Gerontoformica sternorhabda, Boudinot & Richter & Katzke & Chaul & Keller & Economo & Beutel & Yamamoto, 2022

Boudinot, Brendon E., Richter, Adrian, Katzke, Julian, Chaul, Júlio C. M., Keller, Roberto A., Economo, Evan P., Beutel, Rolf Georg & Yamamoto, Shûhei, 2022, Evidence for the evolution of eusociality in stem ants and a systematic revision of † Gerontoformica (Hymenoptera: Formicidae), Zoological Journal of the Linnean Society 195, pp. 1355-1389 : 1364-1369

† GERONTOFORMICA STERNORHABDA SP. NOV.

( FIGS 1C, D , 2C, F, I, L , 4– 6 , 7E, F , 11 (1-2), 12(2-1, 2-3, 2-4), 13(3’-1); SUPPORTING INFORMATION, FIGS S1 , S 2 )

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Type material: Holotype. Wingless female (w). Myanmar, Kachin State: Hukawng Valley [CASENT 0741233 in an amber piece labelled AMNH SY-23 and deposited at AMNH] . Paratypes. Wingless females (w). Synincluded with holotype [CASENT0741234]; same locality as synincluded holo- and paratypes [UFV-LABECOL-009656, deposited in CELC] .

Diagnosis (wingless female)

Conforming to the diagnosis of the Formicidae (see: Boudinot et al. 2020a, b), including the following key conditions: (1) postgenal bridge elongated, thus head ‘prognathous’ ( Fig. 2C ); (2) cranial condyles of the anterior/ dorsal mandibular articulation enlarged ( Fig. 5B ); (3) toruli oriented dorsolaterally rather than simply dorsally ( Fig. 5A ); (4) procoxae elongate, about twice as long as wide ( Fig. 2C ); (5) prodisticoxal foramen ‘closed’ and protrochanter narrowly necked ( Fig. 4F ) [Note 1]; (6) meso- and metathoracicocoxal articulations ‘closed’, i.e. directed ventrally rather than laterally or ventrolaterally ( Fig. 4F ); (7) abdominal segment II completely petiolated ( Fig. 4F ); (8) subpetiolar process present ( Fig. 4F ); (9) prora present ( Fig. 4F ).

I. Among Formicidae , identifiable as †Sphecomyrminae: (1) Mandibles simple and bidentate ( Fig. 5A ), without: (a) being strongly bowed and multidentate as in the † Camelomecia group, (b) the scythe-like blade as in the haidomyrmecine † Haidomyrmex clade, (c) projecting anteriorly with numerous teeth as in the haidomyrmecine † Aquilomyrmex clade, or (d) the strong torsion of † Zigrasimeciini (the state in † Boltonimecia is uncertain); (2) the antennal scrobes on the face do not extend all the way to the compound eye ( Fig. 4B ) (such scrobes observed in †Zigrasimeciinae, including † Boltonimecia ) [Note 2]; (3) anterolateral corners of head not produced as robust triangles ( Fig. 5A ) (such corners observed in † Dilobops of the †Haidomyrmecinae); and (4) (a) scapes shorter than the width of the head ( Fig. 5A ) and (b) clypeus not extending posteriorly between the toruli ( Fig. 2L ) (such extension observed in † Brownimeciinae );

II. Within †Sphecomyrminae, identifiable as † Gerontoformica : (5) anteromedian clypeal margin not produced as distinct medial lobe ( Fig. 2L ); vs. such a lobe present († Sphecomyrma , † G. tendir ) [Note 3]; and (6) (a) mandibles short and fitting against clypeus when closed ( Fig. 5A ) and (b) metanotum developed ( Fig. 2C ); vs. mandibles elongate and metanotum not developed († Myanmyrma );

III. Within † Gerontoformica , with the following unique condition: (7) the anteroventral (‘subpetiolar’) process of the petiolar sternum long, orthogonal to the longitudinal axis of the petiole, and more-or-less rod-like, i.e. with anterior and posterior margins parallel to subparallel ( Fig. 2C ); vs. short and triangular († G. gracilis , † G. occidentalis , † G. pilosa , † G. robusta , † G. rugosa , † G. spiralis , † G. subcuspis , † G. tendir ) [Note 4];

