Zigrasimecia Barden & Grimaldi 2013

Synopsis of the genus † Zigrasimecia Barden & Grimaldi, 2013

† Zigrasimecia Barden & Grimaldi, 2013: 407. Type-species: † Zigrasimecia tonsora, by original designation. † Zigrasimecia incertae sedis in Formicidae: Barden & Grimaldi, 2013;

† Zigrasimecia in †Sphecomyrminae: † Sphecomyrmini: Perrichot, 2014;

† Zigrasimecia in †Sphecomyrminae: † Zigrasimeciini: Borysenko, 2017;

† Zigrasimecia in † Zigrasimeciinae: Boudinot et al., 2020

Species list

† Zigrasimecia boudinoti Chaul, sp. nov. Myanmar (Burmese amber, early Cenomanian).

† Zigrasimecia caohuijiae Chaul, sp. nov. Myanmar (Burmese amber, early Cenomanian).

† Zigrasimecia chuyangsui Chaul, sp. nov. Myanmar (Burmese amber, early Cenomanian).

† Zigrasimecia ferox Perrichot, 2014. Myanmar (Burmese amber, early Cenomanian).

† Zigrasimecia goldingot Zhuang, Ran, Li, Feng & Liu, 2022. Myanmar (Burmese amber, early Cenomanian). † Zigrasimecia hoelldobleri Cao, Boudinot & Gao, 2020. Myanmar (Burmese amber, early Cenomanian). † Zigrasimecia perrichoti Chaul, sp. nov. Myanmar (Burmese amber, early Cenomanian).

† Zigrasimecia thate Chaul, sp. nov. Myanmar (Burmese amber, early Cenomanian).

† Zigrasimecia tonsora Barden & Grimaldi, 2013. Myanmar (Burmese amber, early Cenomanian).

† Zigrasimecia emended diagnosis (females).

Note: all traits are observed in workers and queens, except when noted.

Masticatory margin of mandible formed by a developed apical tooth and a low, long, rectangular lamella which spans most the masticatory margin posterad the apical tooth; the lamella often ends apically in a marked angle or even an indented point; the apical margin of the lamella and the basal margin of the apical tooth form a small V-shaped groove (Note 1; Fig. 2). Inner surface of mandible densely covered with thin, spiny chaetae (Note 2). Body of mandible strongly torqued approximately in between its first and second thirds and mandibular insertion shifted ventrolaterally (Note 3; Fig. 3 and 4). Apical portion of body of mandible spatulate (Note 3; Fig. 7, F; Fig. 10, A). Labrum large, having stout, spiny chaetae basally and medially, distal margin medially with a pair of low lobes (Note 4 and 5; Fig. 7, D and F; Fig. 10, A). Palp formula variable, either 5, 3 or 6, 4 (Note 6). Cross-section of cephalic capsule bean-shaped or omega-shaped (Note 7). Pleurostomal fossa lateroventrally migrated (Note 3, Fig. 4, B). Clypeus transverse, mildly arched. Clypeal lobes strongly developed and anteriorly projected over mandibular insertions (Note 8). Clypeal chaetae blunt, relatively long, and densely arranged across anterior clypeal margin (Note 9). In full-face view, anterior clypeal margin strongly concave. Frons strongly protruded dorsally (Note 10). Frontal carina originating somewhere at the posterior (upper) or medial (inner) torular edge and reaching the perimeter of the compound eyes anteromedially; sometimes poorly marked (Note 11). Ocelli absent in workers. Antenna of worker and alate gyne 12-merous. Scape at least twice the length of pedicel. Mesosoma compact, most sclerites fused; dorsal sutures and lines absent; dorsal margin of mesosoma in profile without abrupt breaks, varying from mildly to strongly convex; mesosomal spiracles (metanotal and propodeal) without hood-shaped protrusions over their openings; metanotal spiracle facing laterally; propodeal spiracle slit-shaped and large; mesonotal and metanotal spiracles very approximated (workers only, Note 12, Fig. 5, but see Zigrasimecia ufv-10, Fig. 19). Calcar with or without a bifid tip (Note 13). Tibial spur formula 2s, 2 (1p, 1s) (Note 14). Tibiae either anteroposteriorly flattened or being approximately cylindrical. Pretarsal claws inner margins toothed. Petiole sessile or subsessile, in most species roughly pyramidal; anterior surface oblique, the longest; dorsal surface sometimes brief (only a curved or sharpened transition to the posterior surface), sometimes properly forming a flattened or round surface; posterior surface descending, shorter than the anterior; subpetiolar process conspicuous, anteriorly inclined (Note 15). Helcium (= articulatory sclerites of abdominal segment III) axial, situated at segment III midheight. Poststernite III having a keel-shaped or acute V-shaped anterior process (the "prora"). Abdominal segment IV lacking or having a soft constriction between its pre- and postsclerites, the cinctus (Note 16). Gyne: most head and metasomal characters similar to workers. Mesoscutum and mesoscutellum demarcated by sulcus; mesosoma musculated for flight; forewing with seven closed cells: the costal, two submarginal, one marginal, the basal, subbasal, and first discal.

