The ant fauna of Late Eocene (Priabonian age, 37.8–33.9 Ma) European ambers (i.e., Baltic, Bitterfeld, Danish, and Rovno) is the richest of any local fossil myrmecofauna in the world: 70 extinct and extant genera from 12 extant subfamilies and 204 extinct species have been recorded, and at least 20 new species and several genera are still awaiting a formal description (AntWeb 2025; unpublished personal data). Among these, the subfamily Myrmicinae, with 26 genera and 83 described species, is the most diverse (Dlussky and Rasnitsyn 2009; Barden 2017; Radchenko 2023, 2024; Radchenko et al. 2024, 2025; Boudinot et al. 2024; Radchenko and Khomych 2025).

Wheeler (1915) established the monotypic extinct myrmicine genus Stiphromyrmex with the type species Stigmomyrmex robustus (Mayr, 1868), described from Baltic amber. Stiphromyrmex is clearly distinguished from related genera found in the ambers mentioned by its 12-segmented antennae instead of the 9-segmented in Enneamerus Mayr, 1868, and the 11-segmented antennae in Pristomyrmex Mayr, 1866 and Thanacomyrmex Chény, Wang & Perrichot, 2019. This genus is otherwise quite peculiar, its main diagnostic features being: total length of 3.25–3.50 mm; head without antennal scrobes; eyes of moderate size, situated in front of the midlength of the sides of the head; frontal carinae absent; frontal lobes poorly developed, only partly covering the antennal sockets, and toruli not concealed by the frontal lobes; median part of clypeus flattened, delineated laterally by coarse longitudinal carinae, anterior clypeal margin with paired long setae, no medial seta present; antennae 12-segmented, scape short, far from reaching the occipital margin, funiculus with a well-developed three-segmented apical club; mandibles with distinct masticatory margin, armed with two larger apical teeth followed by two small denticles; palp formula 4, 3; mesosoma compact, short, and stout, promesonotum convex, promesonotal suture and metanotal groove absent; propodeum with massive curved spines directed upward and backward at an angle of ca. 55°; femora and tibiae somewhat incrassated, meso- and metatibiae with a well-developed simple spur; head and mesosoma with coarse foveolate sculpture (Fig. 3).

Recently, one of the co-authors (HWR) discovered a queen in a piece of Baltic amber that matches the main diagnostic features of workers of Stiphromyrmex robustus. Here, this queen morphotype is described for the first time. We also discuss the taxonomic position of the genus Stiphromyrmex, and we compile a key to the identification of amber genera and species from the Myrmecina genus-group, to which Stiphromyrmex belongs.

We investigated the queen of Stiphromyrmex robustus, fossilized in a piece of Baltic amber, as well as the neotype worker of this species that was designated recently by Radchenko and Dlussky (2017b).

The original photographs were taken with a Leica Z16 APO microscope equipped with a Canon EOS 70D camera and processed by LAS Core software, and with flash lighting and a P-51 Camlift Driver ver. 2.6.1 controlling a Canon EOS K2-SC camera.

The specimens were measured using a Leica MZ6 stereomicroscope with an ocular micrometer at a maximum magnification of ×100. Not all features of the specimen examined were properly measurable, so we measured only well-visible details (calculated to an accuracy of 0.01 mm), particularly:

HL maximum length of the head in dorsal view, measured in a straight line from the anteriormost point of the clypeus to the midpoint of the occipital margin;

SL maximum length of the scape, measured in a straight line from its apex to the articulation with the condylar bulb;

OL maximum diameter (length) of the eye;

OW minimum diameter (width) of the eye;

GL length of the genae, measured from the anterior margin of the eyes to the articulation with the mandible;

MdL length of the mandible, measured from its tip to articulation with the head;

ML diagonal length of the mesosoma (seen in profile), measured from the anterior end of the neck shield to the posterior margin of the propodeal lobes;

MH height of the mesosoma, measured from the upper level of the scutum perpendicularly to the level of the lower margin of the mesopleuron;

PL maximum length of the petiole, measured from the posterodorsal margin of the petiole to the articulation with the propodeum;

PH maximum height of the petiole in profile, measured from the uppermost point of the petiolar node perpendicularly to the lowest point of the petiole;

PW maximum width of the petiole in dorsal view;

PPL maximum length of the postpetiole between its visible anterior and posterior margins;

PPH maximum height of the postpetiole in profile from the uppermost to the lowermost points, measured perpendicularly to the tergosternal suture;

PPW maximum width of the postpetiole in dorsal view;

HTL maximum length of the metatibia;

HFL maximum length of the metafemur;

ESL maximum length of the propodeal spine in profile, measured along the spine from its tip to the deepest point of the propodeal constriction at the base of the spine;

ESD distance between the tips of the propodeal spine in dorsal view.

The approximate total length is calculated as the sum of HL + ML + PL + PPL + length of the gaster.

For simplicity, we give ratios of various measurements (e.g., HL/HW) rather than naming and abbreviating various indices (e.g., CI – cephalic index).

