A unique individual was discovered in a large piece of mid-Cretaceous amber from the Hukawng Valley (Kachin State) in northern Myanmar. Through trimming of the piece, the wasp was isolated into a largely clear, light yellow block of amber, square in shape and with dimensions of approximately 8 mm long, 8 mm wide, and 1.5 mm deep. The piece includes a few isolated plant trichomes and other minor organic debris, but none of these are in a position to obscure the wasp, and there are minimal internal distortions from flow lines within the resin, fractures, or bubbles. There is a sizeable bubble beneath the apex of the metasoma that extends along the left side and expands along the posterior of the mesosoma and around the mesosomal-metasomal articulation (Fig. 1), but really only poses a challenge for views of the apicalmost sterna. Fine internal fractures filled with air also extend around the margins of the metasoma, most prominently along the right side, but these are short and slightly reflective; they do not impose restrictions on structural views of the inclusion. The antennae are extended to the left of the inclusion, and the legs are positioned beneath (but not fully tucked under) and outward from the body. The head is somewhat dipped downward and with the mouthparts positioned toward the propleura, and therefore everything other than the mandibles are rendered impossible to observe.

We have provided the description here with the goal of furthering paleontological work on Hymenoptera, and expanding our knowledge of character diversity among lower Platygastroidea, so that this might inform future phylogenetic inquiry and the discovery of explanatory patterns (sensu Grimaldi and Engel, 2007). Morphological terminology used in the description generally follows that of Masner (1976) and Mikó et al. (2007), while the higher classification adopted is modified from that of Masner (1976), Johnson and Masner (2006), Johnson et al. (2008b), and Masner et al. (2007a). We follow the position of Engel and Krombein (2012) and McKellar and Engel (2012) in the retention of multiple families within Platygastroidea, and advocate for the elevation of at least Nixoniini and Sparasionini, if not other clades, to familial rank alongside Platygastridae s.str. and a more narrowly circumscribed Scelionidae. Measurements were taken with ocular micrometers attached to an Olympus SZX12 stereomicroscope and an Olympus BX41 compound microscope. Microphotographs were prepared with a Canon 7D digital camera attached to an Infinity K-2 long-distance microscope lens.

The amber-bearing deposits of the Hukawng Valley have been studied geologically and mapped by Cruickshank and Ko (2003), while the age has been most extensively covered by Shi et al. (2012). Grimaldi et al. (2002) and Ross et al. (2010) provide further information on the locality, as well as a preliminary overview of the biotic diversity and abundance of various lineages. The taxonomic actions established herein are registered under ZooBank LSID urn:lsid:zoobank.org:pub:2AC79FD9-48AF-41D5-A6C8-FB04960CE84A.

Geoscelionini Engel and Huang, trib. nov.

Type genus: Geoscelio Engel and Huang, gen. nov.

Diagnosis: ♀: Small wasps, less than 2 mm in length, body compact, with ovoid metasoma (Fig. 1); head broad, transverse in dorsal view, without transverse ledge; frons without distinct scrobe or facial depression (Fig. 2), broadly convex, submedian carina present (Fig. 2); lower frons without fanlike facial striae (Fig. 2), upper frons with longitudinal striae extending to occipital carina; striae absent between submedian carina and malar sulcus, and on gena; internatennal process weakly produced, torulus opening slightly laterally; antenna with elongate scape (Fig. 1 and Fig. 2), scape length great than height of compound eye; first flagellomere distinctly longer than pedicel; flagellum not strongly clavate, basal flagellomeres distinctly longer than wide, gradually reducing in length to become about as long as wide, forming a weak clava apically (flagellomeres V–XII); orbital carina present; compound eye glabrous, with inner orbits approximately parallel, interocular width slight greater than compound eye length; lateral ocelli much closer to inner orbit of compound eyes than to median ocellus, yet distinctly separated from inner orbit by about one-half of ocellar diameter; malar sulcus present, extending to lower posterior angle of compound eye (Fig. 2); clypeus appears strongly transverse with minute medioapical emargination; mandible short, stout, broad apically, with three strong apical teeth (Fig. 2); occipital carina present, complete, with anterior crenulae (Fig. 3A), with occiput depressed relative to carina.

