Burmella paucivenosa sp. nov.

The generic name of female gender is a composition of “Burmar” as an ancient term for Myanmar, combined with “ella”, a common ending of generic names in mayflies, and especially so within Ephemerelloidea.

Adults of Burmella gen. nov. differ from other mayfly genera by the following combination of features: forewings (a) with small number of cross veins; (b) pterostigma with simple veins, not anastomosed; (c) CuP smoothly curved towards wing base; (d) two secondary bifurcate veins in cubital field; (e) at least several free marginal intercalaries along ventral margin; hind wings (f) strongly rounded, small, as long as 0.08–0.14 of forewing length; (g) small number of cross veins; (h) triad RS present or absent; no MA and MP triads; (i) no secondary branches of cubital veins; (j) costal process developed, rounded apically, situated centrally; abdomen (k) with vestigial gill sockets recognizable at least on segments II–VI; genitalia (l) with large median projection of styliger plate, widely rounded apically; (m) three distal segments of forceps strongly elongated and slender; segment II longest, 5× as long as segment III; segments III and IV approximately of equal length; segment IV expanding apically; (n) penis lobes widely separated by V-shaped cleft; (o) no trace of paracercus. Additionally, in female (p) anterior part of eyes covered by anterolaterally expanded clypeal shield.

Subimago and larva unknown.

Burmella paucivenosa sp. nov. (SMNS; BU-179); Burmella clypeata sp. nov. (SMNS; BU-321).

Hukawng Valley, Kachin State, Myanmar (Burma); Cenomanian, mid-Cretaceous.

Holotype. Male imago in Mid-Cretaceous Burmese amber, SMNS collection, inventory number BU-179. Well preserved specimen visible in lateral aspect. Due to fragility, the piece of amber is additionally embedded in translucent resin to seal the specimen from oxygen and prevent mechanical damage. Body and both pairs of wings completely preserved (Figs 1, 4–5); most part of right and left foretibiae and both foretarsi missing; most part of caudal filaments missing. For measurements see Table 1.

Figure 1. 

Burmella paucivenosa sp. nov., male imago, holotype, general lateral view A left side of body B right side of body. Scale bars: 2 mm.

The species epithet combines Latin “paucus”, few, and “venosus”, veined, referring to the reduced wing venation of the hind wing.

Male imago: body length 5.75 mm; forewings with 3–4 marginal intercalaries connected with longitudinal veins, two free marginal intercalaries, no cross veins in anal field; hind wings strongly rounded, small, as long as 0.14× of forewing length, three cross veins between C–Sc, three cross veins between Sc–RA, one cross vein between RA–RSa, one cross vein between RA–RSp, RS not forked; penis lobes relatively simple, obliquely truncate apically, nearly tube-like; strong apical tooth on outer margin.

Colouration relatively pale, yellowish-brown to dark brown; eyes and mesonotum darkest, dark brown to blackish; abdominal segments partly translucent; traces of dark brown maculation along of lateral margins of terga (Figs 1, 2, 4, 6).

Figure 2. 

Burmella paucivenosa sp. nov., male imago, holotype A head, right lateral view B compound eye, left lateral view C head and thorax, left lateral view D mesothorax, left lateral view. Blue line – border between portions of compound eye, da – damaged area, lp – lower portion, LPs and red line – lateroparapsidal suture, MNs and green line – mesonotal suture, MPs and yellow line – medioparapsidal suture, up – upper portion. Scale bars: 0.5 mm (A, C); 0.2 mm (B, D).

Head. Compound eyes well-developed, large, widely rounded, medially contiguous; upper portion of compound eyes translucent and slightly yellowish apically, brownish-black basally; border between dorsal and ventral portions of compound eyes well distinguishable; lower portion of compound eyes brownish-black (Figs 1, 2A–C). Facets of compound eyes hexagonal. Ocelli poorly preserved, relatively small, without conspicuous colouration. Facial keel relatively small. Antennae slightly longer than head.

Thorax. General colouration yellowish-brown to brownish-black. Prothorax narrow, light brown. Mesonotal suture transverse, distinctly expressed; medioparapsidal suture relatively straight; lateroparapsidal suture distinctly curved laterally; scutellum not modified; no preserved natural colouration of pigmented area of mesonotum. Mesosternum with brownish basisternum and slightly paler furcasternum; basisternum elongated; furcasternal protuberances distinctly separated. Lateral sides of mesothorax light brown to brown, with blackish maculation. Metathorax brown to dark brown, blackish maculation dorsally (Fig. 2C, D).

