Protozigrasimecia chauli

A northern Myanmar fossil from the lowermost Cenomanian horizon (100.5 to 93.9 Mya), near the Albian boundary.

Identification
Cao et al. (2020) - As for genus: Uniquely identified among all Formicidae by the diagnostic features of the tribe Zigrasimeciini and in the Key to the genera of Zigrasimeciini. The taxon definition provided here includes characters which encompass meaningful variation among all described and many undescribed stem Formicidae, as well as with relevant comparisons with crown groups.

The new genus is defined by the following combination of character states (provided in sequence from anterior to posterior along the body axis.): (1) head massive, omega-shaped (Ω); (2) anterior clypeal margin broadly concave and arcuate; (3) traction setae present on the anterior clypeal margin and labrum; (4) clypeal traction setae not restricted to single row on anterior margin, but doubling and tripling on clypeus toward cranial midlength; (5) basal mandibular margin evenly convex, ending apically at base of basal tooth; (6) masticatory margin bidentate, comprising well-developed, acute basal and apical teeth; (7) flagellomeres elongate, length > 2 × width; (8) diagonal antennal scrobes present (extending from toruli to anteromedian eye margins); (9) ocelli present; (10) mesosoma diagonal, with domed promesonotum; (11) promesonotal articulation clearly defined and probably mobile; (12) mesonotum well-developed, comprising only mesoscutum; (13) mesoscutellum and metanotum not expressed; (14) propodeum rectangular and box-shaped; (15) propodeal spiracle situated low and laterally on segment; (16) propodeal spiracle slit-shaped; (17) tibial spur formula 2b,2(1b,1s) (b = barbirulate, i.e., with fringed margin, but spur not pectinate); (18) pretarsal claws with denticle situated in basal half; (19) helcium apparently infraaxial (anterior articulatory sclerites of abdominal segment III situated below segment midheight); (20) prora of abdominal sternum III in form of longitudinal keel; (21) abdominal tergum III constricted posteriorly, almost nodiform, albeit segment large; (22) abdominal segment IV with cinctus (tergum and sternum IV divided into pre- and postsclerites by transverse sulcus); and (23) sting robust (Fig. 5D).

Distribution
This taxon was described from.

Nomenclature

 * † chauli. †Protozigrasimecia chauli Cao, Boudinot & Gao, 2020: 168, figs. 4-5 (w.) MYANMAR (Burmese amber).

Worker
Worker. All measurements in millimeters. Total body length as defined by the addition of head and mesosomal lengths, plus the lengths of abdominal segments III - V and VII: 11.75. Mesosoma and metasoma covered by variably-dense layers of erect, curved pubescence; longer erect setae present on metasoma and legs; long setae especially dense on sixth (= terminal) abdominal segment; dense setal brushes present apically on fore- and hind-tibiae, plus the tarsomeres of each leg; stout, long setae present on ventral surfaces of tarsomeres, and ring the tarsomere apices; a pair of long stout setae accompany protibial calcar, situated posterad; outer / ventral mandibular margin with six widely-spaced and evenly distributed long setae; malar space apparently without long standing setae.

Head: broad, blocky; head width 1.60; head length to anterior margin of clypeal lobes 1.77, head length to anteromedian clypeal margin 1.42. Eyes bulging, maximum diameter 0.55. Ocelli present. Antenna 12-merous; total length uncertain due to poor preservation of apical antennomeres. Scape with short, curled pubescence; pedicel and flagellum apparently without standing setation or pubescence. Scape length 0.45; pedicel length 0.11; length of flagellomeres I - VII (VIII - X too poorly preserved to measure): 0.43, 0.47, 0.40, 0.40, 0.29, 0.28, 0.32, respectively. Frontal carinae poorly developed, margining antennal scrobe posteriorly / dorsally. Clypeus curved for its entire length; lateralmost portions of clypeus, dorsal to mandibular insertion, expanded as disc-shaped lobes; malar space, continuous with lateral clypeal lobes, apparently explanate laterally; anterior clypeal margin apparently not emarginate medially; anterior clypeal margin margined with an even row of about 44 peg-shaped traction setae; a second, messier row of traction setae begins at about the 8th traction seta from the lateral margin; second row including about 36 traction setae. Labrum of holotype partially concealed by mandibles, which overlap in closure; disc of labrum covered in > 50 traction setae which are similar in size to those of the clypeus, but are acutely pointed apically; labral traction setae absent from a lentil- or lens-shaped area located proximomedially on labral disc; labral traction setae approaching the glabrous lentil-shaped area decreasing in size. Mandibles with comparatively thin and short traction setae on their oral surfaces, these setae apparently too fine for impalement; basal mandibular margin forming even, shallow convexity, ending distally at base of basal / subapical tooth; masticatory mandibular margin short, comprising just the two large, acute teeth; apical tooth larger than subapical / basal tooth; aboral surface of mandible with carina extending from mandibular base onto base of subapical / basal tooth. Maxillary palps long, length of visible palp (obscured at base) 0.95; labial palp and maxillary palp base obscured as preserved.

Mesosoma: relatively compact, mesosoma length (= Weber’s length, from inflection between anterior pronotal face and pronotal neck in profile view to posteriormost point of metapleuron) 4.28; dorsal mesosomal margin interrupted by promesonotal articulation and transversely-impressed mesonotal-metathoracicopropodeal suture; promesonotal articulation apparently unfused and mobile. Pronotal length including neck 1.94; mesothorax anteroposteriorly narrower than pronotum and propodeum, minimum length 0.91, height 2.30; metathoracicopropodeal complex box-shaped, taller dorsoventrally (1.57) than long anteroposteriorly (0.91); dorsal and posterior faces of propodeum rounding into one another along a relatively narrow curve. Legs: mesofemur length 1.49. Pretarsal claws dentate, with tooth occurring in basal half of claw. Apicomedian lobate setae (plantar lobes) of tarsomeres I - IV absent, apparently replaced functionally by apical crown of spur-like setae.

Metasoma: Abdomen with seven exposed segments, of these gaster comprising segments III - VII: segment III length 2.24, postsclerite IV length 2.10, length of remaining abdominal segments as preserved 1.37. Petiole apparently long-pedunculate; petiolar node low and convex, not squamiform; vertex of petiolar node apparently without median emargination; posterior petiolar and helcium long. Helcium apparently infraaxial. Prora massive, longitudinal, keel-like. Abdominal tergum III with anterior face steeply angled; posterior margin constricted, giving tergum nodiform-appearance. Abdominal segment IV with cinctus dividing tergum and sternum into pre- and postsclerites; poststernite distinctly shorter than posttergite; abdominal segment V and VI telescoped within segment IV; tergum and sternum VII long, conical. Sting robust.

Type Material
Material: Holotype: No. CNU-HYM-MA2020001, worker. Locality and horizon: Kachin (Hukawng Valley) of northern Myanmar. The lowermost Cenomanian (near Albian boundary).

Burmese amber, Noije Bum hill, some 18 km southwest of Tanai Village in the Hukawng Valley, of northern Myanmar. Depository: Key Lab of Insect Evolution and Environmental Changes, College of Life Sciences, Capital Normal University (CNU), Beijing, China.

Etymology
The specific name is in recognition and appreciation of Júlio C. M. Chaul, a Brazilian myrmecologist who has undertaken a taxonomic review of ‘’Zigrasimecia’’; his insights have definitively shaped our understanding of the group, and particularly for the new genus.