Mesosoma compact, robust (Fig. 1); pronotum short, transverse, with distinct transverse pronotal carina; pronotal carina interrupted medially (Fig. 3A), posteriorly bordered by single row of areolae except medially, anterolateral corners acute, slightly projecting; vertical epomial carina present; humeral carina present; lateral surface of pronotum slightly depressed; mesoscutum wider than long, almost rectangular, with faint parapsidal lines present; skaphion present, narrow (Fig. 3A), with faint indication of antero-admedian lines; notauli broad and deeply impressed, percurrent (Fig. 1 and Fig. 3); transscutal sulcus prominent, with faint posterior crenulae; mesoscutellum wider than long, unarmed (Fig. 3A), with narrow axilla; metanotum short, largely obscured by posterior border of mesoscutellum, unarmed; legs long, slender, femora slightly crassate (Fig. 1B), tibial spur formula 1-2-2; tarsi pentamerous; pretarsal claws simple, with small arolium; wings lacking (at most represented by tiny, hooked sclerite scarcely exceeding tegular posterior border), with small tegula retained (Fig. 1 and Fig. 3).

Metasoma compact, ovoid (Fig. 1 and Fig. 3); six terga and sterna visible externally, spiracles obscured; terga slightly convex, sterna slightly convex; laterotergites not developed (similar in this regard to Archaeoscelio Brues); terga I–III subequal in length, none greatly enlarged relative to other terga (Fig. 3B); base of terga and sterna I–V crenulate (Fig. 3A), weakly so on segment V.

Remarks: In a multi-familial system of Platygastroidea in which Nixoniidae is considered distinct, the present group apparently would form a tribe or subfamily in that ‘family’ but for now is placed in ‘Scelionidae s.l.’ and pending clarification of basal divergences and clades in among platygastroids. The genus Archaeoscelio Brues might be best incorporated into Geoscelionini owing to its lack of laterotergites and lower facial striae, its broad tridentate mandible, and its primitive antennal form (Brues, 1940 and Masner et al., 2007a). If this is the case, then the circumscription of the tribe should be somewhat emended, as A. rugosus Brues and A. filicornis Brues have a more derived, 1-1-1 tibial spur formula, and lack a skaphion and notauli, among other differences (Brues, 1940 and Masner et al., 2007a). Alternatively, the lack of laterotergites could indicate a position for both of these extinct genera outside and more basal to Nixoniidae, or could be secondarily lost in Archaeoscelio (as is the case with genera such as the mid-Eocene Cobaloscelio and the extant Plaumannion, Huddlestonium, and Tyrannoscelio, all of which are apparently along the stem to Platygastridae s.str.: Masner et al., 2007a and Masner et al., 2007b), and with the 1-1-1 tibial spur formula betraying its more derived placement. Such clarification awaits future paleontological study of Cretaceous Platygastroidea as these taxa should have considerable importance for resolving divergences among the varied groups. The tribal name is registered under ZooBank LSID urn:lsid:zoobank.org:act:A2AFA77F-F18D-48FF-BCB0-3BDDAD55AB20.

Geoscelio Engel and Huang, gen. nov.

Type species: Geoscelio mckellari Engel and Huang, sp. nov.

Diagnosis: As for the tribe (vide supra).

Etymology: The new genus-group name is composed of the Greek prefix geo–, referring to gaia and meaning, “earth”, and Scelio, type genus of the family, and as a reference to the ‘grounded’ aspect of the wasp's putative life history, the result of its complete aptery. The name's gender is masculine. The generic name is registered under ZooBank LSID urn:lsid:zoobank.org:act:3ABF29D3-07C7-492B-BA1F-D47149F5DC3B.

Geoscelio mckellari Engel and Huang, sp. nov.

Fig. 1, Fig. 2 and Fig. 3

Diagnosis: As for the genus (vide supra).

