A Diverse Ant Fauna from the Mid-Cretaceous of Myanmar (Hymenoptera: Formicidae)

A new collection of 24 wingless ant specimens from mid-Cretaceous Burmese amber (Albian-Cenomanian, 99 Ma) comprises nine new species belonging to the genus Sphecomyrmodes Engel and Grimaldi. Described taxa vary considerably with regard to total size, head and body proportion, cuticular sculpturing, and petiole structure while all species are unified by a distinct shared character. The assemblage represents the largest known diversification of closely related Cretaceous ants with respect to species number. These stem-group ants exhibit some characteristics previously known only from their extant counterparts along with presumed plesiomorphic morphology. Consequently, their morphology may inform hypotheses relating to basal relationships and general patterns of ant evolution. These and other uncovered Cretaceous species indicate that stem-group ants are not simply wasp-like, transitional formicids, but rather a group of considerable adaptive diversity, exhibiting innovations analogous to what crown-group ants would echo 100 million years later.


Introduction
Fossils from the Cretaceous are the only windows into ancient morphotypes exhibited by now extinct stem-group ants. The earliest ants known are Late Albian to Early Cenomanian (,99-100 mya) in age from approximately contemporaneous Charentese and Burmese amber of France and Myanmar, respectively [1]. Although there are insect yielding fossil deposits older than 100 million years, no definitive ants have been recovered from these localities. Along with the very oldest ant fossils, there are younger Cretaceous deposits that shed light on early ant history: species have been described from New Jersey amber (92 mya), two Siberian amber deposits (,85 and ,80 mya), and Canadian amber (,79 mya) [1]. Most Cretaceous species are not readily assignable to modern subfamilies and are either incertae sedis or belong to taxonomic groups erected to accommodate presumed basal lineages. These stem-group taxa persist only into Canadian amber from the Late Cretaceous [2]. The majority of stem-group ant taxa belong to the subfamily Sphecomyrminae, which possess some, but not all, of the synapomorphies associated with living crown-group ants. Namely, while sphecomyrmine ants possess a petiole and metapleural gland opening, the scape is shorter than those found in living ants today [3]. Over the past 20 years, new fossil discoveries have demonstrated that ants within the Sphecomyrminae are surprisingly diverse, exhibiting unusual feeding morphology in particular [4][5][6][7][8].
While there are a number of Cretaceous amber deposits, ants from this time period are exceedingly rare and not particularly speciose. In the case of Burmese amber, ants comprise approximately 0.2% of all insect inclusions [9-10] with a total of seven described species [11]. This low species number provides little resolution for describing patterns of early ant diversification and makes a significant novel species radiation particularly noteworthy. Here we describe nine additional species with clear morphological variation coupled with shared characters suggesting a significant radiation of closely related stem-group ants -the first such instance known from Cretaceous ant fauna.
Burmese amber, also called ''burmite'', once thought to be Miocene in age [12], is now known to be 99 myo based on radiometric dating [13]. This amber has been commercially exploited for millennia, the earliest record corresponding to ornate burmite carvings from as early as 600 BC [14]. Over the past decade there has been a revived, acute interest in the insect inclusions in burmite, both scientific and personal, largely because it preserves the most diverse Cretaceous paleofauna in amber relative to other Cretaceous amber deposits [9] [15]. The amber is hard and polishes well, it is abundant, and so it can be commercially marketed. In many cases the scientific endeavor to describe and interpret this paleofauna aligns with the interests of private collectors, allowing for the study of otherwise inaccessible material (e.g., Barden and Grimaldi, 2013). Indeed, natural history museums are replete with specimens that were originally in private collections, such as the Frick collection of fossil mammals, the Rothschild collection of birds, and the Henry Edwards collection of Lepidoptera at the American Museum of Natural History. The specimens reported here represent the largest assemblage of Cretaceous ant specimens known to date. surfaces were then ground slightly and polished using circular, wet emory papers of decreasing grit sizes (400, 600, 800, 1200, 2400, 4000) on a lapidary wheel (Buehler). Embedding in a high quality synthetic resin was necessary only for a few pieces with fractures, following the procedure of Nascimbene and Silverstein [17] (in this study EpoTek 301-2 was instead used as the embedding resin). In order to optimize the observation and measurement of particular structures and views of the ant, the prepared amber piece was lightly pressed into a small (4-5 mm diameter) ball of dental wax or plasticene in the desired orientation. A small drop of glycerine was applied to the upper surface of the amber piece and covered with a glass coverslip, which obscures fine surface imperfections and improves resolution at higher magnifications. Study was generally made using a Leitz Wetzlar stereoscope at magnifications between 48-1446. Measurements and photomicrographs were made using a Nikon SMZ1500 stereoscope with Nikon NIS software; photomicrographs were z-stacked.
