Platythyrea dlusskyi, Aria, Cédric, Perrichot, Vincent & Nel, André, 2011

Aria, Cédric, Perrichot, Vincent & Nel, André, 2011, Fossil Ponerinae (Hymenoptera: Formicidae) in Early Eocene amber of France, Zootaxa 2870, pp. 53-62 : 55-57

Platythyrea dlusskyi sp. n.

( Figures 1 , 3 A–B)

Material examined. Holotype MNHN A32915 (PA7873, worker).

Etymology. The specific epithet is a patronym honoring our colleague Gennady Dlussky , a world authority on fossil ants, for his many contributions to the subject.

Diagnosis. Ponerine ant worker or ergatoid queen with the following characters: head longer than wide, in frontal view with anterior clypeal margin and sides convex, posterior margin nearly straight; posterior clypeal margin strongly convex medially between frontal lobes; mandibles with 10–11 blunt denticles on masticatory margin, with external margin slightly concave medially; antennal sockets widely separated; antenna with scape slightly surpassing the occipital margin, evenly curved; first funicular segment smaller than second. Propodeal dorsum posteriorly with two blunt teeth each prolonged in a concave carina laterally on declivitous face. Petiole longer than high and broad, its dorsal sclerite roughly parallelepipedic in shape, with a small tooth at anterolateral corners, anterior and dorsal margins evenly convex, posterior margin slightly concave, with a pronounced lobe projecting posteriorly above anterior sides of helcium at posterolateral corners.

Measurements. HL 1.65, HW 1.30, MFC 0.45, SL 1.55, ED 0.40, AL 2.45, PW 0.93, PtL 0.90, PtH 0.67, FL 1.57, CI 79, FCI 35, SI 119, OI 24, PtI 74, FI 64, AI 38.

Description. Worker or ergatoid queen. Body length around 8.00 mm; nearly completely glabrous, apparently mostly smooth except rough punctuation of small shallow foveolae limited to mandibles, mesosoma and petiole; head 1.3 times longer than wide, with sides convex, anterior clypeal margin distinctly convex, posterior (occipital) margin nearly straight; central part of head including frontal carinae, frontal triangle and clypeus prominent; posterior clypeal area rather broad and strongly convex medially between frontal lobes; minute setae on mandibles and antennae; mandibles triangular, curved in lateral view, their external margin slightly concave medially, with 10–11 blunt denticles and setae on masticatory margin; clypeus almost glabrous, with only four fine setae along anterior margin; frontal lobes moderately expanded laterally, covering at most half the antennal sockets; frontal carinae well-separated from compound eyes, barely diverging posteriorly. Antenna 12-segmented; scape surpassing the occipital margin for a distance equal to its maximum width, slightly curved; first funicular segment smaller than second, which is a little more than twice as long as broad; other funicular segments decreasing progressively in size. Eyes of moderate size compared to head width, rounded, slightly bulged, situated anteriorly around midlength of head. Ocelli vestigial, removed far posteriorly from compound eyes. Mesosoma long, with dorsal surface rather flat; promesonotal suture conspicuous; anterior margin of mesonotum projecting forward over suture of pronotum; mesonotum and propodeum dorsally fused, mesopropodeal suture present although weakly visible in lateral view; propodeum with a pair of blunt teeth laterally at juncture of dorsal and declivitous faces, the teeth prolonged in concave carina along propodeal declivity; propodeal spiracles relatively small, rounded, situated on metasternal margin, slightly behind midlength of propodeal sides; metapleural lobes large, with rounded orifice. Legs long, glabrous, except scarce setae on tip of tibia and tarsi; protibia with one long pectinate spur, mesotibia with two finely pectinate spurs, one slightly shorter than the other; metatibiae not preserved after first half; claws missing. Petiole 0.75 times as high as long; dorsal sclerite in lateral view roughly parallelepipedic, with a small tooth at anterolateral corners, anterior and dorsal margins evenly convex, posterior margin slightly concave, with a pronounced lobe projecting posteriorly above anterior sides of helcium at posterolateral corners; ventral sclerite with a small ventral tooth at about midlength of petiole. Gaster relatively short, distinctly constricted between abdominal segments III and IV; segments III and IV covering V and partly VI; tip of gaster pubescent; sting not visible, if present.

Discussion. This fossil possesses all diagnostic characters of the Platythyreini as proposed by Bolton (2003), and it falls in the genus Platythyrea after the regional Ponerinae keys of Bolton (1994).