IV. Within † Gerontoformica , identifiable as a member of the pilosa species group: (8) mesoscutum with distinct, raised transverse carina separating mesoscutal and mesoscutellar regions ( Fig. 6B ); vs. such a carina entirely absent or only poorly developed laterally, thus incomplete medially and not forming a distinct angle between the mesoscutal and mesoscutellar regions in profile view ( cretacica , gracilis groups); (9) tergum of petiolar node anteroposteriorly longer than dorsoventrally tall ( Fig. 2C ); vs. petiolar node tergum taller than long ( cretacica group); and (10) abdominal segment IV (metasomal III) with the cinctus distinct and impressed, i.e. divided into pre- and post-sclerites by a transverse sulcus ( Fig. 5E );

V. Within the pilosa species group, distinguished from all species by the following: (11) cinctus developed, but transverse sulci weakly impressed, thus pre- and postsclerites of abdominal segment IV not meeting at strongly oblique angle ( Fig. 5E ); vs. transverse sulci deeply impressed, thus pre- and postsclerites meeting at distinct oblique angle († G. contega , † G. magna , † G. pilosa ); (12) head in full-face view broader lateromedially than long anteroposteriorly, excluding eyes ( Fig. 2L ); vs. head longer than broad († G. contega , † G. magna , † G. pilosa ) [Note 5]; (13) pretarsal claws edentate ( Fig. 5G, I, J ); vs. each claw with a single tooth of variable location († G. contega , † G. magna , † G. pilosa ) [Note 6]; and (14) body small, mesosoma length <1.5 mm ( Fig. 2C ); vs. body large, mesosoma length> 1.5 († G. contega , † G. pilosa ) and> 2.5 († G. magna );

VI. Further distinguished by species in the pilosa group by the following: (15) anteromedian clypeal margin distinctly evenly curved to an ‘incision’ at the point of contact with the rounded lateroclypeal lobes ( Fig. 2L ); vs. anteromedian clypeal margin weakly convex, without distinct incision between the medioclypeus and lateroclypeal lobes, which are themselves anterolaterally angled († G. contega ); (16) in profile, pronotum evenly curved, mesoscutum more-or-less aligned with mesoscutellum and metanotum, and propodeal dorsal and posterior surfaces curving into one another obliquely ( Fig. 2C ); vs. pronotum, mesonotum, and propodeum each shouldered in appearance, i.e. pronotum with strong anterior dorsolateral bulge, mesonotum with mesoscutal and mesoscutellar regions meeting at nearly a right angle, and propodeal dorsal and posterior surfaces also meeting at nearly a right angle († G. contega ); (17) standing setation on body short, relatively sparse (with the exception of the propodeum and petiolar tergum) ( Fig.5 ); vs. longer setation present († G. magna , † G. pilosa ); and (18) anteroventral process of abdominal sternum III (metasomal II) robust but short anteroposteriorly ( Fig. 2C ); vs. prora anteroposteriorly long, sharkfin-like in form († G. pilosa ) [Note 7].

Notes on the diagnosis

Note 1: The right procoxa and protrochanter of the holotype are slightly disarticulated. However, the focal details to evaluate are the circular shape of the prodisticoxal foramen and the crank-like (curved) and thin proximal neck of the protrochanter, which can be evaluated from Figure 4F .

Note 2: The only species of †Sphecomyrminae to have an elongate antennal scrobe is † G. contega . Although the scrobe of this species was observed via direct examination of the holotype, it remains a possibility that this is an artefact because other specimens of † Gerontoformica may have asymmetrical, sulcuslike distortions of the head corresponding to the position of the scrobe (see the ‘preservation’ section of the † G. gracilis description; Fig. 10 ). Conducting a µ-CT scan of the type specimen and/ or the accrual of additional specimens are necessary.

N o t e 3: T h e a n t e r o m e d i a n c l y p e a l p r o c e s s o f † Sphecomyrma is lateromedially thin and at least as long as broad; it is distinctly bordered laterally by the medioclypeus, i.e. the clypeal ‘disc’ between the lateral clypeal lobes. In contrast, the entire medioclypeus of † G. tendir is apparently produced. See Note 5 of the ‘Notes on classification’ section above.

Note 4: The form of the subpetiolar process is unknown for four species within the genus: † G. contega , † G. cretacica , † G. magna and † G. orientalis .

Note 5: Although it is difficult to evaluate the head shape of † G. contega from the holotype due to cut of the amber matrix, the head does appear to be longer than broad. Reevaluation of this condition through µ-CT is recommended.