All measurements were recorded in millimeters. Length and width measurements were taken at their greatest value unless otherwise noted. Due to variable preservation, measurements were taken for the holotype only with exception of total body length, which was obtained for paratypes if possible. Paratype and holotype body lengths were reported as a range in descriptions. Mesosomal length was characterized in two ways: a traditional measurement known as Weber's length taken as a straight line from the anterior margin of the pronotum (excluding any ''neck'') to the posteroventral margin of the propodeum; and as individual measurements of the pronotum (including any neck-like anterior extension abutting the occipital carina), mesonotum, metanotum, and propodeum in lateral view. The pronotal, mesonotal, and metanotal measurements were taken as a straight line along the dorsal margin of each sclerite while the propodeum was measured from the anterodorsal margin of the sclerite to the dorsal-most point of anterior petiole attachment. The additional mesosomal measurements were recorded to better capture the relative proportions of individual sclerites and because some specimens were posed in a manner that artificially decreased Weber's length due to positioning or angle of view (such specimens are noted in descriptions). In cases where exact measurements were not possible due to refractive distortion the structures were not measured, or they were described with approximate relative sizes.

Nomenclatural Acts
The electronic edition of this article conforms to the requirements of the amended International Code of Zoological Nomenclature, and hence the new names contained herein are available under that Code from the electronic edition of this article. This published work and the nomenclatural acts it contains have been registered in ZooBank, the online registration system for the ICZN. The ZooBank LSIDs (Life Science Identifiers) can be resolved and the associated information viewed through any standard web browser by appending the LSID to the prefix ''http://zoobank.org/''. The LSID for this publication is: urn:lsid:zoobank.org:pub:7B4FB94C-DB6F-4441-9414-70CCA9F7431B. The electronic edition of this work was published in a journal with an ISSN, and has been archived and is available from the following digital repositories: PubMed Central, LOCKSS.
WORKER DIAGNOSIS (revised): Species possess a well developed metapleural gland opening and sting; distinct petiole; and shortened scape (,0.25 the total antennal length) like other Sphecomyrminae; differentiated most easily by the small, stout setae along the entire anterior margin of the clypeus. In addition, all known Sphecomyrmodes possess bidentate mandibles, and a subapical tooth on the pretarsal claw. Similar clypeal structures are found in one other sphecomyrmine genus and two Cretaceous genera incertae sedis. In the case of Zigrasimecia, the clypeal setae are accompanied by long, tapered setae on the labrum, which are absent in Sphecomyrmodes. The incertae sedis genera, Myanmyrma and Gerontoformica, can be distinguished from Sphecomyrmodes by a conspicuous medial gap between the clypeal setae and an elongated scape, respectively. COMPOSITION: Species contegus n. sp., gracilis n. sp., magnus n. sp., orientalis, pilosus n. sp., rubustus n. sp., rugosus n. sp., spiralis n. sp., subcuspis n. sp., tendir n. sp., in amber from northern Myanmar, Albian-Cenomanian (ca. 100 myo); species occidentalis in amber from Charente-Maritime, France (Early Cenomanian, ca. 100 myo).
Sphecomyrmodes  0.40 mm in length, with two denticles at apex; apical tooth longer than subapical one, approximately half as broad as subapical tooth [lengths difficult to measure as preserved]; external surface of mandible with sparse setae. Labial palps with two equal-sized segments, maxillary palps four-segmented, first approximately 0.36 the length of segments 2, 3, and 4 individually.
