Burmella clypeata, Godunko & Martynov & Staniczek, 2021

Godunko, Roman J., Martynov, Alexander V. & Staniczek, Arnold H., 2021, First fossil record of the mayfly family Vietnamellidae (Insecta, Ephemeroptera) from Burmese Amber confirms its Oriental origin and gives new insights into its evolution, ZooKeys 1036, pp. 99-120 : 99

Burmella clypeata sp. nov. Figures 7 , 8 , 9 , 10 , Table 1

Material examined.

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 .

Derivation of name.

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

Diagnosis.

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.

Description.

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 ).

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 ).

Affinities.

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).

Figures

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).

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).

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).

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).