Note 6: Teeth on the pretarsal claws have been recorded for most species of † Gerontoformica († G. contega , † G. gracilis , † G. magna , † G. occidentalis , † G. pilosa , † G. robusta , † G. spiralis and † G. subcuspis ). The condition of having reduced teeth on the pretarsal claws, however, is shared with † G. cretacica and † G. tendir . Notably, Perrichot et al. (2008) recorded minute teeth on the pretarsal claws of † G. occidentalis , which suggests the need to re-evaluate the holotype of † G. cretacica .

Note 7: The prora is clearly an important structural feature for distinguishing stem ants but is not clearly visible in most specimens, including † G. contega and † G. magna in the pilosa group, † G. cretacica and † G. orientalis in the cretacica / orientalis group, plus † G. rugosa and † G. tendir . It is extremely small and nearly absent in † G. gracilis and † G. occidentalis , and it is developed, but short and comparatively inconspicuous in † G. robusta , † G. spiralis , and † G. subcuspis .

MEASUREMENTS AND INDICES

Holotype, specimen C-33: HWed = 0.87; HWev = 0.95; EWl = 0.14; HD = 0.55; ML = 1.25; PnLi = 0.50; PnWa = 0.30; MnL = 0.27; AIIILm = 0.48; AIIILl = 0.53; HPI = 0.64; HSI = 0.69; AIIILI = 0.91.

Paratype, specimen C-34: HWed = 0.94; HWev = 1.06; EWl = 0.20; HD = 0.66; ML = 1.39; PnLi = 0.45; PnWa = 0.27; MnL = 0.28; AIIILm = 0.50; AIIILl = 0.64; HPI = 0.70; HSI = 0.67; AIIILI = 0.78.

UFV-LABECOL-009656: HWed = 0.79; HWev = 0.85; EWl = 0.11; HD = 0.46; ML = 1.05; PnLi = (~0.35–0.38); PnWa = –; MnL = 0.29; AIIILm = 0.4; AIIILl = 0.48; HPI = 0.58; HSI = 0.75; AIIILI = 0.83.

Description

Head: The head is broad in facial view, i. e. lateromedially wider than anteroposteriorly long from the anterior clypeal margin to the apparent posterior head margin ( Fig. 2L ); in posterior or lateral view, the head is dorsoventrally narrow, with the vertexal region not being particularly domed; standing setae are present on the vertexal and facial regions [Note 1]; there are short, decumbent setae distributed sparsely on the head capsule, a few longer, and suberect setae medially on the vertex near the ocelli. The compound eyes are situated in the posterior third of the head ( Fig. 2L ); they bulge laterally, breaking the silhouette of the lateral head margins in facial view; their height above the surrounding surfaces of the cranium is comparatively low; they comprise over 100 ommatidia, but not more than 200 ( Fig. 4C ) [Note 2]; they are apparently glabrous, i.e. lacking interstitial setation. The three ocelli are completely developed ( Fig. 2L ). The frontal carinae diverge posterolaterally, toward but not reaching the compound eyes ( Figs 2L , 5A ); they are sinuate in form, i.e. from their anterior margins they are directed relatively medially then bulge laterally before curving laterad; their anterior termini are close to the epistomal line, but not extending on to the clypeus; the minimum distance between the frontal carinae is about 0.29× maximum head width as measured in full face view. The antennal scrobes, or depressed contact surfaces of the face laterad the frontal carinae, are parallel in orientation relative to the frontal carinae and are apparently longer than wide ( Figs 2L , 4B ). The antennal toruli abut but do not indent the posterior clypeal margin [Note 3]; they are in the form of a low, more-or-less even ring. The antennae are 12-merous ( Fig. 4B ). The scapes are somewhat flattened and curved ( Fig. 2C, F, I, L ); they are about three to four times as long as wide ( Fig. 2I ); their length is about half the width of the head, and less than half the length of the head; they bear a vestiture of short subdecumbent to suberect setae. The pedicels are about twice as long as wide ( Fig. 4F ); they are about one-third the length of the scapes, and somewhat more than half the length of the third antennomere; their setation is similar to that of the scapes. The flagellae are longer than the mesosoma and are simple, i.e. not thickening distally ( Fig. 4F ); they bear a range of standing and appressed setae; flagellomeres I are the longest, being about four times as long as wide and more than half the length of the scapes; flagellomeres II–IX are about as long as the pedicel; flagellomeres X are longer than flagellomeres II–IX. The clypeus is about four times as wide (lateromedially) as long (anteroposteriorly), with the length measured from the midpoints of the anterior and posterior clypeal margins and the width measured between the lateralmost points of the clypeus ( Fig. 2L ). The lateroclypeal areas are formed as lateral lobes ( Figs 2L , 5A ). The medioclypeal area is anteriorly convex ( Fig. 2L ); its length at the midline of the head is about 0.21× head length also at midline, as measured in full-face view; it bears five or six long and flexuous setae that are situated near the anterior clypeal margin and anteriorly directed, consisting of one medial seta surrounded by a pair of setae and potentially a second, even more lateral pair [Note 4]; the anterior medioclypeal margin bears a row of chaetae. The mandibles are simple and apically bidentate ( Fig. 5A ). The maxillary palps are 5-merous ( Figs 2I , 4A , 7E, F ) [Notes 5, 6]; they are conspicuously short, with their total lengths shorter than the lengths of either the mandibles or scapes; with the exception of the apical palpomere, they are thick and bulging at about their midlengths; they are adorned with erect and appressed pilosity. The labial palps are 4-merous ( Figs 2I , 4A , 7E, F ) [Note 6]; they are short, being just over half the length of the maxillary palps; with the exception of the apical palpomere, they are more-orless conical and thickening toward their apices; they are adorned with erect and appressed pilosity; the proximal palpomere lacks a distinct process.

