Xoanon

Wedmann, Sonja, Pouillon, Jean-Marc & Nel, André, 2014, New Palaeogene horntail wasps (Hymenoptera, Siricidae) and a discussion of their fossil record, Zootaxa 3869 (1), pp. 33-43 : 35

Xoanon ? eocenicus sp. nov.

( Figs. 1 , 2 )

Material. Holotype specimen MeI 14895, stored in the Messel insect collection at the Senckenberg Research station Grube Messel, part of the Senckenberg Research Institute and Natural History Museum Frankfurt.

Diagnosis. Small siricid, with diagnostic forewing characters only; most basal section of vein Rs reclined, then short fused section of Rs+M, then Rs approximates stigma, crossvein 1r-rs most probably very short; most distal part of vein Rs only faintly visible, probably directed towards anterior wing margin but R1 seemingly not strongly bent at its apex, so apex of cell 3R1 seems to lie very close to anterior wing margin; crossvein 1 cu–a located quite distal to forking of vein M+Cu into M and Cu, at about middle of cell 1M; no vein Cu1.

Age and outcrop. Lower Middle Eocene, Geiseltalian, ca. 47 million year old former Maar lake Messel, near Darmstadt, Hesse, Germany; The specimen was collected in the Messel Formation, year 2009, in grid square F9, in strata 2.5 to 3.5m above local stratigraphic marker level alpha.

Etymology. Named after the Eocene period.

Description. A fore- and parts of a hindwing associated with fragmentarily preserved body remains; body structures poorly preserved, only remains of thorax, abdomen and a probable fragment of head partly visible; length from presumed head remains to preserved wing tip 11.4 mm; length from presumed head remains to preserved abdominal remains 8.2 mm; sex unknown; hindwing in bad condition and uninformative; forewing nearly complete with distal part of pterostigma and wing apex missing; preserved forewing part 9.0 mm long, 2.4 mm wide; part of wing between anterior wing margin and vein R dark infuscate; pterostigma 4.6 mm from wing base, 0.3 mm wide, only basal part preserved; cell 1R1 small, about 0.9 mm long, 0.2 mm wide; crossvein 1r-rs not preserved, but probably very short; basal section of Rs reclined (directed posterobasal); Rs re-emerging from Rs+M very near to junction of Rs and M; cell 2R1 about 1.4 mm long; cell 3R1 at its anterior margin about 2.3 mm long, 0.7 mm wide above crossvein 3r-m; distal part of vein Rs directed towards anterior wing margin, but with R1 not strongly bent at its apex, so apex of cell 3R1 situated very close to anterior wing margin; cell 1Rs maximally 2.1 mm long, at its middle 0.5 mm wide; instead of one crossvein there are two crossveins 2r-m present; one placed distal of crossvein 2m-cu, ending in cell 3M; a second crossvein placed between Rs and M just basal of 2r-rs, aligned with 2m-cu; cell 1M pentagonal, 1 mm long, 0.3 mm wide; crossvein 1cu-a well distally of point of separation between M and Cu, in middle of cell 1M; no crossvein Cu1; cell 2M 1.4 mm long, at its middle 0.6 mm wide.

Discussion. The subvertical to reclined basal section of Rs is a autapomorphy of the Siricidae (e. g. Königsmann 1977, p. 26, Rasnitsyn 1988). This character is clearly present in the fossil. Forewing vein Rs reemerges from Rs+M very near to junction of Rs and M, fore wing vein Cu1 is absent. This fossil resembles the recent genus Xoanon in the fore wing venation, because the distal cross vein 2r–m is placed apically and joined to cell 3M, similar to the autapomorphy of Xoanon proposed by Schiff et al. (2012), already noted by Benson (1943, p. 34). The fossil has a second crossvein 2r-m between Rs and M, aligned with 2m-cu. Duplications and aberrations in the wing venation of Siricidae can be found quite often. For example, Fig. C 24.1 in Schiff et al. (2012) shows a bifurcation of vein 2r-m in extant Urocerus flavicornis . We found also a half additional crossvein 2r-m in an extant specimen of Urocerus gigas ( Fig. 3 A), and strong aberrations in the same region in a Sirex juvencus specimen ( Fig. 3 B), so the presence of an aberrant additional crossvein in this fossil is not unusual. Cell 2R1 is relatively long compared to cell 3R1, which fits with extant Xoanon matsumurae (compare e.g. Fig. C 1.24 in Schiff et al. 2012). On the other hand, the shape of cell 3R1 is very particular for a siricid, with the distal part of vein Rs2 directed towards the anterior wing margin as in Xoanon and in most other siricids, but with R1 not strongly bent at its apex, so that the apex of cell 3R1 is very close to the anterior wing margin. A similar shape of cell 3R1 can be found in most basal Hymenoptera (e. g. Schulmeister 2003), so this might be an atavism to a plesiomorphic character state, perhaps autapomorphic for the species. Mainly because of the peculiar form of cell 3R1 there remains an uncertainty in regard to the generic placement of the fossil, which is indicated by the “?” in the generic attribution, following the recommendations of Bengtson (1988) regarding open nomenclature. In modern Siricidae the body lengths range between 7 to 38 mm and more, the body lengths are highly variable even among one species. So the preserved body length of minimum 8.2 mm for the Messel fossil does not contradict a placement in the genus Xoanon .

Figures

FIGURE 1. Xoanon? eocenicus sp. nov., SMF coll. no. MeI 14895, photograph of whole fossil. Scale bar represents 1 mm.

FIGURE 2. Xoanon? eocenicus sp. nov., SMF coll. no. MeI 14895, detail showing wing venation. A, photograph. B, drawing of fore wing venation. Labelling of longitudinal veins in capital letters, cells in capital italic letters, cross-veins in small letters. Scale bar represents 1 mm.

FIGURE 3. Fore wings of extant Siricidae with aberrations in the wing venation. Black arrows point to aberrations. A, wings of Urocerus gigas with an additional half cross vein 2 r-m. B, wings of Sirex juvencus with strong aberrations in the same wing region. Both specimens are from the Hymenoptera collection of Senckenberg Research Institute and Natural History Museum Frankfurt. Scale bars represent 5 mm.