Zigrasimeciinae

Nomenclature

 * † [subfamily of Formicidae]
 * †Zigrasimeciini Borysenko, 2017: 3. Type-genus: †Zigrasimecia Barden & Grimaldi, 2013: 407.

Taxonomic History

 * †Zigrasimeciini as tribe of †Sphecomyrminae: Borysenko, 2017: 3, 18 (diagnosis).
 * †Zigrasimeciini as subfamily of Formicidae: Boudinot, Perrichot & Chaul, 2020: 29.

Cao et al. (2020) amended and made of the following about the tribe Zigrasimeciini.

Emended diagnosis (female): Among Mesozoic Formicidae, uniquely defined by the presence of a facial scrobe for reception of scape which extends posterolaterally from antennal torulus to compound eye. Zigrasimecia and Protozigrasimecia uniquely identified by the following: head massive, block-shaped, with broadly omega-shaped (Ω) occiput. These two genera additionally identified by rotation of the mandibles within their sockets (Note 1). Otherwise, the tribe shares the following states with Gerontoformica: anterolateral clypeal margins lobate (albeit much expanded), covering mandibular bases; anterior clypeal margin lined by peg-like traction setae (Note 2); worker ocelli present or absent; propodeal spiracle slit-shaped (Note 3); tibial spur formula 2s,2(1p,1s) (Note 4); petiole anteriorly pedunculate and convex-nodiform or sessile / subsessile subsquamiform (latter not observed in Gerontoformica); anteroventral process of petiolar sternum present (Note 5); and cinctus of abdominal segment IV, defining pre- and post-sclerites present or absent.

Note 1: The rotation of the mandibular base allows the mandibles to swing diagonally, which along the body axis is posterolateral during opening and anteromedial during closure, illustrated by Figures 1A, 1B, and 2D.

Note 2: Perioral traction setae of varying form also occur on the labrum of Zigrasimecia, Protanilla, Apomyrma, Amblyopone, among other taxa. Stout setae which may have a frictional function also occur on the ventral mandibular surfaces of various taxa, and are more difficult to delimit morphologically.

Note 3: All currently valid Gerontoformica species were observed to have slit-shaped propodeal spiracles, except for Gerontoformica cretacica, Gerontoformica occidentalis, and Gerontoformica orientalis for which the spiracles are not visible or well-preserved enough for evaluation. Slitshaped spiracles also occur in other stem ants, including Sphecomyrma freyi and various Haidomyrmecinae.

Note 4: The tibial spur formula follows Bolton (2003). Specifically, s, simple; b, barbirulate; p, pectinate; when two spurs differ in formula, the conformation (s, b, or p) is indicated parenthetically. Despite description of Mesozoic Formicidae with two or more protibial spurs, this is a misnomer, as the calcar is the only true spur; it is attended posteriorly by one to two spur-like simple setae.

Note 5: The subpetiolar process is present in the majority of the Formicidae, and occurs in all valid, described stem ants to date.

Comments: Borysenko (2017) provided a list of nine characters to define the tribe Zigrasimeciini, comprising at the time the poorly-preserved Canadian fossil taxon Boltonimecia and two species of Zigrasimecia. However, none of the nine characters uniquely define the tribe relative to other groups of Mesozoic Formicidae, and a number of revisions are necessitated by the discovery of Protozigrasimecia (genus defined below). The strongest character linking all three genera is the presence of the antennal scrobes, which is, surprisingly, unique among known stem ants, as far as their diversity is known to the present day (note that the antennal groove of Gerontoformica contegus is not a scrobe as here used).

Borysenko’s description of the cranium of Zigrasimeciinae as “specialized, shield-like, with dorsal part thick and raised” is a mischaracterization of Zigrasimecia, although it is apparently the case for Boltonimecia – we do note that the preservation of the Boltonimecia canadensis holotype is compromised at a gross scale due to desiccation and compression. Furthermore, Borysenko’s definition of the group as having the “gastral constriction absent” is no longer valid, as Protozigrasimecia has clearly defined pre- and postsclerites of abdominal segment IV. Critical reanalysis of the placement of Boltonimecia, ideally with emphasis on the mandibular articulations, will be possible after the discovery and description of better-preserved material. In the meantime, we redefine the group as above, and provide a key to known generic taxa below.