Mesosoma: The pronotum bears an anteromedian neck process, and lateral and posteromedian flanges ( Figs 2C, F , 5C , 6A ); these flanges are flared in the paratype specimen C-34 ( Fig. 4C ) but not in the holotype or other paratype; the muscular node or ‘disc’ of the pronotum is almost spheroidal in shape, with the lateral margins strongly convex in dorsal view and the dorsal margin strongly convex in lateral view, and with an anteroposterior length approximating its dorsoventral height; pronotal setation is sparse, being represented by a few subdecumbent setae ( Fig. 6A ). The pronotal lobes are well developed ( Fig. 6A ) [Note 7]. The mesonotum is divided into an anterior mesoscutal area and a posterior mesoscutellar area by the transverse mesonotal carina ( Figs 2C, F , 4C, F , 5C , 6B ). The mesoscutal area is approximately in the form of a low saddle (i.e. is a low hyperbolic paraboloid), with a concave dorsal margin in lateral view, and with the anterior rim more upcurved than the posterior rim ( Fig. 2C ). The mesoscutellar area is convex, but sunken relative to the mesoscutal area ( Fig. 2C ). The mesopectus is divided into dorsal and ventral areas by a longitudinal sulcus ( Fig. 2C ); its dorsoventral height is about equal to its anteroposterior length. The dorsal mesopectal area is approximately rectangular in shape, being somewhat more than twice as long anteroposteriorly as tall dorsoventrally ( Fig. 2C ); its dorsoventral height is one-third the dorsoventral height of the lower mesopectal area as measured from the transverse sulcus directly ventrad to an imaginary line drawn parallel to the ventrolateral margins of the meso- and metapecta. The ventral mesopectal area is approximately triangular in shape, being broad along is dorsal margin and narrowing posteroventrally to its ventrolateral margin ( Fig. 2C ); its anteroposterior length along its dorsal margin is approximately equal to its dorsoventral height. The transverse mesometanotal sulcus is anteroposteriorly short/ thin ( Fig. 2C ). The metanotum is developed as a distinct bulge between the metanotal spiracles ( Fig. 2C ). The transverse metanotopropodeal sulcus is anteroposteriorly long/ broad and continues ventrally toward the base of the mesocoxa, completely separating the lateral metapectal area from the lateral mesopectal area ( Fig. 2C ). The metapleural gland orifice is large, hairy, and in the form of a broad subelliptical pit ( Fig. 6B ); it is margined dorsally by a bulge, the metapleural gland bulla, and ventrally by the ventral metapleural gland flange, which itself is a continuation of the sinuate ventrolateral carina of the metapectus and is spiniform and dorsally inclined. The propodeum is boxy ( Figs 2C , 6B ) [Note 8]; it bears standing setae and is the hairiest mesosomal region. The propodeal spiracles are situated distant from the metanotum, but near the dorsal propodeal margin in lateral view ( Figs 2C , 5C ); they are posteriorly to posterolaterally directed and protected anteriorly by the anterior flange of the propodeal spiracles. The propodeal lobes are apparently not developed.