Mesosoma: Cuticle rugose where detail visible. Aside from pronotum, propleuron, and legs, mesosomal segments dessicated and appear flattened in dorsal view. Weber's length 1.65 mm. (some flexibility in mesosoma apparent, reducing the length of this metric). Dorsal lengths: Pronotum 0.76 mm, mesonotum 0.59, propodeum 0.64. Pronotum with distinct neck, hourglass-shaped when viewed from above, 0.26 mm wide at head attachment, 0.14 mm in ''neck'' extension, 0.28 mm at widest point above proxocoxae; in lateral view pronotum with blunt dorsal projection. Dorsal and posterior margins of pronotum almost perfectly linear from lateral view, posterodorsal edge of pronotum a right angle. Pronotal-propleural sulci faint; propleuron reduced, only slightly visible. Mesonotum a trapezoid when viewed laterally, raised above the pronotum and metanotum, the anterior face approximately 0.36 the length of the posterior. Metanotum a distinct sclerite possessing protruding spiracle opening; metanotal groove present as small area of darkened cuticle. Propodeal spiracle visible as oval-shaped opening at the apex of pointed, cuticular projection. Metapleuron covered with small, tapered setae; metapleural gland opening visible, appears as a slight indentation just anterior to petiole attachment. Procoxa with line of small setae along anterior edge, 0.49 mm long, 0.22 mm wide at base, 0.15 mm at lower joint; protrochanter 0.18 mm long, 0.13 mm at widest; profemur 0.89 mm long, 0.15 mm wide, covered in small setae becoming more dense apicad, trochantellus present; protibia 0.93 mm long, 0.10 mm wide, dense patch of setae at lower joint; protibial spur with small subapical projection forming forked tip. Mesocoxa 0.43 mm long, width distorted; mesotrochanter 0.21 mm in length, 0.12 mm wide; mesofemur 1.23 mm long, 0.16 mm wide, small patch of setae at lower joint, trochantellus visible; mesotibia 1.10 mm long, 0.10 mm wide, four stiff, pointed setae and patch of short, tapered setae present at lower joint; two mesotibial spurs present, one slightly pectinate. Metacoxa 0.39 mm in length, width distorted; metatrochanter 0.17 mm in length, width distorted. Meta-femur 1.54 mm in length, 0.14 mm wide, setose near lower joint, trochantellus present; metatibia setose throughout, 1.37 mm in length, 0.10 mm in width; two metatibial spurs present, both finely pectinate. Tarsal segments densely setose, each terminating with four thick, pointed setae. Tarsal claw possessing small subapical point in addition to terminal tooth, resulting in two-pronged claws.
TYPES: Holotype JZC Bu300A. Wingless female (presumed worker). Preserved in a 961263 mm section of transparent yellow amber with a small midge. Some measurements were not possible due to shrinkage and desiccation during preservation. Paratype JZC Bu115-A.
ETYMOLOGY: From the Latin contego meaning to conceal or protect. Referencing the antennal scrobes, which presumably functioned as reservoirs for antenna scapes as in modern ants.
COMMENTS: Paratype JZC Bu115A may be a different species since it has reduced antennal scrobes and the head is less elongate; preservation, however, makes this difference uncertain.
