Usomyrma mirabilis, Dlussky & Radchenko & Dubovikoff, 2014

Dlussky, Gennady, Radchenko, Alexander & Dubovikoff, Dmitry, 2014, A new enigmatic ant genus from late Eocene Danish Amber and its evolutionary and zoogeographic significance, Acta Palaeontologica Polonica 59 (4), pp. 931-939 : 932-935

Usomyrma mirabilis sp. nov.

Figs. 1–3.

Etymology: From Latin mirabilis , wonderful, marvellous, miraculous, to indicate the peculiar morphology of this species.

Type material: Holotype: ZMUC, the label says “Bórge Mortensen, 28.2.1965, no. 380”, male, complete specimen. Paratype: ZMUC, the label says “Bórge Mortensen, 1.11.1964, no. 398”, male, complete specimen.

Type locality: Unknown locality in Denmark .

Type horizon: Danish (= Scandinavian) Amber, late Eocene.

Diagnosis.—As for genus.

Description.—Total length ca. 3.5 mm. Head ca. 1.3 times longer than broad, with distinct, quite long temples, narrowly rounded occipital corners and straight posterior margin. Occiput strongly elevated above the level of frons, and very big ocelli situated on the top of this elevated portion. Eyes very big, their maximal diameter ca. 0.6 of head length; genae very short. Shape of clypeus rather peculiar: it is quite long, its median portion swollen and forms subtriangular elevation narrowed anteriorly, lateral portions of clypeus flattened, subrectangular, with narrowly rounded anterio-lateral corners; posterior clypeal margin widely inserted between frontal lobes. Frontal triangle well developed, clearly separated by sutures from the head surface. Antennae extremely unusual (see generic diagnosis); 2 nd to 12 th funicular segments very long and narrow, cylindrical; length of funicular segments starting from third decreasing to the apex of antenna, except for terminal one; length of funicular segments 1:2:3:4:5:6:7:8:9:10:11:12 are 0.09:0.40:0.20:0.20:0.18:0.18:0.16:0.16:0.13:0.13:0.13: 0.25 mm (holotype) and 0.15:0.37: 0.21:0.21:0.18:0.18:0.16:0.16:0.16:0.16:0.18: 0.26 mm (para-

A type). Scape short, ca. 2.5 times shorter than head length. Fore wing with an extraordinary venation (see also generic diagnosis). Vein section 2 M+Cu more than twice longer than cross-vein cu-a. Vein sections 4Cu and 5M well developed, almost reaching wing margin. Vein A short. Cross-vein cua displaced proximally to the base of wing. Length of fore wing ca. 2.9 mm. Venation of hind wing is invisible. Mesosoma long, more than 1.6 times longer than height, scutum and scutellum slightly convex, propodeum rounded, its dorsal surface somewhat convex, subequal to declivity; mesopleural suture presents. Scutum quite narrow, nearly twice narrower than total length of scutum and scutellum; pronotum completely overlapped by scutum, invisible from above. Petiole with relatively short peduncle and well-developed, rounded node, narrowly attached to first gastral segment. Its ventral surface convex, without tooth or lobe (seen in profile). Tarsal claws simple. Genitalia: pygidium convex, without any additional structures, pointed at the apex. Pygostyles well developed. Stipites long and narrow. Volsellae long and narrow, with basal finger-shaped process that almost reach midlength of volsella. Distal part of volsella with long and pointed ventral and blunt dorsal processes. Sagittae of aedeagus long and straight, ventral denticles invisible. Integument of whole body with very fine superficial microsculpture, appears dull. Body and legs seem to be without standing hairs, only funicular segments with very short subdecumbent pilosity.

Dimensions and indices.— Holotype: HL = 0.64, SL = 0.24, AL = 1.12, AH = 0.61, PL = 0.20, PH = 0.15, HTL = 0.58 mm; SI 1 = 0.38, PI = 1.33, AI = 1.83, HTI = 0.91; paratype: HL = 0.60, HW = 0.45, SL = 0.25, OL 1 = 0.35, OL 2 = 0.28, AL = 1.00, AH = 0.62, SCW = 0.38, SCL = 0.68, HTL = 0.75 mm; CI = 1.33, SI 1 = 0.42, SI 2 = 0.56, OI 1 = 0.58, OI 2 = 0.78, OI 3 = 1.27, AI = 1.61, SCI = 1.78, HTI = 1.25.

Remarks.—The wing venation, the antennal structure and the general appearance of the Usomyrma males are so amazing and unusual for any other ants, that when we first investigat- ed the holotype and paratype specimens, we supposed that they may represent a new ant subfamily. Nevertheless, on further study we recognized that in many respects this genus resembles and most probably related to the peculiar dolichoderine genus Leptomyrmex by the general body shape, by the structure of antennae, and by the wing venation. For these reasons we place it to the tribe Leptomyrmecini .

Nevertheless, Usomyrma well differs from Leptomyrmex by a set of important features, that we consider either as plesiomorphic or as autapomorphies (compared to Leptomyrmex ). One of the distinct plesiomorphies of Usomyrma is the peculiarities on the fore wing venation: the wings having the well developed pterostigma, the much wider closed cell 3r, and the big central closed cell that formed by the fusion of the cells (1+2r)+mcu that forms based on the loss of the vein section Rs+M (the latter also can be considered as a unique autapomorphy compare to all other known ants) (for details of the putative reconstructions of evolution of the wing venation in Leptomyrmex see Emery 1913; Wheeler 1934). Another plesiomorphy is the structure of mandibles: they are elongate-triangular, with a well developed masticatory margin, similar to that in Leptomyrmex , but except that in Usomyrma the masticatory margin possesses a set of the well developed, relatively long and sharp teeth.

The most distinctive autapomorphy of Usomyrma is the structure of antennae: the second funicular segment is extremely elongate, much longer than any other segment, including the apical one, while antennae remain 13-segmented. An analogous structure of the antennae is known only in two New Guinean species ( Leptomyrmex puberulus Wheeler, 1934 and L. flavitarsus [Smith, 1859]), where the second funicular segment is similarly elongate, but the antennae have become 12-segmented! In other words, the second funicular segment in Usomyrma is elongated itself, but in the two Leptomyrmex species it represents the fusion of the second and third segments. The depressed frons, the peculiar shape of clypeus, the reniform eyes also may be considered as autapomorphies in Usomyrma (for details see description, above).

Stratigraphic and geographic range.—Late Eocene Amber, Denmark.