Legs: The legs are developed as expected for the Formicidae , with some notable characters and states. They are densely hairy, appearing shaggy, with the setae suberect ( Fig. 4A, D, G–J ). Apparently, the apical protibial foramina are open, i.e. without a bridge of sclerite dividing the calcar from the probasitarsus ( Fig. 5G ) [Note 9]. The mesoprefemora and metaprefemora are well developed and are broader ventrally than dorsally ( Fig. 5D ). The protibia bears and anterior brush of dense suberect setae in their apical third, near the calcar ( Fig. 4A ). Each of the mesotibia and metatibia bear a pair of apicoventral spurs ( Figs 4I, H , 5H ); the anterior tibial spurs are pectinate; the posterior tibial spurs are barbirulate to simple. The calcar is apparently bifid apically, with one point being the apex of the elongate velum and the other point a small array of hairs ( Fig. 5G ); two stout setae are developed (inserted) posterior to the calcar. The plantar lobes of the tarsi are not developed (absent), but the tarsomeres have a brush of dense ventral setae, and are apically margined by thick, coarse chaetae ( Figs 4G–J , 5G–J ) [Note 10]. The fourth tarsomeres of each leg are deeply notched distally, thus appearing V-shaped (probasitarsi) or arrowhead-shaped (meso-, metabasitarsi) ( Fig. 4G–J ). The pretarsal claw teeth are extremely small, only visible as weak bulges on the ventral margins of the claws ( Fig. 5G, I, J ). The aroliae of all legs are welldeveloped but comparatively small ( Fig. 5G, I, J ); they are ≤ 0.5× the length of the pretarsal claws.

Metasoma: The petiole is nodiform and lacks tergosternal fusion between its postsclerites ( Figs 2C , 4F , 5F ) [Note 11]. The petiolar tergum is anteriorly narrowed, has the spiracles situated on anterolateral bulges, and has a collar posterior to its node ( Figs 2C , 5F ); it bears several relatively long setae; setae on the remainder of the metasoma are gradually denser and longer from AIII–VII, with the greatest concentration around the sting. The petiolar node is anteroposteriorly longer than dorsoventrally tall ( Fig. 2C , 5F ); its anterior margin is longer than its posterior margin in lateral view, and these margins curve evenly into one another. The laterotergites are developed (present) ( Fig. 5F ), approximately wedgeshaped and broadening posteriorly. The petiolar sternum is anteriorly flat, with this region bearing stiff proprioceptor setae ( Fig. 5F ); its main portion is about twice as long anteroposteriorly as tall dorsoventrally ( Figs 2C , 5F ); it is broadly convex in cross-section at its midpoint ( Fig. 2I ); anteroventrally it is produced as the subdigitate to almost triangular subpetiolar process, which is at least twice as long dorsoventrally as wide anteroposteriorly ( Figs 2C , 4F , 5F ); posteriorly, the sternum is concave, appearing notched, with the concavity receiving the prora. The helcium (presclerites of the third abdominal segment) is narrow relative to the third abdominal postsclerites ( Figs 2C, F , 4C, F ) [Note 12]; the helcial tergite conceals the helcial sternite in lateral view ( Fig. 5F ). The abdominal posttergite III is somewhat constricted posteriorly and is not fused with the third abdominal poststernite ( Fig. 2C ); it is distinctly necked anteriorly, with a dorsoventrally short but distinct anterior surface which curves to the distinct ‘node’ of the sclerite; its ‘node’ is ‘shouldered’ in appearance, bulging anterolaterally around its ‘neck’, with the ‘shoulders’ visible over the ventrolateral tergal margins in ventral view ( Fig. 2I ) [Note 13]. The abdominal tergosternal margin III is weakly curved, without distinct ‘shouldering’ as observed in various Formicinae and Dolichoderinae ( Fig. 2I ) [Note 14]. The abdominal poststernite III is weakly constricted posteriorly, weakly angled lateromedially, and bears the prora anteroventrally ( Fig. 2I ). The prora is subdigitate in lateral view, being dorsoventrally long and anteroposteriorly thick ( Fig. 2C ); lateromedially, it is relatively narrow, and approximately wedge-shaped in ventral view

( Fig. 2I ). The abdominal segment IV is neatly divided into pre- and post-sclerites by the cinctus, or transverse sulci on the tergum and sternum ( Fig. 5E ); its constriction is comparatively shallow. The abdominal presclerites IV are slightly narrower dorsoventrally and lateromedially relative to the postsclerites ( Fig. 5E ). The abdominal poststernite IV is shorter than the abdominal posttergite IV. The abdominal segments V and VI are undivided and are homonomous in form, i.e. highly similar in shape, size, and other qualities of appearance ( Figs 2C , 4A, C , 5E ). The abdominal tergum VII is approximately dome-shaped; in situ, its external surface is oriented dorsoventrally relative to the preceding segments ( Fig. 5E ). The abdominal sternum VII is lateromedially cupped and narrowed distally ( Fig. 5E ). The sting is long and narrow ( Figs 4A , 5E ). The third valvulae (sting sheaths) are digitate in form and exserted to highly exserted as preserved ( Figs 4A , 5E ) [Note 15].

