon in the Luberon Region, France. It represents the third species of this extinct genus, which was first described from the late Eocene Florissant Formation in Colorado, US and later found in Aix-en-Provence, France, from the late Oligocene. The taxonomic placement of this genus in the context of tribal classification is analysed and discussed, based on geometric morphometrics of the fore and hind wing venation of fossil and extant Pimplinae species. The results suggest that Lithoserix does not belong to the same group as the extinct genus Crusopimpla, but rather represents a more basal genus within Pimplini or belongs to an extinct separate tribe, closely related to Pimplini.

Calcaires de Campagne-Calavon Formation, compression fossils, Darwin wasps, fossil record, geometric morphometrics

Pimplinae is a species-rich subfamily of Darwin wasps that began to diversify in the Cretaceous (Kopylov 2009; Kopylov et al. 2010; Spasojevic et al. 2021). Currently, Pimplinae is regarded as paraphyletic (Spasojevic et al. 2021) and consists of four tribes. Pimplini, the tribe that branches off first, is much older than the other pimpline tribes and most other subfamilies in the informal group Pimpliformes (except Diplazontinae). The other three tribes group together, with Ephialtini branching off first, resulting in Delomeristini and the recently resurrected Theroniini being sister groups (Klopfstein et al. 2018). Notably, while no extinct tribes exist in Pimplinae, there are some extinct genera, such as Crusopimpla, which was first described from the Tadushi Formation in Russia (Eocene) (Kopylov et al. 2018) and later found in the Fur Formation in Denmark (Eocene) (Klopfstein 2022). Another extinct pimpline genus is Lithoserix, which was first described from the late Eocene Florissant Formation in Colorado (Brown 1986). It was originally placed in Siricidae, but later moved to Ichneumonidae (Kasparyan and Rasnitsyn 1992). The second species of Lithoserix, L. antiquus (Saussure, 1852), was found in Aix-en-Provence in France (late Oligocene) and only transferred to the genus recently (Spasojevic et al. 2022). Within Darwin wasps, Lithoserix was initially placed in Ephialtini by Kasparyan and Rasnitsyn (1992); however, this placement was questioned due to its resemblances to some Delomeristini and Theroniini, as well as to Xanthopimpla (Spasojevic et al. 2022). Moreover, many important body characters are not preserved in the Lithoserix fossils, making their tribal placement even more difficult (Spasojevic et al. 2022).

In contrast to body characters, which often exhibit varying degrees of preservation in compression fossils, wings and their venation are often consistently well-preserved, making them an important character system for identifying fossils. Moreover, recent studies have demonstrated that fore wing characteristics can be used to distinguish different subfamilies of Darwin wasps (Li et al. 2020; Meier et al. 2022; Viertler et al. 2022). Within Pimplinae, some tribes are known to possess specific hind wing vein characteristics, such as the interception of the nervellus, which is clearly above the middle in Pimplini, while in Ephialtini, it is often around or below the middle (Gauld et al. 2002). However, it can also be clearly above in quite a few Ephialtini such as Dolichomitus, Ephialtes etc.

The newly-described fossil species is from the Calcaires de Campagne-Calavon Formation in the Luberon Region of south-eastern France. This formation is around 31–30 million years old (early Oligocene) and includes numerous fossiliferous localities (Ducreux et al. 1985; Duhamel and Louchart 2020; Coster and Legal 2021). Its sedimentary deposits consist mostly of laminated limestones (Coster and Legal 2021) and its paleoenvironment is considered a quiet, calm and shallow lacustrine setting (Duhamel and Louchart 2020). The formation is known for its rich fossil finds, ranging from birds to fish, insects and plant remains (Duhamel and Louchart 2020; Coster and Legal 2021). Most fossil insects are Coleoptera and Diptera (Skartveit and Nel 2017; Nel et al. 2023), while descriptions of Darwin wasps from this location are rather scarce. There is one major publication by Nicolas Théobald (1937), providing an overview of insects in Oligocene formations of France, including two described Darwin wasps (Pimpla aquensis Théobald, 1937 and P. anomalensis Théobald, 1937) found in Céreste, a locality of Calcaires de Campagne-Calavon. However, P. anomalensis probably does not belong to Pimplinae since the areolet seems pentagonal, a feature not found in Pimplinae. Additionally, P. aquensis is difficult to assign, but it appears to have a rather petiolate or strongly tapering first tergite, which would be uncommon in Pimplinae.

In this study, I describe and illustrate the third species of the extinct genus Lithoserix. Based on a geometric morphometric analysis of wing venation, including extant and fossil species of the four pimpline tribes and the unplaced fossil genus Crusopimpla, I discuss the taxonomic placement of Lithoserix.