DESCRIPTION: Head: [One side of head partially missing, lost at surface of amber]. Head with sparse, tapered setae. Head capsule gradually rounded at posterior and anterior ends in frontal view, length 1.14 mm from posterior margin of head to anterior margin of clypeus, greatest width 1.10 mm. Head in profile drop-shaped, 0.54 mm deep at occipital carina; 0.70 mm depth from median axis of eyes, 0.30 mm depth from anterior margin of clypeus. Ocelli present, situated just above posterior margin of eyes, circular, approximately 0.02 mm diameter, slightly obscured by fractures in cuticle. Midline of eye positioned 65% of the distance from anterior margin of clypeus to occipital foramen, bulging in anterior view, inner margins approximately 0.80 mm apart; eye 0.33 mm deep, 0.20 mm wide. Cuticle raised between antennal bases; bases embedded but exposed, internal margin with small, curved, frontal carina running to just lateral to anterior margin of eyes; tentorial pits present just below anterior terminus of frontal carina. Scape 0.48 mm, pedicel 0.18, funicular segment I 0.42, II 0.34 mm, III 0.31, IV 0.35, V 0.35, remaining segments missing; scape broad at base (0.15 mm), abruptly narrowed to 0.07 mm. Clypeus 0.22 mm long, posterior margin concave, anterior margin convex, lateral margin linear and meeting posterior suture at 145u angle. Anterolateral margin of clypeus covers mandible base, slightly upturned; dorsal surface covered with short, tapered setae. Entire anterior margin of clypeus with row of numerous (.20) small, pointed, peg-like setae. Mandible with two denticles, apical tooth slightly larger of the two. [Mandibular measurements not possible as preserved]. Maxillary palps with four palpomeres, I 0.14 mm, II 0.20 mm, III 0.20 mm, IV 0.16 mm. Labial palps not visible.
Mesosoma: Long and gracile, Weber's length 2.34 mm; height 0.32 mm, measured as a perpendicular straight line at mesonotum/mesopleuron. Segment lengths in lateral view: pronotum 0.89 mm, mesonotum 0.57 mm, metanotum 0.28 mm, propodeum 0.69 mm. Anteriorly, pronotum extends into neck, in dorsal view appears as a rounded rectangle meeting occipital carina; carina oval in dorsal view; pronotum gradually widens posteriad from 0.28 mm at head attachment point to 0.55 mm above procoxae. Pronotal-propleural sulci well developed. Mesonotum and mesopleuron very narrow in lateral view, greatest depth 0.12 mm and 0.21 mm, respectively. Mesopleuron extremely long, 0.77 mm, widely separating fore-and mid-legs. Small depression between mesonotum and metanotum, which are at same dorsally height, both slightly rugose. Metanotal spiracle opening small and turret-like, surrounding cuticle dorsally pointed.
Metasoma: Petiole nodiform, with peduncle short (0.15 mm), total length 1.00 mm, attached to propodeum at a height of 0.15 mm in lateral view, broadened to 0.52 mm at apex of node;   capsule slightly elongate from frontal view 1.256longer than wide; ocelli clearly visible; petiole sessile; darkened and prominent metanotal groove.
DESCRIPTION: Head: Elongate in frontal view, length 1.49 mm from anterior margin of clypeus to posterior margin of head, width 1.18 mm below eyes. In profile, head capsule drop-shaped, narrowing toward anterior end, with cuticular buldge at medial axis. Head somewhat flattened dorsally from medial margin of eyes to occipital carina, gradually rounded at apex. Occipital carina oval, gradually rounded with no apparent deviations in shape. Eyes very large, 0.44 mm long, 0.35 mm wide; high on head capsule, at 80% of the distance from anterior margin of clypeus to occipital foramen; distance between inner margins of eyes wide, 0.95 mm; eyes bulging significantly in frontal and dorsal views. Ocelli present, oval, 0.10 mm long, 0.06 mm wide, spaced 0.17 mm apart on flattened cuticle between posterior margin of eyes. Cuticle raised between antennae. Antennal bases exposed, 0.15 mm in diameter, seated in oval cuticular depressions approximately 0.50 mm long, 0.40 mm wide; tentorial pits present near anterior margin of these depressions; depressions surrounded along interior curvature by frontal carina, which runs from 0.11 mm below antenna base to ,0.20 mm below anterior margin of eyes. Antenna: total length 6.66 mm, scape 0.70, pedicel 0.18, funicular segment I 0.61, II 0.51, III 0.65, IV 0.64, V Mandibles obscured by fissure, appear narrowed for apical 2/3, possibly bidentate. Maxillary and labial palps not visible.