Wings. Forewings hyaline, translucent, relatively narrow; venation well recognizable, light brown to dark brown; veins darker proximally and slightly paler distally; relatively small number of cross veins, especially in medial, cubital, and anal fields; no jagged edge along of ventral margin of forewings. Pterostigma with 3–4 simple veins. Vein sections between C and RA slightly frosted-brown distally; veins C and Sc brown to dark brown, visible all over their length; RS forked near base, after 0.14 of its length; iRS well-developed, connected with RSp by 5 cross veins, not approximated to RSa₁; MA fork slightly asymmetrical, forked after 0.60–0.62 of its length; MA₁ and MA₂ connected with iMA by 2–3 cross veins; MP asymmetrical, forked after 0.25 of its length, MP₁ and MP₂ basally connected by a single cross vein; iMP relatively short, connected with MP₁ and MP₂ by single cross veins from each side; CuP smoothly curved toward wing base, basally connected with CuA by cross vein cua–cup, CuP connected with A₁ by cross vein cup-a₁, cua–cup located distally from cup-a₁; in cubital field two secondary bifurcate veins iCu₁₊₂ and iCu₃₊₄ arising from CuA (i.e. four veins iCu₁–iCu₄ each reaching basitornal margin of forewing); basal end of CuP closely approximated to CuA base; A₁ closely approximated to A₂; no cross veins in anal field. Several intercalaries (iRSa, iRSa₂, iMA, iMP) connected to longitudinal veins by crossveins; two small, basally free marginal intercalaries in R and MP fields; no free intercalary veins in cubital and anal fields (Figs 1, 4).

Hind wings hyaline, translucent, strongly rounded, small, as long as 0.14 of forewing length; venation light brown to brown; venation significantly simplified, with strong reduction of number of longitudinal and cross veins; ventral margin of hind wings without jagged edge. Few cross veins between C–Sc (3 veins), Sc–RA (3 veins), RA–RSa (one vein), and RA–RSp (one vein); no triads of RS, MA and MP; MA connected with R; MP approaching CuA; no secondary branches of cubital veins; no free marginal intercalaries; costal process rounded apically, markedly protruding above anterior wing margin, situated at nearly middle of hind wing length (Fig. 5A, B).

Legs well preserved, except of tarsi missing in both forelegs; margins of preserved leg segments without visible strong spines or setae. For measurements of leg segments see Table 1.

Right foreleg: length ratio of femur/tibia = 1/2.02; left foreleg: length ratio of femur/tibia = 1/1.96. Right middle leg completely preserved: length ratio of femur/tibia/tarsus = 1/0.71/0.21; length ratio of tarsomeres: 1/1.25/1.25/1.38/1.75 (5 > 4 > 3 = 2 > 1). Left middle leg much shorter than right one, probably re-grown after previous injury, therefore with changed proportions of tarsomeres. Right and left hind legs completely preserved; right hind leg: length ratio of femur/tibia/tarsus = 1/0.91/0.51; length ratio of tarsomeres: 1/1.29/1.43/1.71/2.00 (5 > 4 > 3 > 2 > 1). Left hind leg: length ratio of femur/tibia/tarsus = 1/0.66/0.41; length ratio of tarsomeres: 1/1.40/1.40/2.00/2.20 (5 > 4 > 3 = 2 > 1). Patellotibial suture present on middle and hind legs, absent on forelegs. First tarsomere of middle and hind legs fused with tibia. Claws ephemeropteroid on preserved middle and hind legs, with outer claw hooked and inner claw blunt (Figs 1A, B, 3A–D).

Figure 3. 

Burmella paucivenosa sp. nov., male imago, holotype A, B tarsus of right middle leg C right middle leg D left hind leg. I–V – tarsal segments, pts – patellotibial suture. Scale bars: 0.1 mm (A, B); 0.5 mm (C); 0.2 mm (D).

