Zigrasimeciini, Borysenko, 2017

Borysenko, Leonid H., 2017, Description of a new genus of primitive ants from Canadian amber, with the study of relationships between stem- and crown-group ants (Hymenoptera: Formicidae), Insecta Mundi 2017 (570), pp. 1-57 : 18-19

Тribe Zigrasimeciini trib.n.

Diagnosis (workers). (1) head capsule specialized: shield-like, with dorsal part thick and raised (putative apomorphy); (2) anterolateral clypeal margins produced over mandibular bases in rounded lobes (putative apomorphy); (3) peg-like setae on anterior clypeal margin present; (4) ocelli absent; (5) relative length of antennomeres variable; (6) neck long; (7) protibia with three spurs: one pectinate and two simple; (8) petiole pedunculate; (9) gastral constriction absent.

Comment. Under the PhyloCode, the taxon should be named “ Zigrasimeciini P, new clade name” and defined as “the clade consisting of Zigrasimecia Barden and Grimaldi and all species that share a more recent common ancestor with Zigrasimecia Barden and Grimaldi than with Sphecomyrma freyi Wilson and Brown, 1967 or Haidomyrmex cerberus Dlussky, 1996 ”.

Composition. Genera Zigrasimecia Barden and Grimaldi, 2013 (type genus), Boltonimecia gen.n..

Discussion. A short scape, long flagellum, two spurs on meso- and metatibia, pretarsal claws with a preapical tooth, and the clypeus posteriorly not inserted between the antennal sockets indicate that Boltonimecia belongs to stem-group ants and the Sphecomyrminae . The only problem here is the form of the metapleural gland orifice and the presence of the mesoscutum and scutellum; these characters have been listed in the diagnosis of the Sphecomyrminae ( Bolton 2003) , but cannot be observed in Boltonimecia due to compression of the mesosoma.

The antennal indices of Boltonimecia and Zigracimecia (type species: Z. tonsora ) are similar to those of the Sphecomyrmini , although some important peculiarities need to be mentioned. Boltonimecia ’s SL/ HL is one of the greatest among the Sphecomyrminae and is close to that of the crown-group ants. Both genera have a longer pedicel compared with the Sphecomyrmini ; in Boltonimecia , the pedicel is so elongated that PL>F1L, as in the crown-group ants ( Sphecomyrmini always have F1L>PL). In Boltonimecia and Z. ferox #1, PL=F2L (in Sphecomyrmini, PL <F2L, except for Gerontoformica occidentalis ). Also, in Boltonimecia , F1L<F2L (in Sphecomyrmini , F1L>F2L without exception).

Therefore, the statistical analysis also leaves no doubt about the assignation of Boltonimecia and Zigrasimecia to stem-group ants and the Sphecomyrminae . These two genera, however, seem to be closer to each other than to the Sphecomyrmini , thus I propose that they be placed into a separate tribe.

The shield-like head, anterolateral clypeal margins produced over mandibular bases in rounded lobes, and three protibial spurs are the most important characters of the new tribe. Noteworthy, these are found also in some Gerontoformica species ( Barden and Grimaldi 2014). In particular, the rounded anterolateral clypeal margins are present in G. spiralis and G. tendir ; three protibial spurs (called “stiff setae” by the authors) - in G. spiralis , G. subcuspis , G. magnus , G. rubustus ; at least some Gerontoformica have the shield-like head. Also, like Zigrasimeciini , G. rugosus and G. spiralis have a very long pedicel; all Gerontoformica have the peg-like setae on the anterior clypeal margin. These peculiarities are starting points for a thorough revision of a seemingly non-monophyletic genus Gerontoformica , which hopefully will be achieved in the near future.

Subclades of Formicidae P

The last part of a proposed higher classification is the subclade composition of the crown group Formicidae P. Since the composition of the subclades that correspond to monophyletic subfamilies is stable, a phylogeny-based classification is straightforward and congruent with the Linnaean system. In a formal phylogenetic definition of the subclades given in Table S18, attention was paid to the recommendation that the type species (Article 11.7 of the PhyloCode) and the species used in the reference phylogenies (Article 11.8 of the PhyloCode) should preferably be used as specifiers. For most of the clades, a node-based definition is used, except for the clades consisting of only one extant species, for which a branch-based definition is provided.

The vast majority of Cenozoic fossil crown-group ants fit nicely into the clades that correspond to tribes or genera in the Linnaean system; some Cretaceous crown-group species ( Brownimecia ) are most probably stem taxa to a clade of a higher (supra-subfamilial) taxonomic level; and only a few fossil crown-group species are related to modern subfamilies but do not fall into any of the tribes (i.e., they are stem taxa to those subfamilies). Most likely, only five taxa (genera) can be considered as forming pan-clades with three recent subfamilies, a formal definition of which is provided below (diagnoses after Bolton (2003), with modifications).