Enneamerus costatus

The holotype worker of E. costatus has been trapped in resin together with an ant larva, and such findings are very rare in ambers. We suppose that the worker was carrying the larva while changing the nest site. Perhaps this species was able to form polycalic colonies and had quite high level of sociality. On the other hand, we cannot exclude the alternative, perhaps less likely, possibility that it was a larva of another ant species taken as prey. (Radchenko and Dlussky 2017)

Identification
Radchenko and Dlussky (2017) - Enneamerus costatus clearly differs from Enneamerus reticulatus by the much coarser, costate sculpture on the frons, the much less abundant and shorter standing hairs on the antennal scape, and the generally less abundant pilosity on the body.

Distribution
This taxon was described from Baltic Amber (Eocene).

Nomenclature

 * † costatus. †Enneamerus costatus Radchenko & Dlussky, 2017: 6, figs. 4-6 (w.) BALTIC AMBER (Eocene).

Worker
HL 0.86, HW 0.68, SL 62, OL 0.21, ML 0.78, PNW 0.33, PL 0.30, PW 0.18, PH 0.29, PPL 0.13, PPW 0.22, PPH 0.17, ESL 0.18, ESD 0.27, HTL 0.51, total length ca. 3. HL/HW 1.27, SL/HL 0.72, SL/HW 0.92, PL/HL 0.35, PL/PH 1.03, PPL/HL 0.15, PPL/ PPH 0.76, ESL/HL 0.21, ESL/HW 0.26, ESD/ ESL 1.50.

Head widely oval, with convex sides and occipital margin and widely rounded occipital corners. Eyes of moderate size, subcircular, situated behind midlength of sides of head. Antennae 9-segmented, with distinct, large 3-segmented club that is somewhat longer than total length of 2nd to 5th funicular segments together; each of the latter segments somewhat longer than wide; scape strongly curved at base, without any trace of lobe or carina, relatively long, almost reaching occipital margin. Antennal scrobes well developed, deep, extend above eyes. Clypeus with two coarse longitudinal medial carinae, surface between them distinctly longitudinally concave (depressed); anterior clypeal margin very shallowly concave, without teeth or denticles. Man-dibles quite wide, masticatory margin with long apical tooth and 5 small teeth of the same size.

Mesosoma short and stout, narrowed posteriorly, promesonotal suture absent, metanotal groove distinct, wide and deep. Anterior margin of pronotum slightly convex, humeri very narrowly rounded. Dorsal surface of propodeum ca. twice shorter than posterior one. Propodeal spines not long (but not dentiform), widened at base, acute at tips, directed more upward than backward and divergent laterally. Petiole with distinct but relatively short peduncle, subequal to its height, its anterior surface strongly concave; petiolar node high, quite thin, with narrowly rounded dorsum and rather steep anterior and posterior surfaces; seen in profile, node subconical, narrowing to apex, with slightly convex crest. Postpetiole short and wide, rounded dorsally.

Frons with coarse longitudinal, slightly sinuous six costae between frontal carinae at level of eyes; surface between costae with fine reticulation. Occipital area, temples and sides of head with coarse reticulation. Mesosomal dorsum with coarse reticulation, mesopleura and sides of propodeum with coarse, short longitudinal rugae. Petiolar node and postpetiolar dorsum smooth and shiny. Gaster smooth and shiny. Mandibles longitudinally rugulose.

Body with sparse, not long, somewhat curved erect to suberect hairs. Scape with sparse, rather short suberect hairs, which are subequal or shorter than width of scape; funiculus with short subdecumbent hairs, legs with sparse short decumbent pilosity.

Etymology
The name is derived from the Latin word “costa” – rib, edge, and refers to the costate sculpture on the frons.