Description: ♀: Total length (as preserved, excluding antennae and ovipositor) 1.57 mm; integument generally brown to reddish brown throughout, generally impunctate and smooth except as noted, and without distinct setae over body. Head broad, width across compound eyes 0.64 mm, interocular distance 0.34 mm; integument of gena and between malar sulcus and submedian carina appearing weakly granulose. Antenna elongate, scape length 0.42 mm, slightly widening from base to apex; pedicel about twice as long as apical width, distinctly shorter than first flagellomere (Fig. 2); flagellomere I longest, about 4.5 times as long as apical width, flagellomeres II and III longer than wide, progressively shorter than flagellomere I; flagellomere IV slightly longer than apical width; flagellomeres V–XI forming weak clava, each about as long as apical width except flagellomere XII, which is slightly longer than wide and tapering to acute apex. Mandible stout, wider apically than at base, with three prominent, heavily sclerotized apical teeth visible on right mandible, teeth broadly separated by deep V-shaped incisions and arranged transversely along apical margin; anterior border with slight hump at base of first marginal tooth. Labrum not visible.

Pronotum strongly transverse, with exceptionally short posterior, elevated, dorsal-facing surface bordering mesoscutum, entirely consisting of single row of areolae interrupted medially against skaphion and anteriorly demarcated by transverse carina; otherwise pronotal surface sloping strongly to short medial collar between vertical epomial carinae; epomial carinae extending dorsally toward acutely pointed, dorsal anterolateral corners; lateral surface of pronotum slightly depressed and somewhat wrinkled, with large areolae ventrally along border with propleuron and mesepisternum (netrion lacking). Mesoscutum short, broad, medial width 0.63 mm, medial length 0.26 mm, surface weakly convex; narrow, short, skaphion present medially; notauli deeply and broadly impressed, not composed of areolae and not widening posteriorly, percurrent, separated by distance slightly more than their length, parapsidal line faintly indicated between notaulus and lateral border, mesoscutal lateral border with weak crenulae; transscutal sulcus deeply impressed, posteriorly with deep crenulae on mesoscutellum; mesoscutellum broader than long, medial length 0.19 mm, anterior width 0.35 mm, posterior border gently and broadly rounded, with overall hemispheric shape, and with posterior border composed of single row of deep areolae; tegula small, ovoid, with a small sclerotized hook slightly projecting beyond apical margin from beneath (highly reduced forewing?); mesepisternum weakly depressed, apparently smooth in area of depression but areolate anteriorly and posteriorly, apparently without sternaulus; metanotum short, surface composed of row of areolae and sloping, largely obscured by mesoscutellar posterior border in dorsal view; propodeum strongly declivitous, surface weakly areolate. Legs slender, with femora slightly crassate and longer than tibiae (less so for metafemur); inner surfaces of tibiae in apical halves and on tarsomeres with numerous, minute, fine setae; basitarsi longest tarsomeres, but shorter than combined lengths of remaining tarsomeres.

Metasoma compact, ovoid, medial length 0.66 mm, maximum width 0.64 mm (at apical margin of tergum II and base of tergum III); tergal posterior margins largely straight; sternal posterior margins somewhat concave; terga laterally slightly overlapping sterna; tergal bases with prominent, deeply impressed crenulae, weaker on tergum V and apparently lacking on tergum VI; tergum I with dorsal carina anterior to crenulae and defining sharp angle between tergal dorsal surface and precipitously declivitous anterior-facing surface bearing narrow articulation to propodeum; sterna with similar pattern of basal crenulae as on terga; apical segments not forming telescopic ovipositor tube; ovipositor exserted (as preserved), thin and about as long as medial length of terga II and III combined, without ovipositor tube, sheaths thin and about as long as ovipositor.

♂: Latet.

Holotype: ♀, NIGP 163293, Lowermost Cenomanian (near Albian boundary), Hukawng Valley, Kachin State, northern Myanmar; deposited in the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing, China.

Etymology: The specific epithet honors Dr. Ryan C. McKellar of the Royal Saskatchewan Museum, a prominent paleontologist who has contributed much to the study of amber as well as the Cretaceous record of Hymenoptera. The species name is registered under ZooBank LSID urn:lsid:zoobank.org:act:7A15E287-D0D2-4C63-BBA7-C7BAC9819E05.