Abdominal segments completely preserved, partly translucent, relatively pale, yellow to brown, with intensively brown maculation on terga laterally and sterna posteriorly. Vestigial gill sockets, not finger-like, recognizable on segments II–VI, poorly visible on segment VII due to influx of resin and cracks. Abdominal segments without large and prominent posterolateral projections; abdominal segments VIII–IX not elongated compared to previous segments. Abdominal sterna slightly paler than terga. Cerci brown, partly preserved; no trace of paracercus (Figs 1, 4, 6A, B).

Figure 4. 

Burmella paucivenosa sp. nov., male imago, holotype A right and left wings position in amber B right forewing venation and size ratio of fore and hind wings. Scale bars: 1 mm.

Genitalia well preserved, light brown to brown, darker maculation on forceps. Styliger plate angulate, mediocaudally deeply incised; median projection large, widely rounded apically, markedly protruding above anterior margin of styliger. Basal segment I of forceps short, with rounded inner margin, slightly wider than long; segment II of forceps strongly elongated, slender distal segments III and IV much shorter, approximately of equal length; segment IV expanding apically; length ratio of forceps segments II–IV: 1.00/0.20/0.18 (Fig. 6C, D). Penis lobes widely separated by V-shaped cleft, relatively simple, obliquely truncate apically, nearly tube-like; structure of left penis lobe poorly visible; inner side of right penis lobe probably partly damaged or lost (i.e. looks semicircular from ventral side); strong apical tooth on outer margin; titillators not distinguishable (Fig. 6C, D).

Burmella paucivenosa sp. nov. exhibits a combination of morphological characters allowing its attribution to Vietnamellidae, namely the presence of strongly rounded hind wings in combination with the presence of short intercalaries distally connected with longitudinal veins. Compared to other representatives of Vietnamellidae, Burmella paucivenosa sp. nov. is characterized by the presence of only two short free marginal intercalaries, while the number of these intercalary veins in all extant species and also in Burmella clypeata sp. nov. is significantly higher.

Within Vietnamellidae, Burmella paucivenosa sp. nov. can be attributed to the newly described genus Burmella gen. nov., as defined in Diagnosis (see above), mainly based on the following diagnostic characters: shape and structure of venation of hind wings, with reduced cross venation and distinct costal process situated centrally; lack of furcation of RS, MA, MP, CuA, and CuP in hind wings (Fig. 5); shape of male genitalia with deeply diverted penis lobes (Fig. 6C, D).

Figure 5. 

Burmella paucivenosa sp. nov., male imago, holotype A right hind wing in amber B right hind wing venation. Scale bars: 0.2 mm.

Figure 6. 

Burmella paucivenosa sp. nov., male imago, holotype A abdomen, left lateral view B abdominal segments III–VII, lateral view C, D genitalia, ventrolateral view. III–VII – numbers of segments, white arrows mark remnants of gill sockets, pink area – penis lobes, light blue area – gonobasis (styliger plate), brown area styliger segment I, yellow area –styliger segment II, light green area – styliger segment III, purple area – styliger segment IV. Scale bars: 1 mm (A); 0.5 mm (B); 0.2 mm (C, D).

In the latter character, the male imago of Burmella paucivenosa sp. nov. differs from all other known male adults of Vietnamellidae. The genus Vietnamella is characterized by the presence of a club-shaped, elongated penis that is medially fused along its longitudinal axis, with only a small, V- or U-shaped incision apically (Tshernova 1972: 613, fig. 7; Hu et al. 2017: 385, figs 4C, 5C; Auychinda et al. 2020a: 8, fig. 4G; 2020b: 28, figs 7J, K, 8J, K). In contrast to Vietnamella, the tubular penis of Burmella paucivenosa sp. nov. is medially deeply split, with lobes strongly stretched laterally (Fig. 6C, D). Obvious differences are also visible in shape and proportions of forceps segments. In Burmella paucivenosa sp. nov. the 4-segmented forceps is strongly elongated and slender, with segment II being the longest, with the same width distally as segment III basally, while distal segment IV is markedly elongated and nearly subequal to segment III (Fig. 6C, D). On the contrary, in Vietnamella the forceps is only 3-segmented with segments significantly different in shape and proportions: Segment I is the longest one, while shortest segment III is small and rounded, which is typical for many species of Ephemerelloidea (see Kluge 2004).