Class Insecta Linnaeus, 1758

Order Hymenoptera Linnaeus, 1758

Family Formicidae Latreille, 1802

Subfamily Myrmicinae Lepeletier de Saint-Fargeau, 1835

Genus Stiphromyrmex Wheeler, 1915

The taxonomic position of Stiphromyrmex is not yet definitively resolved. Wheeler (1915) placed this genus in the tribe Myrmicini Lepeletier de Saint-Fargeau, 1835, and considered it to be related to the Paleotropical genus Pristomyrmex. It should be noted that Wheeler (loc. cit.) treated this tribe too broadly and included many unrelated genera in it. Later, Dlussky and Fedoseeva (1988) treated Stiphromyrmex as incertae sedis in Myrmicinae, and more recently Bolton (1994, 1995, 2003) considered it incertae sedis in the tribe “Myrmecinini” Ashmead, 1905, which was simply repeated by Radchenko and Dlussky (2017b). Then Ward et al. (2015) synonymized many tribal names, including “Myrmecinini,” with Crematogastrini Forel, 1893, and assigned Stiphromyrmex to this tribe.

The generic composition of the tribe “Myrmecinini” has changed over time (e.g., see Ashmead 1905; Emery 1914; Forel 1917; Emery 1924; Chapman and Capco 1951; Dlussky and Fedoseeva 1988), and as a result Bolton (2003) attributed four extant (Acanthomyrmex Emery, 1893; Myrmecina Curtis, 1829; Perissomyrmex M. Smith, 1949; and Pristomyrmex) and two extinct genera (Enneamerus and Stiphromyrmex) to this tribe. Finally, based on molecular phylogenetic analysis, Blaimer et al. (2018) established an informal Myrmecina genus-group within Crematogastrini, including all extant genera of the tribe “Myrmecinini” sensu Bolton (2003) with the addition of the genus Dilobocondyla Santschi, 1910, and supported the monophyly of this genus-group, with Dilobocondyla as sister to the other four genera.

However, morphologically the former tribe “Myrmecinini” (even without Dilobocondyla) is not so strictly defined, and its taxonomic position within the subfamily Myrmicinae remains somewhat uncertain (Bolton 2003). For example, Emery (1914, p. 37) noted about “Myrmecinini”: “Caratteri negativi; generi non rientranti in nessuna delle tribu seguenti” [Negative characteristics; genera not belonging to any of the following tribes], and Bolton (2003) hesitated to assign “Myrmecinini” to any of the tribal groups he proposed.

Nevertheless, Bolton (loc. cit.) emphasized several specific features of the labrum as apomorphies of this tribe, but these features are practically invisible in most fossil specimens. At the same time, Emery (1924: 230) noted that “Les parties latérales de l’épistome sont étroites et forment á leur bord postérieur une arête transversale, limitant, comme chez les Tetramoriini, la fosse antennaire” [The lateral parts of the clypeus are narrow and form a transverse ridge at their posterior edge, bordering, as in Tetramoriini, the antennal sockets]. Bolton (2003) confirmed this character and added several more.

As a result, the Myrmecina genus-group can be, at least tentatively, defined by the following characters: a) the lateral parts of the clypeus are raised into a ridge in front of the antennal insertions (similar to Tetramorium Mayr, 1855); b) the median part of the clypeus is broadly inserted between the antennal sockets; c) the frontal lobes are strongly reduced, so that the antennal sockets and toruli are largely or entirely exposed (except for Myrmecina and Dilobocondyla); d) the antennae have 9, 11, or 12 segments and a 3-segmented apical club.

Twenty-six Myrmicinae genera, both extant and extinct, are currently recorded from the Late Eocene European ambers, and workers and/or gynes are known for 21 of them (Wheeler 1915; Dlussky and Perkovsky 2002; Dlussky and Radchenko 2006a, b, 2011; Radchenko et al. 2007; Radchenko and Dlussky 2011, 2012, 2015; Chény et al. 2019; Radchenko 2023, 2024; Radchenko and Khomych 2025).

Among these, only four genera correspond to the proposed characteristics of the Myrmecina genus-group: the extinct Enneamerus, Stiphromyrmex, and Thanacomyrmex, and the extant Pristomyrmex. In particular, their frontal lobes are strongly reduced, and the antennal sockets are completely or almost completely exposed; at least the toruli are not concealed by the frontal lobes. The median portion of the clypeus is broadly inserted between the antennal sockets, and the lateral parts of the clypeus are raised into a ridge in front of the antennal sockets. They have already been assigned, at least tentatively, to the tribe “Myrmecinini” or, formally, to the Myrmecina genus-group of the tribe Crematogastrini (Bolton 2003; Blaimer et al. 2018; Chény et al. 2019), and we agree with this opinion.

In any case, the three fossil genera mentioned above already displayed some derived characters and cannot be considered the putative ancestors of any genera of the Myrmecina genus-group or even the stem of this group.

Below we compile a key to the identification of the genera and species attributed to the Myrmecina genus-group. Two species from Baltic amber are known in Enneamerus Mayr, 1868 (Radchenko and Dlussky 2017a); one species each from the same amber is known in Stiphromyrmex and Thanacomyrmex, and three species of Pristomyrmex Mayr, 1866 were found in Danish, Bitterfeld, and Rovno ambers (one species based on the male only) (Dlussky and Radchenko 2011; Radchenko and Dlussky 2018).