In addition to the 37 extant species, five fossil species have been described in the genus Platythyrea . The Late Eocene Baltic amber P. primaeva Wheeler, 1915 , redescribed by Dlussky (2009) , differs from our fossil by its smaller size (body length of worker and gyne 5.5–6 mm, respectively, versus 8 mm in P. dlusskyi ), its cubic petiole with a postero-dorsal tooth-like projection (parallelepipedic and without projection in P. dlussky ), and the antennae with second funicular segment relatively shorter ('only slightly longer than broad', versus twice as long as broad in P. dlusskyi ). The four other fossil species are from Miocene Dominican amber ( Lattke 2003, De Andrade 2004): P. dlusskyi sp. nov. is easily distinguishable from P. scalprum Lattke, 2003 and P. pumilio De Andrade, 2004 in its larger size (body length 3.8 mm in workers of P. scalprum and P. pumilio ), its scape and second funicular segments relatively longer (SI 67 in P. scalprum , 58.6 in P. pumilio , 119 in P. dlusskyi ), and from P. scalprum in its petiolar shape ('petiole elongate', 0.44 times as high as long in P. scalprum , 0.75 times in P. dlusskyi ). The dorsally-pointing propodeal denticles on the third species, P. procera Lattke, 2003 , seem to represent a significant structural difference from our fossil, P. dlusskyi . Finally, P. dentata Lattke, 2003 , although based on a wingless queen, is similar to our fossil in its dentate mandibles, but it differs by having a nearly straight anterior clypeal margin, a posterior petiolar margin with a dorsal lobe, and the metacoxa with a triangular dorsal tooth.

Our fossil does not fit either with any of the extant Platythyrea species in Brown's (1975) identification keys to Africa (petiole neither armed nor toothless) or Indo-Australia (petiolar node neither higher than long, nor bearing three teeth). The key to New World Platythyrea species by De Andrade (2004) would place P. dlusskyi near the extant P. l e n c a De Andrade, 2004, P. p r i z o Kugler, 1977 and probably the extinct P. d e n t a t a. The synapomorphy uniting these species is the presence of teeth on the mandibles, which has been proposed as a possible plesiomorphic state ( Lattke 2003). The fact that the oldest Platythyrea species displays mandibular teeth may support this hypothesis, although convergence is still highly probable for traits of this type. Furthermore, the size and length of the funiculus in P. dlusskyi resemble that of the extant P. l e n c a and P. p r i z o rather than P. dentata .

The presence of ocelli among Platythyrea is uncommon, although present in the queen of the extinct morphotype P. dentata , and 'probably present' in the ergatoid female of P. primaeva (see Wheeler, 1915: p. 37). The possession of ocelli is often related to an improved flight vision (Kastberger 1990), either in alate sexuals or vestigially in ergatoid specimens, as could be the case in our fossil. There is only one Platythyrea species with ergatoid queens in extant faunas, P. conradti Emery, 1899 , but all individuals lack ocelli according to Molet and Peeters (2005) who present a brief morphological comparison between the ergatoid and the worker, which we cannot use since we only have one specimen. Therefore, it is difficult to decide whether this fossil is a worker, an ergatoid sexual or even a gamergate. We leave the caste in the diagnosis as an open question.

Platythyrea are predominantly humid intertropical ants living in twigs, fallen dead trees and other cavities or, for larger subtropical African species, underground ( Brown 1975). Members of this group actively run over tree trunks during foraging activities, which may be what the fossil specimens were doing in the Early Eocene forest ecosystem of Oise, leaving them susceptible to capture by fresh resin. Platythyrea probably played an important role in the ecosystem and may have competed with other arboreal ants such as Gesomyrmex or Tetraponera , also found in Oise amber (Aria et al. in prep.). These associations reinforce the hypothesis that a humid, warm forest was spread near the northern coast of France during the earliest Eocene.

Figures

FIGURE 1. Platythyrea dlusskyi sp. n., reconstruction of the holotype (worker or ergatoid queen) MNHN A 32915. A. Left profile view of propodeum, petiole and first segment of gaster. B. Right dorsolateral view showing the articulation of the central body part. C. Head in frontal view.

FIGURE 3. Photographs of the ponerine ants from Early Eocene French amber. A – B: Platythyrea dlusskyi sp. n., holotype worker or ergatoid queen MNHN A 32915; A. Head in frontal view. B. Right profile of the body, excluding head. C – E: Male undetermined, specimen MNHN A 32917. C. Habitus in profile view. D. Detail of the central part of body in left lateral view. E. Apical part of gaster in lateral view. F. Male undetermined, specimen MNHN A 32918, detail of head in ventral view. All scale bars: 0.5 mm.