The new fossil species is the third species of the extinct pimpline genus Lithoserix, which lived from the late Eocene to the late Oligocene. In this study, I described the fossil species Lithoserix oublierus sp. nov. and evaluate the placement of Lithoserix within Pimplinae using of geometric morphometrics of the fore- and hind wings of extant and fossil pimpline taxa.

While the fore wing is useful to distinguish Pimplinae from other subfamilies (Viertler et al. 2022), it does not seem as straightforward between the tribes of extant Pimplinae, which all partially overlap in the bgPCA analyses. In the hind wings, however, some tribes exhibit a rather specific venation in the analysis. This is in agreement with previous suggestions that some groups can be distinguished through hind-wing venation: in Crusopimpla, the nervellus is broken below the middle (Klopfstein 2022) and, in Pimplini, it is broken way above the middle (Gauld et al. 2002). The fact that Pimplini splits in both bgPCA analyses from most Ephialtini, Delomeristini and Theroniini might support the notion that it is not closely related to the remaining Pimplinae (Klopfstein et al. 2018).

Considering the species diversity of Crusopimpla (Klopfstein 2022; Viertler et al. 2022; Manukyan 2023) and its substantial dissimilarities of the wing venation from extant tribes and Lithoserix (see results), it is plausible that Crusopimpla may belong to an ancestral or basal pimpline tribe.

For Lithoserix, important characteristics, such as the clypeus shape or the tarsal claws, are missing to make a confident tribal assignment, as was already discussed with the two previously-described Lithoserix species (Spasojevic et al. 2022). However, based on the wing analyses conducted herein, Lithoserix should not be placed in the same tribe as Crusopimpla and also an affiliation to Ephialtini seems improbable. The wings of Lithoserix species are most similar to Pimplini. Both groups change shape similarly with increasing size and no significant shape differences were obtained in either fore- or hind wing. The hind wing with the nervellus intercepted high up, as found in Lithoserix williamsi and Lithoserix oublierus sp. nov., is often found in the tribe Pimplini, although it is less pronounced in Lithoserix antiquus. An argument against Pimplini is the ovipositor length of Lithoserix, which is not completely preserved in this fossil, but very long in Lithoserix williamsi and L. antiquus. This is rather rare in Pimplini, but frequent in Ephialtini (Gauld et al. 2002). However, because Pimplinae probably originated in the Cretaceous (Spasojevic et al. 2021), it would not be surprising to find Lithoserix belonging to a stem-lineage within Pimplini or even an extinct tribe, possibly closely-related Pimplini.

To confidently place Lithoserix in Pimplini or to propose a new tribe, a more robust collection of fossils and comprehensive morphological evidence is imperative. The classification of Lithoserix presents challenges, as it appears to be situated closest to Pimplini, when considering wing venation, but not according to ovipositor length. To gain information on the evolutionary path of Pimplinae or Darwin wasps, in general, more fossils need to be evaluated.

However, this might be a difficult task. There is not only a lack of researchers studying the astonishing diversity of extant Darwin wasp, but even more so of people working with their fossil taxa. It is probably not rare that undescribed Darwin wasp fossils are labelled “wasp” or “Hymenoptera” in natural history collections and we can only speculate how many fossils in this group are currently overlooked.

I want to thank Tamara Spasojevic (Natural History Museum Basel, Switzerland) and Seraina Klopfstein (Natural History Museum Basel, Switzerland) for valuable feedback and comments on the species Lithoserix oublierus sp. nov. Special thanks to Bastien Mennecart (Natural History Museum Basel, Switzerland) and Loic Costeur (Natural History Museum Basel, Switzerland) for taking Tamara Spasojevic and me along to the excavation site in Murs, where we could experience to “dig out” fossil insects first-hand. I am grateful to Pauline Coster (Réserve Naturelle Nationale Géologique du Luberon, France), who helped us on the site and provided access to this fossil, which was waiting for discovery. Many thanks to André Nel (Muséum National d’Histoire Naturelle Paris, France) and Corentin Jouault (Muséum National d’Histoire Naturelle Paris, France) for identifying the fossil as Darwin wasp. A special thanks also to Fabrizia Ronco (University of Basel, Switzerland) for valuable feedback on the wing analyses, to Emmanuel Robert (University of Lyon, France) for providing images of the holotypes of Pimpla (?) anomalensis and P. aquensis, enabling a comparison with this new fossil species and to Adranik Manukyan and Tiago Belintani for their useful comments, which improved the manuscript.