Eocenomyrma groehni

The seventh Eocenomyrma  species to be described, it is the fourth member of this fossil genus known from Baltic Amber.

Identification
Radchenko (2020) - Among all known Eocenomyrma species, Eocenomyrma groehni may be confused only with Eocenomyrma orthospina, but it well differs from the latter species by structure of the frontal carinae, which merging with the rugae that extend to the occipital margin; in contrary, the frontal carinae in E. orthospina are short, curve outwards and merge with the rugae, which surround antennal sockets. Additionally, entire head dorsum in E. groehni is with the slightly sinuous longitudinal rugae, but in E. orthospina the only frons is with the not coarse longitudinal, slightly sinuous rugae, and the lateral parts of the head dorsum and occiput are with reticulation.

Distribution
This taxon was described from.

Nomenclature

 * † groehni. †Eocenomyrma groehni Radchenko, 2020: 110, fig. 1 (w.) BALTIC AMBER (Eocene).

Worker
HL 0.66, HW0.60, SL 0.41, FW 0.25, FLW 0.30, OL 0.14, ML 0.75, PNW 0.38, PL 0.31, PW 0.23, PPL 0.21, PPW 0.29, HTL0.43, ESL 0.29, ESD 0.30; HL/HW 1.11, SL/HL 0.69, SL/HW 0.76, FW/HW 0.41,FLW/FW 1.21, OL/HL 0.22, ML/PNW 2.00, PL/HW 0.52,PW/HW 0.39, PPW/PW 1.28, PPW/HW 0.50, PPW/PPL1.44, ESL/HW 0.48, ESD/ESL 1.05.

Total length ca. 2.7 mm. Head somewhat longer than wide, with slightly convex sides, widely rounded occipital corners and slightly concave occipital margin. Eyes relatively big, situated some-what in front of sides of head. Frontal carinae moderately curved, frons quite wide; they merging with rugae that extend to occipital margin, do not curve outwards and do not merge with rugae that surround antennal sockets; frontal lobes extended laterally, covered base of antennae. Anterior clypeal margin widely and shallowly concave medially. Antennae 12-segmented, with rather big 3-segmented apical club that subequal to total length of remainder funicular segments; scape quite long, almost reaching occipital margin, smoothly curved at base, without lobe or carina. Shape of mandibles barely visible.

Mesosoma quite slender, twice longer than pronotal width, humeri rounded, promesonotal suture absent, propodeal groove deep. Propodeal spines long, straight and acute, ca. 0.5 times of head width, strongly diver-gent (seen from above). Petiole with long peduncle, anterior surface of its node strongly concave, node quite high, with rounded dorsum. Postpetiole wide, ca.1.4 times of its own length and ca. 1.3 times of width of petiole. Spurs on middle and hind tibia obscured. Whole head dorsum with slightly sinuous longitudinal rugae. Mesosomal dorsum with coarse reticulation, only surface between propodeal spines with fine trans-versal rugosity; petiole, postpetiole and gaster smooth and shiny. Surface between rugae on head and mesosoma with fine superficial punctation, but appears shiny.

Temples and occipital margin of head with a few straight, quite long and thin erect to suberect hairs, mesosoma with sparse hairs, waist and gaster with more abundant similar hairs. Antennal scape and legs with decumbent to subdecumbent pilosity. Body colour dark brown.

Type Material
Holotype worker, complete specimen, GPIH No. 4945, coll. Gröhn No. 6800 (AntWeb CASENT No. 0917556). Baltic amber, Late Eocene, Priabonian stage, 33.9–37.8 Ma. Deposited in the Geologisch-Paläontologisches Institut des Universität Hamburg, (GPIH), collection of C. Gröhn (now Centrum of Natural History, CeNak).

Etymology
The species is dedicated to Mr. Carsten Gröhn, who collected the holotype specimen.

Taxonomic Notes
Radchenko (2020): Eocenomyrma orthospina: Radchenko and Dlussky 2016: 407 (only specimen No. 6800, misidentification, nec Dlussky and Radchenko, 2006: 563).