Sussudio boreus

A fossil ant from the Baltic amber.

Identification
Definition (worker) (Boudinot et al., 2022).
 * 1) With characters of the Formicinae (see Boudinot et al. (2022), Note 1).
 * 2) Cranium size variable, width somewhat broader than to about twice that of mesosoma (see Boudinot et al. (2022), Note 2).
 * 3) Mandible triangular, with 7–8 teeth.
 * 4) Dorsal mandibular groove extending along lateral mandibular margin in dorsal view.
 * 5) Palps reduced, not reaching midlength of postgenal bridge.
 * 6) Frontal carinae raised above antennal toruli.
 * 7) Antennal toruli abutting posterior clypeal margin.
 * 8) Antennomere count unknown (see Boudinot et al. (2022), Note 3).
 * 9) Antennomere III longer than broad.
 * 10) Compound eyes situated posterior to head midlength.
 * 11) Compound eyes not enormous and long axes subparallel.
 * 12) Ocelli minutely present or absent.
 * 13) Promesonotum domed, raised well above propodeum.
 * 14) Metanotal groove well-developed.
 * 15) Petiolar foramen in profile view clearly raised and margined with conspicuous thickened carina or lip.
 * 16) Petiolar node squamiform, tall; dorsoventral height equal to or possibly exceeding dorsum of propodeum.
 * 17) Abdominal segment III without raised tergosternal fusion.

†Sussudio boreus (the only known species within this genus) was previously placed in the Lasiini (Emery, 1925, Dlussky & Fedoseeva 1988, Bolton, 1994), and has been considered to be a member of the genus Pseudolasius (Wheeler, 1915; LaPolla & Dlussky, 2010). Based on an examination of a syntype (GZG-BST04646) and a nontype (NHMW1984-31-211) worker, as well as the original description from Wheeler (1915), we reject both placements. Neither specimen can be mistaken due to the unique combination of characters they display, including setation, high petiolar nodes, massive crania, and mesosomal form. †Sussudio boreus does not display the modifications of the third abdominal segment characteristic of the Prenolepis genus group, also rejecting its original placement in Pseudolasius (see diagnostic Note 2 above). †Sussudio is excluded from the Lasiini overall by two additional characters: (i) The petiolar node, which is completely vertical and very tall dorsoventrally, is as tall as or possibly taller than the propodeum; and (ii) the raised and conspicuously lipped petiolar foramen. No lasiine, extant or extinct, has such a tall petiolar node, nor do any species display the petiolar foramen conformation observed in †Sussudio.

We cannot confidently place †Sussudio in any extant tribe, therefore, we consider †Sussudio incertae sedis in the Formicinae. As with other fossil taxa, which are of uncertain placement in the Formicinae, micro-CT scanning could be used to evaluate the form of the helcium and third abdominal sternum.

Distribution
This taxon was described from and is also known from  and  (Perkovsky, 2016).

Nomenclature

 * † boreus. †Pseudolasius boreus Wheeler, W.M. 1915h: 133, fig. 64 (s.w.) BALTIC AMBER (Eocene).
 * See also: LaPolla & Dlussky, 2010: 270.
 * Combination in Sussudio: Boudinot et al., 2022: 143.