Mesosoma: Robust, of similar height and length; setose, especially on dorsal surface. Weber's length 2.51 mm (angle of view likely reducing this measurement slightly). Segment lengths in lateral view: pronotum 1.20 mm, mesonotum 0.63, metanotum 0.29, propodeum 0.90 mm. Pronotum with anterior extension, terminus rounded to meet and accommodate rounded occipital carina; extension concealed in lateral view, sclerite gradually rounded dorsally. Pronotal-propleural suture well developed, propleuron only slightly exposed. Pronotal cuticle concave laterally just above procoxa. Mesopleuron broad, 0.88 mm greatest width. Dorsal surface of metanotum lower than mesonotum and propodeum; metanotal groove visible as darkened region. Propodeum shaped as a rounded right angle posterodorsally; spiracle opening near anterior sulci, protruding with surrounded cuticle pointed; patch of long, tapered setae (up to 0.17 mm) on dorsal surface; metapleural gland opening visible as small pit just above metacoxa. Legs thick, long, covered in tapered setae of varied lengths. Procoxa 0.95 mm long, 0.50 mm wide at base, 0.36 mm wide at apex; protrochanter 0.38 mm long, 0. 18  WORKER DIAGNOSIS: Distinguishing features include high developed pilosity over much of body, with setae as long as ,0.25 mm; pronotum elongated into narrow neck; metanotal and propodeal spiracles protruding approximately 0.04 mm; petiole with flattened, long, ventral projection; sternal projection on gaster segment I (abdominal segment III) greater than 0.10 mm and slightly hooked; gaster segment I and II with narrow but deep dorsal constriction between them. Approximate body length 4.31 mm.
DESCRIPTION: Head: Slightly elongate, length 0.91 mm from anterior margin of clypeus to posterior margin of head, 0.69 mm width in frontal view. Head capsule teardrop-shaped in profile, 0.58 mm at widest point just posterior to eyes, narrowing to 0.23 mm at anterior margin of clypeus. Ocelli minute, diameter 0.03 mm, but significantly raised above surface of surrounding cuticle. Occipital foramen completely flat along medial axis. Dorsal surface of head with long, tapered setae in patch between ocelli and occipital foramen. Occipital carina not clearly visible, appears to be oval with smooth edges. Eye 0.18 mm in length, 0.14 mm wide, inner margins far apart (0.64 mm); eyes bulging laterally when viewed frontally, set approximately 2/3 from anterior edge of clypeus to occipital foramen. Stout, tapered setae      propleuron-pronotum sulci well developed. Metanotal spiracle opening produced; metanotal groove visible as region of darkened cuticle. Propodeum possseses large, oval-shaped metapleural gland opening 0.06 mm wide above anterior margin of metacoxa. Propodeal spiracles present at tips of dorsal cuticular projections near anterior suture. Petiole attaches high near dorsal margin of propodeum. No trochantellus present on legs. Procoxa 0.42 mm long, width distorted; protrochanter 0.15 mm long, 0.11 mm wide; profemur 0.85 mm long, 0.14 mm wide; protibia 0.80 mm long, 0.11 mm wide with small, tapered setae near anterior joint and single spur with small subapical point; spur accompanied by a stiff basal seta ,30% as long as spur itself. Mesocoxa 0.32 mm long, width distorted; mesotrochanter 0.13 mm long, width 0.12 mm; mesofemur 0.63 mm long, 0.13 mm wide with small tapered setae near apex joint; mesotibia 0.97 mm long, 0.09 mm wide, possessing numerous tapered setae near apical joint and single pectinate spur. Metacoxa 0.38 mm long, width distorted; metatrochanter 0.11 mm long, width 0.10 mm; metafemur 0.69 mm long, 0.12 mm wide with small tapered setae at apex; metatibia 1.19 mm long, 0.09 mm wide with numerous tapered setae near apex and paired spurs (one pectinate, one simple) of equal length. Tarsi setose, each with four stiff, pointed setae at terminus; pretarsal claw with subapical tooth.
