Two new species of wood wasps (Anaxyelidae), Orthosyntexis elegans gen. et sp. nov. and Orthosyntexis thanti sp. nov., are described from mid-Cretaceous Kachin amber. Orthosyntexis gen. nov. exhibits characters and character combinations unique to the family, such as the combination of two mesotibial spurs, a length ratio of forewing 1Rs to 1M<1, a length to width ratio of forewing cell 1mcu ≥ 2, and the presence of 2M+Cu in the hind wing. The new species and morphological characters allow for an exploration of anaxyelid phylogeny. The phylogenetic results indicate that Anaxyelidae are monophyletic and it is suggested to classify the genera in two subfamilies, Syntexinae and Anaxyelinae, the latter including Kempendajinae and Dolichostigmatinae.

Apomorphy, Kachin amber, morphological characters, phylogenetic analysis, Syntexinae, wing venation

Anaxyelidae are a small family of wood wasps, comprising a single extant species, Syntexis libocedrii Rohwer, 1915, commonly called the incense cedar wood wasp, which occurs in western North America (Rohwer 1915; Wickman 1967). Syntexis libocedrii lays eggs in the sapwood of coniferous trees, e.g., yellow juniper, giant red cedar, incense cedar, and probably Douglas fir, preferring recently burnt timber. The larvae bore through the wood, sometimes wreaking economically significant levels of damage (Wickman 1967; Smith 1979; Grimaldi and Engel 2005). While the family today consists of a single species, during the Cretaceous, particularly the Early Cretaceous, a significant diversity of anaxyelids has been documented. This diversity has been arranged into four subfamilies: Syntexinae Benson, 1935 including S. libocedrii and several fossil species, and three other subfamilies, i.e., Dolichostigmatinae, Kempendajinae and Anaxyelinae, known exclusively from fossils (Rasnitsyn 1968, 1969, 1980, 1990; Zhang and Rasnitsyn 2006). To date, 18 genera and 44 species of Anaxyelidae have been described (Rasnitsyn et al. 1998; Rasnitsyn and Martínez-Delclòs 2000; Zhang and Rasnitsyn 2006; Ortega-Blanco et al. 2008; Kopylov 2018, 2019; Wang et al. 2018; Kopylov et al. 2020; Wang et al. 2020: listed in Suppl. material 1). Despite this considerable compression fossil diversity, only two specimens have been described in amber, one from the Early Cretaceous of Spain (Eosyntexis parva Ortega-Blanco, Rasnitsyn & Delclòs, 2008) and the other from the Late Cretaceous of northern Myanmar (Sclerosyntexis hirsuta Wang, Ren, Kopylov & Gao, 2020). In addition, seven undescribed specimens were recorded in Zhang et al. (2018).

Most morphological studies have recovered Anaxyelidae as basal among Siricoidea (Rasnitsyn 1988; Ronquist et al. 1999). The molecular analysis of Schulmeister et al. (2002) recovered a clade Anaxyelidae + Siricidae, while Vilhelmsen et al. (2010) found the same in their analysis using implied weights. Similarly, Heraty et al. (2011) recovered Anaxyelidae as sister to Siricidae with strong support in all analyses. Most work has supported Anaxyelidae as comprising a superfamily, Siricoidea, with Siricidae, and together as the sister group to Xiphydrioidea + Euhymenoptera (i.e., Orussoidea + Apocrita) (Sharkey et al. 2012). However, all of these studies concerned only extant taxa and there have been no reports concerning the phylogenetic relationships of the many fossil anaxyelids.

Herein, we describe a new genus, with two new species, of Anaxyelidae from Kachin amber, which have a single rs-m in the fore- and hind wings and a long pedicel. Orthosyntexis gen. nov. is attributed to the subfamily Syntexinae based on the wider pterostigma, 1r-rs longer than 2r-rs in the forewing, and 1m-cu lacking in the hind wing. More importantly, 2M+Cu is present in the hind wing and two apical mesotibial spurs are also present in these fossils, representing new morphological features for Anaxyelidae. In addition to providing formal descriptions of the species, we present a phylogenetic analysis of living and fossil Anaxyelidae to document the placement of the fossils, explore relationships among the various fossil species and S. libocedrii, clarify the suprageneric classification, and explore the early evolution of the family.

The placement of Orthosyntexis in Anaxyelidae is well-supported, most notably by the fore- and hind wings each with a single rs-m and the pedicel length to width ratio ≥ 2. Moreover, the genus can be attributed to Syntexinae mainly based on the following combination of characters: forewing with the maximum width of pterostigma not shorter than 2r-rs, 1r-rs absent and hind wing m-cu absent. Given that most of the available fossil anaxyelids are preserved as compressions, often with body structures poorly discernible, there is a natural reliance on wing traits and so most characters currently supporting nodes are derived from the venation. Naturally, as more amber fossils become available, it is hoped that a finer comparison of body structures can be made across living and fossil taxa in the future.

Hitherto, no attempt has been made to explore relationships among anaxyelids in a cladistic framework, likely owing to the fact that there is only one extant species and that many of the known fossils are quite incomplete. Recent phylogenetic treatments of the families of Hymenoptera (e.g., Vilhelmsen 2000, 2001; Sharkey et al. 2012), identified a single putative apomorphy for the family, specifically the longitudinal subdivision of the pronotum, easily observed in our amber fossils. Our analysis recovered a monophyletic Anaxyelidae, even when including the considerable fossil diversity and the lack of critical information from many compression fossils. In addition, our analysis indicated the pedicel length to width ratio ≥ 2 and the loss of forewing 2rs-m and hind wing 3rs-m to be additional putative apomorphies for the family (Fig. 8).