Note 1. This character varies slightly among the species in terms of how tall the lamella is and how sharply defined its apical angle is.

Note 2. Somewhat similar mandibular chaetae on the oral surface are found in Camelomecia Barden & Grimaldi, 2016 (apparently thinner than those of Zigrasimecia), and Brownimecia Grimaldi et al., 1997 (relatively smaller than in Zigrasimecia); among extant ants, in Tatuidris Brown & Kempf, 1968 (very similar to Zigrasimecia) and in some leptanillines, such as Protanilla izanagi Terayama, 2013 (strikingly similar).

Note 3. The ventral mandibular articulation, and the corresponding pleurostomal fossa, in Zigrasimecia is strongly ventrally shifted when compared to Gerontoformica and, in fact, most other ants (Fig. 4, see also Fig. 3 of Richter et al., 2022). Because of that shift, the expected plane of movement for the closing mandibles if they were not torqued would be from an anterolateral to posteromedial, similar to what is understood for the Haidomyrmecinae (Barden & Grimaldi, 2012; Barden et al., 2017), instead of a regular lateral to medial plane. The torsion of the body of the mandible, however, prevents its flat apical portion to rise posterad during the shutting movement and makes the apical torqued portion to stay parallel and beneath the labrum when both are in full adduction. This generates an increase in the contact zone between the mandible inner surface and the dorsum of labrum, both surfaces covered on spiny chaetae (Fig. 3, D). The flat, expanded, apical portion of the mandible further increases this contact surface between mandibles and labrum. This mechanism presumably allowed Zigrasimecia to have a strong grip, which could be an adaptation for hunting arthropods, maybe prey much larger than the ants itself, as do the amblyoponines, which prey on centipedes much larger than the individual ants; or, alternatively, for hunting prey which has powerful escaping devices (e.g. the furca of springtails).

Note 4. A hypertrophied labrum is unusual in Formicidae. In Sphecomyrminae it occurs in Myanmyrma maraudera (Barden & Grimaldi, 2016). In extant ants, robust and considerably developed labra are observed in the Dorylinae. Highly modified, but not necessarily hypertrophied, labra are seen in the closely related ants of the genus Strumigenys, the phalacromyrmecine genera, and the basicerotine genera, with a few species, Eurhopalothrix spectabilis Kempf, 1962 and Rhopalothrix jtl015 (USNMENT00688794) having very large labra, the latter, indeed, resembling considerably that of Zigrasimecia in terms of shape.

Note 5. Although labral chaetae occur among other Cretaceous ants (e.g., Myanmyrma Engel & Grimaldi, 2005, some Gerontoformica Nel & Perrault, 2004, Dhagnathos Perrichot et al., 2020 and Chonidris Perrichot et al., 2020), the exact conformation seen in Zigrasimecia, with the chaetae forming a dense patch formed by various rows on the basal-medial portion of the labrum disc (not restricted to one or two transversal basal row) and being stout and pointy is unique to the Zigrasimeciinae. In extant genera, labral chaetae are usually disposed as a basal, transverse row and are seen in the following genera: Protanilla Taylor, 1990, Anomalomyrma Taylor, 1990, Opamyrma Yamane et al., 2008, Apomyrma Brown et al., 1971, Amblyopone Erichson, 1842 and Onychomyrmex Emery, 1895.

Note 6. The palpal formula of Z. ferox is 5, 3 (Perrichot, 2014). The only species which could have its palpal formula established in this paper, Z. chuyangsui sp. nov., showed an undoubtful 6, 4 count (Fig. 10, B), indicating variation in the palpal formula within the genus. The queen Zigrasimecia YKLP-AMB-002 also has the palpal formula 6, 4 (although presented as 4, 3 in Zhuang et al., 2022b, see below).