Metasoma: Petiole cylindrical, with slight dorsal projection, longitudinal sculpturing present throughout, attaches to propodeum at a height of 0.09 mm, increasing to height of 0.17 mm at apex of rounded node, total length of 0.47 mm, attaches to gaster segment I (abdominal segment III) at a height of 0.12 mm; no helcium sclerite visible. Gaster segments heavily sculptured and rugose. Gaster tergite segments disarticulated, sclerites not visible as preserved. Gaster segment I 0.29 mm long, II 0.60 mm, III 0.59 mm, segments IV and V largely obscured by fissures.
TYPES: Holotype JZC Bu1648, wingless female (presumed worker) in transparent yellow piece of amber, rounded and polished to 1961564 mm. Specimen surrounded by numerous small bubble inclusions as well as fibrous plant material.
ETYMOLOGY: From the latin word ''rugose'' meaning wrinkled, in reference to cuticular sculpturing. COMMENTS: Specimen is heavily desiccated in some regions, however the characteristic rugosity is unlikely a result of this desiccation as many sclerites, as well as the head capsule, remain more glabrous in appearance. The two additional palpomeres on the maxillary palps is a significant difference, as is the frontal lobe and cylindrical petiole with high attachment.
Sphecomyrmodes spiralis, new species urn:lsid:zoobank.org:act:EBEA426D-72BF-4942-8AE7-58669B B8C5B8 Figures 7A-C. WORKER DIAGNOSIS: Most similar to S. orientalis but distinguished by fewer setae on exterior mandible margin surface and presence of sternal projection on anterior margin of gaster segment I (abdominal segment III) -some other characters not observable in S. orientalis holotype. Can be separated from other Sphecomyrmodes species by sinuous frontal carina spiraling out from antennal base; head slightly drop-shaped in profile view with greatest height/ width posteriorly. Body length 4.22-5.11 mm.
DESCRIPTION: Head: gradually widened antero-posteriorly and dorsoventrally. Frontal view: width just below eyes ca 1.26greater than width at clypeus; laterally, depth just below eyes nearly twice depth at clypeus; head width just below eyes 0.90 mm, height from anterior margin of clypeus to posterior margin of head ca 0.90 mm. Head slightly flattened dorsally. Ocelli present, small, 0.05 mm diameter, located between center of eyes when viewed dorsally. Occipital foramen gradually rounded, oval-shaped.
Center of eyes positioned above anterior ridge of clypeus at 70% the distance between ridge and occipital foramen; eyes bulging when viewed frontally, inner margins 0.80 mm apart. Eye length ca 1.256 the depth. Antennal bases exposed, oval, longer radius twice that of shorter radius. Cuticle raised between antennae, with semi-circular frontal carina originating at anterior ridges of toruli, curving and terminating lateral to bottom margin of eyes. Total antenna length 3.31 mm: Scape 0.48, pedicel 0.17, funicular segment I 0.35, II 0.26, III 0.25, IV 0.28, V 0.25, VI 0.24, VII 0.26, VIII 0.32, IX 0.18, 60.27 mm. All segments narrow and devoid of long setae. Clypeus a rounded trapezoid, with convex, parallel anterior/posterior margins and straight lateral margins converging at a 140u angle just below antennae; tentorial pits visible just lateral to this union on head capsule. Clypeus length 0.21 mm along medial axis, sparse tapered setae approximately 0.12 mm in length present on dorsal surface. Clypeal cuticle raised slightly in center and near lateral margins, entire anterior margin possesses ca 25 stout (0.02 mm long) setae. Anterolateral margin of clypeus with slight anterior projection, obscures mandible base when viewed from above. Mandibles falcate, 0.36 mm from base to tip of apical tooth, gradually hooked, with slight groove along inner curvature. Mandibles with two denticles at terminus, apical tooth approximately 1.46 longer and slightly more narrow than basal tooth. Mandibles with sparse setae on outer margin (mandibles preserved slightly extended but appear to retract flush with anterior margin of clypeus). Two labial palpomeres present, basal palpomere slightly longer. Maxillary palps with four segments, basal one 0.36 size of remaining equal-sized segments.
ETYMOLOGY: In description of distinctive spiraling frontal carina.