Zhang and Rasnitsyn (2006) and Kopylov (2019) organized the diversity of Anaxyelidae into four subfamilies. The establishment of Dolichostigmatinae and Kempendajinae was based on four fossil specimens from three species: Dolichostigma tenuipes Rasnitsyn 1968, Kempendaja jacutensis Rasnitsyn 1968 and Mangus magnus Kopylov 2019. The diagnosis of the subfamily Kempendajinae (Rasnitsyn 1980) focused generally on characters of the body and hind wing, which are plesiomorphies; for example, desclerotized pterostigma in the forewing and rs-m located in the distal third of hind wing cell r. Desclerotized pterostigma is also observed in two anaxyelids, Anaxyela gracilis Martynov 1925 and Sphenosyntexis pallicornis Rasnitsyn 1968. In addition, rs-m in the hind wing is also located in the distal third of cell r in U. undosa. Thus, the features used to justify the subfamily Kempendajinae are unreliable and our analysis robustly placed the group within Anaxyelinae. Similarly, the feature used to differentiate Dolichostigmatinae, i.e., large forewing pterostigma also exists in Dolichosyntexis transbaikalicus Kopylov 2019. Again, our analysis indicated that the subfamily Dolichostigmatinae is merely a subgroup of Anaxyelinae. Accordingly, we recognize only two monophyletic subfamilies in Anaxyelidae: Syntexinae and Anaxyelinae. Of course, this is only an analysis based on our phylogenetic results, and many fossil specimens provide few useful features due to preservation reasons. Therefore, it is hoped that more fossils of Anaxyelidae will provide more favorable evidence for our analysis in the future.

Recently, the anaxyelid Sclerosyntexis hirsuta was described from a single specimen in Kachin amber (Wang et al. 2020). Here, we add two new species, O. elegans and O. thanti, from the same deposit, based on three specimens. They have thick legs, particularly inflated metatibiae, and a pattern of forewing venation similar to Sclerosyntexis, such as 2r-rs joining the pterostigma proximal to pterostigmal midlength, 1r-rs absent, and 2Rs+M present. Furthermore, Sclerosyntexis + Parasyntexis was recovered as the sister group to Orthosyntexis in our phylogenetic estimate for the family.

Although Orthosyntexis belongs to the same clade as the modern Syntexis, these genera are relatively distantly related within the subfamily, and many features differ significantly between them, e.g., flagellomere I length to width ratio < 3, mesotibia with two apical spurs, 1r-rs absent, 2Rs+M present, hind wing cell r closed, and hind wing 2M+Cu present. However, they do share some similar morphological traits, such as notauli close to the mesoscutellum and 1rs-m at the base of Rs in the hind wing. In addition, the new genus, like extant Anaxyelidae, has the ovipositor apically modified with marginal serrations. Extant Siricoidea use their ovipositor to insert their eggs and spores of a symbiotic fungus into dead or dying trees (Goulet 1993; Grimaldi and Engel 2005). However, there has been no evidence of fungal spores in extinct Anaxyelidae, so a symbiotic relationship between extinct Anaxyelidae and fungi has not been conclusively established, although the distribution of this trait across wood wasps tends to suggest that the extinct diversity had a similar association (e.g., Morgan 1968; Kajimura 2000; Grimaldi and Engel 2005). Nonetheless, conclusive evidence of a fungal symbiosis in the fossils is lacking and requires further study as more fossil Anaxyelidae are discovered. Indeed, µCT scans of amber-preserved anaxyelids would be critical to look for potential fungal spores carried on adult females or the presence of distinct mycangia. To date, only seven symphytan species have been described in Kachin amber, of which five are syspastoxyelids, one is an anaxyelid, and one is an orussid (Engel et al. 2016; Zhang et al. 2018, 2020; Zheng et al. 2019; Wang et al. 2020, 2021). And eight symphytan specimens have not been described in Kachin amber, of which seven are anaxyelids and one is an orussid (Zhang et al. 2018). Compared with Mesozoic fossil deposits of China and Russia, the number of Symphyta in Kachin amber is quite small. The reason for such a low diversity and abundance may be that the mid-Cretaceous fauna of Kachin amber is characterized by high levels of endemism indicative of an insular biota (Grimaldi et al. 2002; Rasnitsyn and Öhm-Kühnle 2018; Lin et al. 2019; Zhao et al. 2020), as well as its high termophilic (tropical) nature (Grimaldi et al. 2002; Zhang et al. 2018).

We are grateful to Hongru Yang, Qiong Wu, Xiangbo Guo, and Yuanyuan Guo (Capital Normal University, Beijing, China) for their helpful advice. We thank the editor Dr. Michael Ohl, and two reviewers, Dr. Lars Vilhelmsen and Dr. Alex Rasnitsyn, as well as an anonymous reviewer for their critical review of the manuscript, which gave considerable insights and improvements. D.R. was supported by grants from the National Natural Science Foundation of China (No. 31730087 and 32020103006). T.P.G. was supported by the National Natural Science Foundation of China (31872277) and the Fok Ying-Tong Education Foundation for Young Teachers in the Higher Education Institutions of China (171016). The authors declare that there are no financial competing interests (political, personal, religious, ideological, academic, intellectual, commercial), nor are there other competing interests in the production of this manuscript.