Geoscelio mckellari is a fascinating discovery among Cretaceous platygastroid wasps, and reflects the considerable variety within this lineage that was already in place by the mid-Cretaceous. The retention of 12 flagellomeres not forming a pronounced apical clava (the clava is at best weakly indicated in G. mckellari), a 1-2-2 tibial spur formula, absence of a frontal depression on the head, and six largely equal metasomal terga are all putatively plesiomorphic characteristics, shared with only the genus Nixonia Masner among the modern fauna (Johnson and Masner, 2006, Masner, 1976 and van Noort and Johnson, 2009). Indeed, these are likely groundplan features for the superfamily, with multiple independent reductions in the line leading to the Platygastridae s.str. as well as the Scelionidae, a pattern that would accord with current phylogenetic evidence (e.g., Murphy et al., 2007).

While in the modern fauna it is only Nixonia that retains a full complement of 12 flagellomeres, there are various genera in the fossil record, particularly the Cretaceous, that have this same condition. Of special interest are the genera Proteroscelio Brues in Early Cretaceous Lebanese amber and Late Cretaceous Canadian amber (Brues, 1937 and Johnson et al., 2008a), and Proterosceliopsis Ortega-Blanco et al. and Bruescelio Ortega-Blanco et al. in Early Cretaceous Spanish amber (Ortega-Blanco et al., 2014). All of these genera share the plesiomorphic retention of 12 flagellomeres, a 1-2-2 tibial spur formula, a convex frons (lacking a frontal depression), and, at least in the case of Proteroscelio, also retain a malar sulcus. Unlike Geoscelio, all are fully winged and have distinct laterotergites. Proteroscelio also lack an occipital carina, a submedian carina, a skaphion, and notauli, and have a more distinct clava, short basal flagellomeres, and a more elongate metasoma lacking basal crenulae on the terga and sterna (Johnson et al., 2008a). Cretaxenomerus Nel and Azar in Early Cretaceous Lebanese amber also have 12 flagellomeres and 1-2-2 tibial spurs, but this genus has the lateral ocelli positioned close to the median ocellus, lacks notauli and a skaphion, and has a rather elongate metasoma with a prominent ovipositor tube and tergum III enlarged relative to the other segments (Nel and Azar, 2005).

The compact form, 12 flagellomeres, and primitive absence of laterotergites are shared with Archaeoscelio, described from mid-Eocene Baltic amber (Brues, 1940 and Masner et al., 2007a). Indeed, aside from the aforementioned features, there are many traits similar between these two genera, despite their separation by approximately 50–55 million years. Both have an overall compact habitus; have a broad, stout, tridentate mandible; have the apical seven flagellomeres forming a weakly delimited, non-abrupt clava; lack a facial depression; lack a sternaulus; and have basally crenulate terga and sterna. Nonetheless, there remain significant differences, such as the 1-1-1 tibial spur formula, lack of submedian and orbital carinae, lack of a malar sulcus, lack of a skaphion and notauli, and more convex mesosomal dorsum in Archaeoscelio, as well as the general integumental sculpturing of the body in both genera. In all of these respects, G. mckellari appears more plesiomorphic than Archaeoscelio, and the latter may be a stem-group to the platygastrid lineage.

For the time being, G. mckellari stands as an outlier among Cretaceous Platygastroidea. Nonetheless, it provides initial clues to specializations already in place by the mid-Cretaceous, likely associated with some aspect of parasitoidism on a particular host. Scelionids are endoparasitoids of spider and insect eggs, and some of the most noted genera with apterous females are those of the Baeini, compact spider parasitoids (Austin et al., 2005 and Masner, 1976). Interestingly and not surprisingly, where wing reduction has taken place among modern platygastroids, it is almost universally associated with a shift to a soil or litter habitat (Austin et al., 2005), and this is likely the case for Geoscelio as well and suggests that their hosts were among the debris on the floor of the Burmese amber forest.