COMMENTS: This species appears to be the most common in Burmese amber, comprising the greatest percentage (,25%) of currently known and identifiable Burmese ant workers. Segments of roughly equal thickness throughout, devoid of macrosetae. Posterior margin of clypeus broadly notched, gradually joined with lateral margin at very wide (,160u) angle. Clypeal sclerite raised along medial axis, with numerous setae of varied lengths (as long as 0.15 mm) on dorsal surface. Entire anterior margin of clypeus convex, with row ca. 30 slightly pointed denticles, approximately 0.03 mm in length, these denticles appear as two rows in some sections of clypeal margin. Anterolateral margin of clypeus raised as a slightly upturned shelf above base of mandible in frontal view. Mandibles bidentate and falcate, 0.69 mm long from base to tip of apical tooth; apical tooth 0.16 mm long, 0.06 mm wide at base, basal tooth measurements 0.08 mm, 0.09 mm. Labial palps comprised of two equal-sized palpomeres, maxillary palps not visible.
Metasoma: Petiole nodiform, with shortened peduncle and rounded dorsal node. Propodeal-petiolar attachment height 0.16 mm. Petiole length 0.59 mm in lateral view (peduncle 0.11 mm.) Node gradually rounded, height 0.36 mm, shortened to 0.18 mm at gaster attachment point. Ventral projection of petiole present, originates approximately at center of petiole, gradually increased in height anteriad to 0.06 mm at its apex, with sheer anterior face. Gastral segment I (abdominal segment III) extended 0.10 mm anteriorly, meeting petiole with projection pointed ventrad (projection 0.08 mm long). Segment I length 0.55 mm, including anterior extension, gastral segment II length 0.60 mm, segment III 0.35 mm, IV 0.20 mm, V 0.29 mm. Pygidium setose; sting extruded, 0.95 mm exposed.
TYPES: Holotype JZC Bu304. Wingless female (presumed worker). Specimen preserved in 2961364 mm piece of very clear yellow amber. Also preserved are particles of detritus, bubbles, a partial spider, and an insect larva (not an ant). Paratypes JZC Bu305, JZC Bu121A.
ETYMOLOGY: From cuspis, Latin for point, in reference to the petiolar projection; the prefix in reference to the ventral position of this point.
Sphecomyrmodes tendir, new species urn:lsid:zoobank.org:act:81A4DB69-1E5E-4418-9578-A212E79 6790F Figures 9A-C. WORKER DIAGNOSIS: Differs from congeners based on the clypeal setae, which do not cover the entire anterior margin as in other Sphecomyrmodes, but rather only a medial portion of the anterior margin where there is also a prominent medial lobe; a reduced subapical tooth on the pretarsal claw. Body length 6.93 mm.
DESCRIPTION: Head: elongate, even in width throughout. Height from anterior ridge of clypeus to posterior margin of head 1.22 mm, width greatest below eyes, 0.78 mm. Medial margin of eyes positioned ,55% the distance from anterior margin of clypeus to posterior margin of head; eyes small, bulging when viewed head-on, 0.11 mm deep, 0.06 mm wide. Cuticle heavily darkened, fine sutures and details obscured, ocelli not visible. Two small, pointed lobes present on vertex of head. Cuticle raised between antennae, antennal bases exposed (measurement not possible due to darkening). Funicular segments IX and X missing. Scape 0.42 mm long, pedicel 0. 16 ETYMOLOGY: From the latin word ''tendo'' meaning to extend. Referring to the medial clypeal projection.
COMMENTS: The severely darkened cuticle of the specimen renders resolving some detailed characters impossible with light microscopy. The distinctive medial lobe on the anterior margin of the clypeus is similar to that of Sphecomyrma mesaki Engel and Grimaldi, however more broad.

