Placement

Haas, Michael, Burks, Roger A., Janšta, Petr & Krogmann, Lars, 2020, Redescription and phylogenetic placement of the Cretaceous wasp Parviformosus wohlrabeae (Hymenoptera: Proctotrupomorpha), Palaeontologia Electronica (a 05) 23 (1), pp. 1-10 : 5-8

Family Placement

In the original description, Barling et al. (2013) argued that the posterior end of the metasoma as well as the shape and morphology of the ovipositor allow placement within the Pteromalidae . Principally, offering a solid and universally accepted morphological definition of Pteromalidae is not possible because the group is polyphyletic (Heraty et al., 2013) and therefore lacks synapomorphic characters (Graham, 1969; Bouček and Rasplus, 1991; Grissell and Schauff, 1997). Therefore, a phylogenetic placement of pteromalid lineages makes only sense on the subfamily level. Barling et al. (2013) claimed that because of 1) the “length and shape of the ovipositor”, 2) the “height relative to width of the gaster”, 3) the “short pronotum”, 4) the “shortened head” and 5) the “apically expanding ovipositor sheath” the specimen might belong to the pteromalid subfamily Sycophaginae , now placed in Agaonidae (Heraty et al., 2013) . None of the abovementioned characters is diagnostic nor represents a synapomorphy for any pteromalid or chalcidoid subfamily. The two defining apomorphies of Sycophaginae , i.e., the presence of grooves framing the mesoscutellum and a deeply sinuated margin of the eighth metasomal tergite, leading to a thumbnail-like medial flap (Cruaud et al., 2011) are not visible on the specimen. Placement of Parviformus wohlrabeae in Sycophaginae would also have challenged the current and wellestablished view that the age of this subfamily and their association with figs ( Ficus , Moraceae ) are of post-Gondwanan origin (Cruaud et al., 2011) and therefore roughly 60 m. y. younger than the fossil. Based on RØnsted et al. (2005) the evolutionary root of figs might lay at the boundary of the Early to the Late Cretaceous, about 100 m. y.a., establishing a time frame of 60 – 100 m.y. for the first development of the mutualism between figs and their chalcidoid pollinators. More recently, the origin of this mutualism was narrowed down to roughly 75 m. y.a. (Cruaud et al. 2012). It is therefore highly unlikely, that P. wohlrabeae , with an age of about 110 m. y., had any association with figs.

Superfamily Placement

As Parviformosus wohlrabeae possesses a deeply incised wasp waist and discernable wing articulations with fragments of wings attached, it can therefore be classified as an apocritan Hymenoptera . Furthermore, possession of a long exserted ovipositor sheath eliminates the possibility of placement in Aculeata. Positioning of the metasoma excludes the parasitoid superfamilies Evanioidea and Stephanoidea. The superfamilies Trigonaloidea and Megalyroidea can also be dismissed by the structure of the pronotum, which is dorsally extremely shortened in the mentioned superfamilies. Additionally, the metasoma of these groups has more apparent sterna than that of P. wohlrabeae , where the observable part of the metasoma is largely covered by terga. Due to severe damage, the metasomal sterna of P. wohlrabeae are hardly discernable, let alone countable. Metasomal characters also contradict placement in the Ichneumonoidea, which possess a metasoma that is usually elongate and petiolate with spiracular openings on each tergum. The metasoma in Ceraphronoidea is also quite different with their first tergite covering most of its length. Moreover, the structure of the mesosoma in P. wohlrabeae , like the notauli and the pronotum, does not agree with the morphological features presented in Ceraphronoidea. The only remaining alternative is a placement within the infraorder Proctotrupomorpha, sensu Rasnitsyn (1988) , which is comprised mostly of minute parasitoids, being morphologically highly diverse and species rich. Extant superfamilies included in Proctotrupomorpha are the Cynipoidea, Platygastroidea, Chalcidoidea , Mymarommatoidea, Diaprioidea, and Proctotrupoidea with addition of the fossil groups Serphitoidea, Cretacoformicidae incertae sedis, and the genus Koonwarrus incertae sedis. A more precise phylogenetic placement of P. wohlrabeae is highly speculative due to its preservational state. Reliable evidence would arise from detectable synapomorphic characters. Such characters, however, are not visible, missing, or subject to vague interpretation, because of distortion and damage.

As it was described as a member of Pteromalidae , Parviformosus wohlrabeae should exhibit the synapomorphies of its superordinate superfamily, the Chalcidoidea . Those include 1) possession of a free prepectus, separating the pronotum from the tegula, 2) the position of the mesothoracic spiracle (Gibson, 1985, 1999; Gibson et al., 1999), 3) morphologically unique multiporous plate sensillae on the flagellar segments (Barlin and Vinson, 1981; Gibson, 1986; Basibuyuk and Quicke, 1999). As Barling et al. (2013) state, the prepectus cannot be identified in the fossil ( Figure 1.2, 1.6 ). Barling et al. (2013) further hypothesize that the prepectus “… is probably internalized, as in many pteromalid subfamilies.” Internalization of the prepectus is so far not reported in Pteromalidae , although miniaturization might occur, e.g., Macromesinae (Gibson, 1986) . This miniaturization also appears in other chalcidoid families and is often linked to a small body size, sometimes leading to an almost fully reduced prepectus (Gibson, 1986). Only the chalcidoid family Rotoitidae was reported to completely lack a visible prepectus (Bouček and Noyes, 1987), but subsequent examination revealed a very slender, independent prepectus, which might be concealed by the pronotum if it is directly adjacent to the mesepisternum (Gibson and Huber, 2000). In P. wohlrabeae , the posterolateral corner of the pronotum is distinctly overlapping the mesopleuron, with no indication of a hidden prepectus underneath ( Figure 1.6 ). Evaluating the position of the mesothoracic spiracle is not possible due to the poor conservational state of the specimen. Presence or absence of multiporous plate sensilla on the flagellar segments cannot be assessed as well, because the antennae are missing. Conclusively, none of the aforementioned synapomorphies of Chalcidoidea can be used for placing the fossil in this superfamily. The long ovipositor is the most obvious feature of the fossil and represents a character state that is common in various chalcidoid lineages but it does not unequivocally support placement in this superfamily, with many parasitoid groups including taxa with an exerted ovipositor.

Based on the limited morphological evidence, we propose to treat Parviformosus wohlrabeae as Proctotrupomorpha incertae sedis. Placement in Chalcidoidea is tempting, mostly due to the lack of satisfactory alternative options, but based on the poor preservation and the absence of any synapomorphies, a reliable superfamily placement of the fossil is not possible. Considering the morphological diversity observed in proctotrupomorphan wasps, P. wohlrabeae might as well belong to a yet unknown "morphotype", not bearing an exact resemblance to any known representative of this clade.

Figures

FIGURE 1. Scanning electron microscopic images of Parviformosus wohlrabeae Barling et al., 2013. (1) Complete habitus. (2) Mesosoma. (3) Metasoma excluding ovipositor. (4) Ovipositor (arrow) engulfed in mineral ridge. (5) Metasoma, broken posterior tip with individual layers of Mt7 and Mt8 (arrow) visible. (6) Anterolateral view of mesosoma, head to right, dashed line marks posterior margin of pronotum. Scales (1) 1 mm; (2, 3) 0.1 mm; (4–6) 0.02 mm. Abbreviations: msc = mesoscutum, Mt6/7 = metasomal tergite 6/7, no1 = pronotum, pl2 = mesopleuron.