Note 7. This feature is widespread in the genus, although more conspicuous in Z. chuyangsui sp. nov., Zigrasimecia ufv-08, when compared to species like Z. thate sp. nov. or Z. perrichoti sp. nov..

Note 8. Smaller clypeal lobes, which are only slightly projected over mandible insertions, are seen in some species of Gerontoformica (e.g. Gerontoformica ufv-01, ANTWEB1032418), Myanmyrma, and Boltonimecia.

Note 9. Among fossils, clypeal chaetae are seen in Gerontoformica and Myanmyrma, Aquilomyrmex, Dhagnathos and Chonidris. Among extant ants such setae are present in Opamyrma, Apomyrma and in most amblyoponine genera. However, the Zigrasimecia type of relatively elongate with subparallel sides and round/truncate apex chaetae is very rare or unseen in other stem ants or extant lineages, with a similar shape only seen in Apomyrma, Opamyrma and Stigmatomma pluto (Gotwald & Lévieux, 1972).

Note 10. This feature is best seen in profile view, where anterior half of the head dorsum outline is strongly oblique from the anterior clypeal margin to the frons level (Figs. 6, E; Fig. 13, B). Despite having a prominent frons, there is no sign of an elevated platform on the frons and vertex as that of Boltonimecia and which was suggested as a character shared between both genera (Borysenko, 2017, see section 3. 4 below).

Note 11. In most Gerontoformica the frontal carina is crescent-shaped and around the torulus. In Myanmyrma maraudera, the frontal carina resembles that of Zigrasimecia, however, it is more developed and reaches the anterior portion of the torulus. In Zigrasimecia the carina may be poorly marked (e.g. Z. chuyangsui sp. nov.), only represented by a wave of the cuticle which forms a somewhat shallow scrobe.

Note 12. The mesosomal conformation of Zigrasimecia sets it apart from other stem ants. Dorsally it has no signs of sutures, so that notal and propodeal limits can be only guessed (usually by the position of the spiracles on the lateral surfaces). It is particularly difficult to determine whether Zigrasimecia has a developed metanotum as other stem lineages or if it shrunk in between the mesonotum and the dorsum of the propodeum, resembling modern ants, where this sclerite is reduced. Mesosoma as compressed and fused as that of Zigrasimecia is found in some lineages of the Myrmicinae, Agroecomyrmecinae, some Dorylinae, and in the Proceratiinae, being particularly similar to Tatuidris and Discothyrea Roger, 1863.

Note 13. The calcar in many Cretaceous ants is considerably different from extant ants (personal observation). In many species of various genera it has a bifid tip, it does not have a well-developed comb across its length, the inner margin is most of the time concave, sometimes lamellate along the main axis (sometimes there are projections, but they are not as tidily organized, not forming a comb-like structure as in extant ants). In Zigrasimecia, the tip is rarely bifid.

Note 14. All Formicidae, including Mesozoic ones, have only 1 protibial spur, the calcar. The calcar can be associated with 1 or 2 posterior, subapical, spur-like setae (Lattke & Melo 2020; Perrichot et al. 2020; Cao et al. 2020a) (Fig. 8, G), which explains previous reports of Zigrasimecia having more than one protibial spur. Therefore, the formula accounts only for the meso- and metatibiae.

Note 15. The petiole of Zigrasimecia is quite unique among the stem lineages. Considering all Zigrasimecia specimens examined, the pedunculate state, considered as a trait shared by Zigrasimecia and Boltonimecia (Borysenko, 2017), is not likely correct (see section 3. 4 below). In Zigrasimecia, in profile view, the anterior petiole margin is straight and inclined posterodorsally across its length, never forming a proper anterior peduncle clearly distinct from the node. Among the species in the genus with longer petioles, this is maintained, although the impression of a pedunculate petiole is greater. Moreover, not rarely the petiole is slightly disarticulated from the mesosoma, reinforcing this impression. This petiole shape is seen in some modern species (e.g. Eurhopalothrix bruchi (Santschi, 1922) and related spp., see images of CASENT0173970 at Antweb).

Note 16. The cinctus between pre- and postsclerites of fourth abdominal segment, when present, is always mild in Zigrasimecia (see discussion under Z. ferox). Among other Cretaceous ants, the constriction goes from entirely absent to the formation of a conspicuous postpetiole (Barden & Grimaldi, 2014; Miao & Wang, 2019).