Preservation: Both the holotype and the synincluded p a r a t y p e (Fi g. 4) h av e d e c o m p o s i t i o n b u b b l e s captured in the process of emanating from the head. The non-synincluded paratype ( Fig. 5 ) has no such bubbles on the head, but some are present on the surface of the petiolar tergum, on some of the gastral tergites, and apparently a large one protrudes from the abdominal apex. The decomposition bubbles are associated with deformation of the cuticle, particularly for the holotype, where the affected cheek and fore tibia have bulged into the lumen of the bubble, as if displaced from compression. Three fracture lines are present around the head of the synincluded paratype, one approximately in the frontal plane, another at the base of the left antenna, and one between the head and pronotum. Finally, the metasoma of the synincluded paratype appears weakly desiccated (wrinkled); it is surrounded by a bubble that makes rendering difficult, but it is otherwise preserved well enough to determine fine details and structural proportions. Neither the holotype nor the non-synincluded paratype have fracture lines or apparent desiccation.

Notes on the description

Note 1: The setation of the body, in general, could not be described in fine detail.

Note 2: The ommatidia were counted for one eye of the paratype from a high-resolution photograph. It was not possible to count all of the ommatidia for this eye. Likewise, we counted 85 ommatidia in the left eye of the non-synincluded paratype; about 15% of this eye is concealed, so we expect that there are indeed> 100 ommatidia, but certainly less than 150.

Note 3: The position of the antennal toruli relative to the posterior clypeal margin is known to be of classificatory value above the species level ( Bolton, 2003). We observe that the toruli contact the epistomal line in † G. sternorhabda , but do not provide this in the diagnosis because the posterior limit of the clypeus is difficult to evaluate in some of the fossils, particularly in the type specimens of the gracilis group. However, among the pilosa group, we observed that the toruli are close to or abutting the clypeus in † G. contega and † G. pilosa . It is difficult to determine the posterior extent of the clypeus in † G. magna , thus no confident statement can be made at this time.

Note 4: The setae on the medioclypeal area could only be evaluated on the non-synincluded paratype. The lateralmost seta is probably paired, but the second hair could not be observed.

Note 5: We have recorded the maxillary palps as 5-merous based on scrutiny of the non-synincluded paratype ( Fig. 5B, C ) and our renders of the synincluded types ( Fig. 7E, F ). It is possible that there is a poorly developed proximal sixth palpomere (‘pm?’ in Fig. 7E, F ). The palps are remarkably short compared to those of † G. gracilis , and they lack the process of the proximal labial palpomere which we observed in the other species.

Note 6: The shape of the palpomeres is notable, as the proximal ones appear flattened to some degree. Extended µ-CT sampling is recommended to understand palpomere shape variation.

Note 7: The pronotal lobes of † Gerontoformica are large compared to crown ants. Systematic evaluation of the development of these lobes among stem ants is recommended.

Note 8: The shape of the propodeum appears to differ between the two synincluded types and the nonsynincluded type.While the propodeum is rounded in the non-synincluded specimen, it is somewhat rectangular in the synincluded specimens, with the dorsal and posterior margins oriented nearly perpendicularly and narrowly rounding into one another in lateral view, and its posterior surface nearly flat.

Note 9: We have stated ‘apparently’, as we are not totally certain about this state. We have included it, however, in order to encourage future examination of this structure, which we know to be variable across the Formicidae and other groups of Aculeata (B. E. B., pers. obs.).

Note 10: Plantar lobes are developed in † G. gracilis , which has far sparser ventral tarsomeral setae and proportionally larger aroliae. Because tarsal setae, lobes and aroliae are functional traits related to traction (e.g. Beutel & Gorb 2001; Boudinot et al. 2021a; Wöhrl et al. 2021), the distinctions observed here suggest, or otherwise indicate, that there is niche separation between † G. sternorhabda and † G. gracilis . Among pilosa group species, the fourth tarsomeres are notched in † G. contega , are gracilis -like in † G. pilosa , and are of uncertain form in † G. magna .

