Protoceratomyrmex revelatus

Identification
As for the genus by monotypy.

Distribution
This taxon was described from.

Castes
Known from workers.

Specimens
Originating from amber mines located near Noije Bum Village, Tanai Town, Myitkina District of Kachin State, northern Myanmar. A radiometric dating of zircons from the amber-bearing bed gave a maximum age of 98.79 ± 0.62 Ma (Shi et al., 2012), thus corresponding to the latest Albiane-earliest Cenomanian (mid-Cretaceous), which may not be very different from the age of the amber itself (Smith and Ross, 2018; Yu et al., 2019).

Holotype NIGP172002. Worker. A specimen exposed in profile views, without apparent distortion, missing apices of left antenna and left mid- and hind tarsomeres. In a piece of amber measuring 9 x 8 x 4 mm, with a mite. NIGPAS, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Science, China.

Nomenclature

 * † revelatus. †Protoceratomyrmex revelatus Perrichot, Wang & Barden, 2020: 12, figs. 6A-B, 8B (w.) MYANMAR (Burmese amber).

Worker
Measurements (in mm) (holotype NIGP172002). HL 0.82; EL 0.20; HoL ca. 0.09; MDbL 0.36, MDtL 0.09, MDaL 0.41; length of antennomeres: I (scape) 0.42, II (pedicel) 0.08, IIIeXII 0.27, 0.13, 0.14, 0.12, 0.08, 0.10, 0.10, 0.11, 0.10, 0.12; WL 1.21; PL 0.33, PH 0.25.

Total length 4.30 mm. Cuticle generally glabrous throughout.

Head. Vertex broad, gradually rounded posterolaterally, medially flattened; head capsule tapered gradually toward anterior margin with maximum width at vertex approximately 2 x that at mandibular insertion; head flattened anteriorly, with steep elevational incline present dorsally to oral opening; ventral surface of head severely depressed around occipital foramen. Ocelli absent, oval-shaped eyes reduced, positioned near midlength of head in lateral view, with dorsal margin abutting vertex of head. Mandibles scythe-like, laterally flattened, dorsoventrally expanded, appearing broad in lateral view; medial margin of mandibles slightly bowed anteriorly, producing cup-like curvature; dorsally developed apical tooth arising gradually, producing curved dorsal mandibular margin from basal arm of mandible, triangulate blade present anteroventrally at “elbow” junction of basal mandibular margin and apical tooth, fine setae present on the lateral margin of this expansion; dorsal margin of mandible unarmed; length of apical tooth and basal margin of mandibles approximately equal. Maxillary palps elongate, ca 0.75 x length of head capsule, comprising six equally sized palpomeres; labial palps stout, roughly equal in length to two maxillary palps, comprising four equally sized palpomeres. Clypeus elongate and steeply elevated, sclerite is well defined by sulci, posterior and lateral margins meet broadly as gradually rounded suture; small, triangulate clypeal horn present, slight anterior widening visible from oblique view; long, fine seta present at base of clypeal horn, approximately 2 x length of horn itself, reaching beyond apex of mandibles as preserved (second, symmetrical hair flanking horn presumably lost). Antennal sockets present just dorsad posterior margin of clypeus; antenna with scape elongate, third antennomere more than twice as long as following one. Cuticle raised between antennal sockets into laterally flattened projection, likely homologous with “frontal triangle” of other haidomyrmecine taxa; projection with sharp anterior face, slightly declined dorsal face, and gradually sloping posterior face, 0.23 mm in length and 0.08 at greatest height.

Mesosoma. Pronotum and propleuron anteriorly extended into neck to meet and accommodate depression of head; propleuron reduced in lateral view, only faintly visible; pronotum broadly arched and dome-like; expanded ventrally in lateral view, with maximum height approximately equal to pronotal length in dorsal view; posterior margin of pronotum demarcated by well-defined mesonotum, resulting in a sharply circular posterior margin; in lateral view, posterior margin of pronotum nearly extending to posterior margin of mesonotum. Maximum height of pronotum, mesonotum, metanotum, and propodeum all approximately equal, with metanotum and propodeum slightly lower in elevation. Propodeum height and length approximately equal, broadly rounded dorsally; large propodeal spiracle situated high, circular and gaping; metapleural gland opening slightly oval-shaped. Procoxa approximately twice as long as mid- and hind coxae; femur and tibia of each leg coated in fine setae; protibia with large calcar and two stiff setae approximately 0.33 x length of calcar; mid- and hind legs with two tibial spurs of equal size; trochantellus present on mid- and hind legs; tarsomeres with fine, stiff setae on underside; conspicuous pretarsal claw present.

Metasoma. Petiole longer than high, node-shaped, coated in short, fine setae; peduncle short, comprising approximately 0.2 x length of petiole itself; petiole node broadly rounded, with anterior surface gradually increasing in elevation, slightly flattened dorsally; posterior surface of petiole attaches to gastral segment I (abdominal segment III) broadly, following a slight decrease in elevation; ventral margin of petiole appears unarmed, without any process or tooth. Abdominal segment III with significant helcium, approximately equal in length to petiole peduncle; sternite possessing a slight ventral keel, triangulate and projecting just ventral to petiole as preserved. Darkened lateral sulci visible on each gastral segment, present along lower one-third of the gaster. Deep constriction present as circular banding between abdominal segments III and IV, this circular constriction interrupted by slight vshaped posterior expansion of abdominal tergite III, visible from above. Pygidium with elongate, tapered setae; third valvula visible above sting; sting curved near apex.

Type Material
Holotype. NIGP172002, worker. Horizon and locality. Upper Cretaceous, upper Albianelower Cenomanian (ca. 99 Ma); in amber from the Hukawng Valley, Kachin State, Myanmar.

Etymology
The specific epithet derives from revelatus (Latin, meaning ‘reveal’ or ‘show’), and refers to the clypeal margins, horn, and frontal triangle, which informed interpretations of morphological development in hell ants.