Holotype. Female imago in Mid-Cretaceous Burmese amber, SMNS collection, inventory number: BU-321. Well preserved specimen visible in dorsal/ventral aspect. Body and left forewings preserved except of lost distal part of C and Sc; left forewing twisted, covering dorsal side of abdomen; right forewings twisted, only partly preserved, distal part missing; foretibiae damaged; right antenna, foretarsi, right middle leg and left cercus missing (Figs 7, 8, 10). Left hind wing not visible. For measurements see Table 1.

Figure 7. 

Burmella clypeata sp. nov., female imago, holotype A general dorsal view B general ventral view C, D head, dorsal view E head and thorax, dorsal view. BA – basal sclerite (basalare), LPs and red line – lateroparapsidal suture, MNs and green line – mesonotal suture, MPs and yellow line – medioparapsidal suture, rc – right cercus. Scale bars: 2 mm (A, B); 0.2 mm (C, D); 0.5 mm (E).

The species epithet refers to the laterally expanded clypeus that partly covers the eyes.

Figure 8. 

Burmella clypeata sp. nov., female imago, holotype A tarsus of left hind leg B tarsus of right hind leg C left forewing in amber D left forewing venation and size ratio of left fore- and right hind wings E distal part of left forewing F preserved basal part of right forewing. I–V – tarsal segments, ci – basally connected intercalary vein, fi – basally free intercalary vein. Scale bars: 0.1 mm (A, B); 1 mm (C, D); 0.5 mm (E, F).

Female imago: body length 7.00 mm; forewings with at least four short marginal intercalaries in MA–MP field basally attached to longitudinal veins, six free marginal intercalaries in RS field; hind wing strongly rounded, small, as long as 0.08× of forewing length, two cross veins between C–Sc, two cross veins between Sc–RA; RS forked; subgenital plate more than 2.00× as wide as long, convex and widely rounded apically; subanal plate triangular, elongated, rounded apically without cleft.

General colouration of body relatively pale, light brown to dark brown. Ventral side of body slightly darker than dorsal side. Body covered by blackish maculation (Figs 7A, B, 10).

Head. Clypeus expanded anterolaterally, partly covering anterior part of eyes. Eyes brown, elongated, relatively large, widely separated medially; facets of eyes hexagonal. Distance between eyes 0.73× of head width. Ocelli well preserved, large, without conspicuous colouration. Facial keel small. Antenna brown, approximately as long as head; segmentation hardly distinguishable, therefore not depicted (see Fig. 7C–D).

Thorax. General colouration brown to dark brown. Lateral aspect of thorax not visible. Prothorax narrow, brown. Mesonotal suture transverse, expressed; medioparapsidal suture poorly visible, straight; lateroparapsidal suture distinctly curved laterally; no preserved natural colouration of pigmented area of mesonotum. Ventral side of mesothorax poorly visible; basisternum relatively short and wide distally, furcasternal protuberances distinctly separated. Metathorax brown to dark brown, blackish maculation dorsally (Fig. 7E).

Wings. Forewings hyaline, translucent, relatively narrow; venation poorly recognizable due to wing deformation, pollution on surface and resin influxes [left wing], and damage of distal part [right wing]; venation well visible from dorsal, and partly from lateral side. Veins light brown to brown; relatively small number of cross veins; no jagged edge along of ventral margin (Fig. 8C–F).

General pattern of forewing venation similar to those of male imago of Burmella paucivenosa sp. nov., except for the following features: six free intercalary veins at least in RS field and CuA–CuP; at least four intercalaries in MA–MP field basally attached to longitudinal veins (Fig. 8C–F).

Hind wings hyaline, translucent, small, as long as 0.08 of forewing length; preserved wing is deformed due to embedding, but most probable was naturally strongly rounded, with shallow costal process; venation brown, significantly simplifies; strong reduction of number of longitudinal and cross veins; no jagged edge along of ventral margin. General structure and pattern of hind wing venation similar to those in male imago of Burmella paucivenosa sp. nov., except for the following features: a few cross veins between C–Sc (2 veins), and Sc–RA (2 veins); fork RS present, iRS short, no cross veins in RS field; costal process not prominent (Fig. 9A–C).

Figure 9. 

Burmella clypeata sp. nov., female imago, holotype A right hind wing, dorsal view B right hind wing, ventral view C right hind wing venation. Scale bars: 0.2 mm (A, B); 0.5 mm (C).