Note 11: Whereas it was possible to determine whether or not the postsclerites of the metasoma were fused, it was not possible to evaluate such fusion for the presclerites of the petiole and third abdominal segment.

Note 12: The helcium of † G. sternorhabda is distinctly broader than that of † G. gracilis . However, it was not possible to evaluate this condition for the other previously described † Gerontoformica species.

Note 13: Abdominal tergum III of the non-synincluded paratype is apparently not similarly shouldered. We are, therefore, less certain about the development and distribution of this state, i.e. whether it is artefactual or not, or whether the additional paratype is indeed conspecific. We choose to recognize the additional paratype as conspecific due to the high degree of shared conditions, as indicated by the diagnosis. We also consider the tergal shouldering to likely be a true developmental state of † G. sternorhabda , as we observe it in both of the synincluded specimens, and as the synincluded † G. gracilis clearly does not display this condition. The third abdominal postsclerite is more-or-less evenly curved from anterior to posterior († G. pilosa ) or is entirely evenly curved from anterior to posterior († G. gracilis , † G. robusta , † G. spiralis and † G. subcuspis ); it could not be confidently determined for † G. occidentalis or for the four nomina dubia species.

Note 14: Shouldering of the third abdominal tergosternal margin was used for generic identification and the tribe-level classifications of the Formicinae and Dolichoderinae by Bolton (1994, 2003). We perceive no difference in the form of these margins between † G. sternorhabda and † G. gracilis ; we encourage evaluation of the form of the margins for other stem Formicidae .

Note 15: We anticipate that study of the sting apparati of stem ants using µ-CT to be fruitful. The degree to which the third valvulae are exserted is unusual.

Remarks on the description

The non-synincluded paratype (UFV- LABECOL-009656) may not be conspecific with the synincluded type specimens. It differs from the synincluded specimens by the following states: (1) possibly in maxillary palpomere count (5 vs. 6; see Note 5 of the description); (2) the propodeal spiracle appears to be slightly higher on the propodeum; (3) the propodeum appears to be more rounded; (4) the petiolar node appears to be shorter; (5) the subpetiolar process appears to be straighter; and (6) the third abdominal posttergite does not appear to be shouldered. We note that these are ‘apparent’ differences because of the limited set of available specimens. Based on expectations from the neontological fauna, it is possible that the differences we observe may be infraspecific variation, but it will be necessary to evaluate more specimens. Because the specimens from the two pieces of amber are otherwise highly similar and equally diagnosable, we conservatively designate them as conspecific.

Etymology

The specific epithet sternorhabda combines the Ancient Greek words στερνών (sternum) and ράβδος (rod) in reference to the form of the subpetiolar process, which is unique among † Gerontoformica . The name is adjectival and feminine in form to match the gender of the genus.

Figures

Figure 1. Overview of the Kachin amber piece and its syninclusions.A, the pupa of †G. gracilis (CASENT01741231). B, the adult of †G. gracilis (CASENT0741232). C, the holotype of †G. sternorhabda sp. nov. (CASENT0741233). D, the paratype of †G. sternorhabda sp. nov. (CASENT0741234). E, the entire amber piece (AMNH SY-23).

Figure 2. Volume renders of the pupa and two of the adult specimens. The black arrows indicate the subpetiolar process, while the white arrows indicate the prora. A–C, lateral view. D–F, dorsal view. G–I, ventral view. J–L, facial view. A, D, G, J. the pupa of †G. gracilis. B, E, H, K, the adult of †G. gracilis. C, F, I, L, the holotype of †G. sternorhabda sp. nov.. Abbreviations: AIII–VII, third through seventh abdominal segments; AIIIas, anterior surface of third abdominal segment; AIIIps, posterior surface of third abdominal segment; ce, compound eye; cl, clypeus; fc, frontal carina; lbr, labrum; md, mandible; msn, mesonotum; mspl, lower mesopectal region; mspu, upper mesopectal region; mtn, metanotum; oc, ocellus; pd, pedicel; pnt, pronotum; ppd, propodeum; pst, prosternum; pt, petiole; sc, scape.