Key to Sphecomyrmodes Workers
This key is intended for identification of Sphecomyrmodes workers presently known from Burmese and Charentese amber. --Head and body cuticle rugose; maxillary palps with six palpomeres… rugosus 9. Mesopleuron extremely elongate such that the distance between the pro-and mesocoxae is roughly twice the procoxal length… gracilis -Mesopleuron fairly stout, distance between pro-and mesocoxae equal to or less than width of procoxae… spiralis 10. Setae present on external margins of mandibles… orientalis -External margins of mandibles entirely glabrous… occidentalis

Comparison with Modern and Other Cretaceous Taxa
Each species described here exhibits a unique assemblage of synapomorphic ant features, bygone primitive morphology, and characters not before known in stem-group taxa (figures 10,11). Moreover, all of these animals possess a clearly discernible shared character, suggesting surprising diversity among closely related species. Described species possess a metapleural gland opening, petiole, and geniculate antennae, as in modern ants. Unlike crown-group ants, these species possess a shortened scape, comprising #0.256 the total antennal length, a feature of stemgroup ants [3] [18]. Members of the extinct genus Sphecomyrmodes also possess a comb of stout setae along the anterior clypeal margin ( figure 12). A similar, although possibly homoplastic, condition is found in some modern ants, such as Adetomyrma, Amblyopone, Martialis, and Stigmatomma [19][20][21], and, aside from long trigger hairs found in trap-jaw ants, the function of clypeal setae is unknown. A comb of clypeal setae is also known from the Cretaceous genera Gerontoformica and Zigrasimecia [11] [22]. Zigrasimecia possesses an additional set of setae on the labrum, and Gerontoformica, while similar in many respects to Sphecomyrmodes, can be distinguished by an elongated scape, a character that may have importance with regard to the crown-or stem-group affinities. Despite the diagnosable differences among clypeal setae bearing ants, it seems likely that these groups share close relationships.
The genus Sphecomyrmodes is readily placed in the putative stemgroup subfamily Sphecomyrminae. However, new Cretaceous diversity along with other incertae sedis taxa such as Myanmyrma [6] and Gerontoformica [22] highlight the need for a rigorous phylogenetic analysis of Cretaceous taxa and stable classification. Some group features, such as the gastral constriction present in S. pilosus (figure 4a), indeed appear to vary within stem-group ant lineages.

Unexpected Diversity
With a total of 11 species -nine described here, two described previously [6][7] -the genus Sphecomyrmodes represents significant diversification among stem-group ants united by a distinct trait. It should also be noted that the discovery of two Sphecomyrmodes workers in slightly older French amber [7] indicate that the genus had considerable geographic range. Anterior clypeal setae appear to have been useful for these early species, as they remain a consistent feature among a varying stock of morphological diversity (figures 10,11,12). Although these individuals have been placed within the same genus some of the features that vary among them are used in present-day taxa to separate higher-level groups. Sphecomyrmodes species are now known to vary with regard to scrobe presence, frontal carinae structure, petiole form, and palpomere count. There is also considerable diversity among comparable species-level traits such as pilosity, fine cuticular sculpturing, and body form. The range of overall body size and proportionality is quite large, with the smallest specimen of S. spiralis at just 4.22 mm in length, contrasted with the largest individual S. magnus at a total length of 8.64 mm. Mesosomal elongation varies as well, with a ratio value (length/height) twice as high in S. gracilis as S. magnus. In terms of gaster length, proportionally, the fully extended gaster of S. contegus comprises just 25% of total body length, while the gastral segments of S. gracilis make up 50% of total body size.
Ancient ant stem-groups were not morphologically stagnant. Indeed, ant species from the Cretaceous exhibited species-level radiations comparable to modern day taxa with similar variations of common features. This apparent homoplasy may be highly informative in understanding patterns of ant evolution as we now know that stem-group (that is, of Cretaceous age not possessing an elongated scape) ants appear to have the same innovations that their modern counterparts exhibit such as: feeding morphologies (mandibular, palp structure, setae patterning), head sculpturing (elongate, equilateral, widened), body form (gracile, robust, sculptured, setose), fine sculpturing (rugosity, pilosity, spiracle placement, sub-petiole and gastral projections), leg morphology (claw morphology, setae patterns). There is no clear single ''linking-ant,'' instead it is apparent that there was significant Cretaceous diversity and innovation, in this case curiously united by a clypeal peg structure that is known in extant basal lineages.