Legs well preserved, except for both forelegs with partly missing tibiae and tarsi; no visible strong spines or setae on margins of leg segments. Preserved part of forelegs darker than middle and hind legs, brown to intensively brown (Fig. 8A, B). For measurements of leg segments see Table 1.

Forelegs partly preserved [due to damage of foretibiae the ratio of femur/tibia is not calculated]. Left middle leg completely preserved: length ratio of femur/tibia/tarsus = 1/2.23/0.70; length ratio of tarsomeres: 1/0.88/0.88/1.00/1.50 (5 > 4 > 3 = 2 < 1). Right hind leg much shorter than left one, probably re-grown after previous injury, therefore with changed proportions of tarsomeres: length ratio of femur/tibia/tarsus =1/0.79/0.62; length ratio of tarsomeres: 1/0.89/0.78/0.89/1.33 (5 > 4 > 3 < 2 < 1). Left hind leg: length ratio of femur/tibia/tarsus = 1/1.02/0.57; length ratio of tarsomeres: 1/1/0.91/0.91/1.27 (5 > 4 = 3 < 2 = 1) (Figs 8A, B, 10A). Other leg characters similar to those in male imago of Burmella paucivenosa sp. nov.

Abdominal segments completely preserved, light brown to brown, with blackish maculation on terga and sterna; ventral side of abdomen paler than dorsal side. Vestigial gill sockets, not finger-like, well recognizable on segments II, V, and IV; on other segments gill sockets not distinguishable due to body position in amber. Abdominal segments without large and prominent posterolateral projections; no conspicuous elongation of distal segments compared to proximal ones. Subgenital plate relatively broad, more than 2.00× as wide as long, convex and widely rounded apically. Subanal plate triangular, elongated, moderately narrow and rounded apically without apical cleft. Right cercus completely preserved, brown, darker proximally, approximately as long as body (Fig. 10A–D).

Figure 10. 

Burmella clypeata sp. nov., female imago, holotype A body, ventral view B, C apical part of abdomen, ventral view D basal segments of abdomen, dorsal view. I–III and V–IX – numbers of abdominal segments, gsr – remnant of gill socket, pp – paraproct plate. Scale bars: 1 mm (A); 0.5 mm (B–D).

Attribution of Burmella clypeata sp. nov. to the newly described genus is confirmed based on the shape of hind wings, and specific venation.

On the other hand, some aspects of the venation of fore- and hind wings differ between Burmella clypeata sp. nov. and Burmella paucivenosa sp. nov. The forewings of Burmella clypeata sp. nov. differ by the presence of numerous free marginal intercalaries between iRS and CuP, as well as the presence of at least one cross vein between A₁ and A₂. In the hind wings differences between the extinct species described here refer to the number of cross veins between C–Sc and Sc–RA. The presence of RS furcation and blunt costal process in Burmella clypeata sp. nov. are also suitable for the separation of both species. In contrast to all other representatives of Vietnamellidae, the clypeus in the female of Burmella clypeata sp. nov. is anterolaterally expanded, as a result the anterior portion of eyes is partly covered by this clypeal shield (Fig. 7C–D; compare with e.g., Auychinda et al. 2020a: 9, figs A–E; 2020b: 30, fig. 9A).

We do however not per se exclude a possible conspecifity of both fossil specimens. This may be supported by a similar, small body size of both specimens, with similar proportions of male/female body length as in extant Vietnamellidae (for Burmella gen. nov. the ratio is 0.82; for Vietnamella between 0.92 and 0.96). Also, the anterolaterally expanded clypeus in B. clypeata may not exclude their conspecifity. Similar clypeal expansions present in one sex only have been reported in several extant and fossil species of Heptageniidae (e.g. in the subgenus Ecdyonurus (Nestormeus) Godunko, 2004), representing a morphological trait independently occurring in several unrelated taxa within the family (see Godunko 2007: 66, figs 1, 2; Hrivniak et al. 2018: 199, 204–205, figs 2–5). However, a clear difference in the venation of fore- and hind wings between B. paucivenosa sp. nov. and B. clypeata sp. nov. rather points to the presence of two different fossil species.

In any case, unless specimens of different sex are syninclusions and fossilized in mating position, we tend to describe males and females of the same genus as different species also to maintain nomenclatural stability (see e.g. Staniczek and Godunko 2016; Godunko et al. 2019).