Figure 4. Photomicrographs of †G. sternorhabda sp. nov. showing setation and certain key features. A–D, G–I, paratype, CASENT0741234. E, F, J, holotype, CASENT0741233.A, body, ventrolateral oblique view. B, head, dorsolateral oblique view. C–F, body and left legs: D, dorsolateral oblique; (F) is slightly offset from (E), revealing the prora and subpetiolar process. G, protarsus, posterodorsal oblique view. H, mesotarsus, dorsal view. I, metatarsus, dorsal view. J, metatarsus, posterior view. Abbreviations: ad, antennomere distorted, i.e. visibly distorted antennomere associated with decay bubble; cn, cinctus; ms, metanotal spiracle; pr, prora; psf, anterior flange of propodeal spiracle; ptb, probasitarsal brush; spp, subpetiolar process; ss, standing setae; tnc, transverse mesonotal carina.

Figure 5. Photomicrographic details of †G. sternorhabda sp. nov. Paratype specimen (UFV-LABECOL-009656). A, head in approximate facial view. B, mouthparts in anterolateral oblique view. C, head and mesosoma in lateral view. D, proximal segments of metathoracic leg in approximate anterior view. E, metasoma in approximate lateral view. F, petiole in lateral view. G, prothoracic tarsus, oblique posterior view. H, metatibial apex and metabasitarsal base in posterior view. I, propretarsus in oblique distal view. J, metapretarsus in posteroventral oblique view. Abbreviations: AI, propodeum; AII– AVII, second through seventh abdominal segments; ca, calcar; cav, velum (lamella) of calcar; ce, compound eye; cl, clypeus; cll, lateral lobe of clypeus; cns, cinctus of fourth abdominal sternum; cnt, cinctus of fourth abdominal tergum; fc, frontal carina; hetg, helcial tergite; msctl, mesoscutellar area (mesoscutellum); msctm, mesoscutal area (mesoscutum); msnttc, transverse

Figure 6. Detailsof thepronota andmetapleural glandsof †Gerontoformica.A,B,fluorescent lightmicroscopy;C–G,standard photomicroscopy. A, B, †G. sternorhabda sp. nov. paratype (UFV-LABECOL-009656). C, †G. pilosa (ANTWEB1038931). D, †G. sternorhabda sp. nov. non-type (JWJ-BU21). E, F, †G. gracilis holotype (JZC-Bu324A). G, †G. near or conspecific with gracilis (ANTWEB1032649). Abbreviations: msnttc, transverse carina of mesonotum; mtplglb, metapleural gland bulla; mtplgldf, metapleural gland dorsal flange; mtplglvf, metapleural gland ventral flange; mtvlc, ventrolateral carina of metathorax; mtntsp, metanotal spiracle; mtnt, metanotum; pntl, pronotal lobe; pntfl, lateral pronotal flange; pntfm, medial pronotal flange; ppdsf, anterior flange of propodeal spiracle; ppl, propleuron; pt, petiole.

Figure 7. Maxillary and labial palps of †G.gracilis and †G.sternorhabda sp. nov.. Small black stars indicate each palpomere on the right-hand side of the body.A, B, †G.gracilis adult, CASENT0741232, in anterior dorsolateral oblique view (A) and anterior view (B).C, D, †G.gracilis pupa, CASENT0741231, in anterior view (C) and anterior dorsolateral view (D).E, F, †G.sternorhabda sp. nov. holotype, CASENT0741233, and paratype, CASENT0741234, in anteroventral oblique view. Abbreviations: pl, labial palp; plp, process of the proximal labial palpomere; pm, maxillary palp; pm?, possible maxillary palpomere.

Figure 10. Features hidden by appendages, including artefacts caused by close proximity of limbs. †Gerontoformica gracilis: A, B, D, adult, CASENT0741232; C, pupa (CASENT0741231). A, C, mesosoma in lateral view with the legs distal to the coxae digitally removed, with the exception of the fore leg of (C), which is not cropped from the render. B, D, head in facial view with the scapes digitally rendered (B) and removed (D). Abbreviations: mspa, mesopectal artefact; sca, scapal artefact.

Figure 11. Couplet 1: mesonotum with (1-1) or without (1’-1) transverse ridge; abdominal segment IV with (1-2) or without (1’-2) cinctus; prora large (1-3) or small (1’-3). (1-1) †G. ufv-05, ANTWEB103848 (J. Chaul, AntWeb), mesosoma in profile view. (1’-1, 1’-3) †G. indet., JWJ-Bu19 (V. Perrichot, AntWeb), mesosoma in profile view. (1-2) †G. sternorhabda sp. nov. paratype, UFV-LABECOL-009656, metasoma in ventrolateral view. (1’-2) †G. gracilis, CASENT0741232, metasoma in lateral view. (1-3) †G. pilosa holotype, JZC-Bu225 (P. Barden, AntWeb), anterior metasoma in profile view.