Male imago: (1) eyes large, medially contiguous; (2) outer margin of foretibia with several stout, pointed setae; (3) cubital field with three long intercalary veins: one long intercalary vein connected with CuA by one or several crossveins; one pair of intercalary veins situated close to CuP and at least connected with CuA (or additionally with CuP) by a crossvein; (4) plate angulate, mediocaudally only shallowly incised; incisions with relatively small pronounced triangular projections; with or without medial projection, if present, small and bluntly pointed apically; (5) forceps relatively long; basal segment conical, without apical hump on inner margin; (6) penis stem elongated; (7) penis significantly surmounting distal end of plate, reaching 0.25−0.30× length of forceps segment 2; (8) penis lobes elongated, apically triangular or rounded, medially contiguous, with V-shaped cleft apically.

Siphloplecton jaegeri Demoulin, 1968, Siphloplecton landolti sp. nov., Siphloplecton studemannae sp. nov., Siphloplecton sp. 5, Siphloplecton sp. 6.

Male imago in Baltic amber (Eocene), MNB, MB.I 7370, specimen labelled as: “6. Pseudoneuroptera III Ephemeridae”; “Museum für Naturkunde Berlin”; “Paläontologisches Museum”; “Slg.: Künow Inv. Nr.: Nr. 268–294 nur noch 9 Stück vorgefunden”; “Ephemeriden”; “Siphloplecton cf. jaegeri ♂ imago Nr.: 271” (Figs 1A–E, 2).

Well preserved specimen, visible in dorsoventral aspect. Wings completely preserved (Fig. 1A, B); posterior margin of left forewing and hind wings twisted. Ventral side of head and prosternum not visible, view obstructed by resin influx and cracks in stone. Foremargin and distal part of left forewing and entire left hind wing dirty brownish coloured; several dark spots on remaining part of left forewing. Such irregular pigmentation is a side effect of the specific conditions of fossilization, and must not be confused with the natural pigmentation of Siphloplecton wings (the right wings of the same specimen are colourless and translucent). Right fore- and left middle legs lost. Cerci partly damaged.

For measurements see Table 1.

Male imago in Baltic amber (Eocene), MNHN, 4655 BA (Figs 3–4).

The specimen is visible in dorsoventral and, partly, lateral aspect. Head and thorax ventrally with “Verlumung”. Right fore- and hind wings fully preserved; left pair of wings partly twisted; details of cubital field not discernible. Cerci lost.

Colour yellow to yellowish-brown with darker thorax, but generally paler than all other known specimens of S. jaegeri. Wings hyaline, translucent, without any pigmentation.

For measurements see Table 1.

General colouration from pale (yellow to yellowish-brown), to dark brown (yellowish-brown to intensively brown); details of wing colouration are described above.

Head uniformly brown. Eyes large, well developed, medially contiguous (Figs 1C, 2, 3A, 4A), slightly flattened laterally (MNB specimen); ocelli of MNHN specimen with slightly paler apical part and darker basally; antennae light brown, longer than head (Fig. 3A); ocelli and antennae of MNB specimen not visible due to resin influxes (Fig. 1A–C).

Thorax brown with dirty maculation dorsally (MNB specimen), yellowish-brown with markedly darker sterna (MNHN specimen); mesonotal suture stretched backwards medially; lateroparapsidal suture elongated, without surrounding pigmentation; furcasternal protuberances of mesothorax fused.

Wings hyaline. Pterostigma with at least 6 anastomosed veins. Cubital field of right forewing with well-developed intercalary vein (iCu1) close to CuA and basally directly connected to it, followed by one pair of intercalaries (iCu2, iCu3) basally connected to each other and connected to CuA (in MNHN specimen also to CuP) by a short crossvein (Figs 1D, 2, 4A). Hind wings with triads RS, MA and MP (Fig. 4A); preserved part of hind wings 0.39× forewing length (MNB specimen). Hind wings of MNHN specimen with triads RS, MA and MP, poorly visible distally; preserved part of hind wings 0.35× forewing length. Costal process small.

Legs brownish; tibiae and tarsi darker than femora; structure and proportions of leg segments similar to those of holotype of S. jaegeri; outer margin of foretibia with pointed setae; measurements of leg segments in Table 1.

Abdominal segments well preserved, paler than thorax. Shape of styliger and penis lobes (Figs 1E, 3D, 4B) similar to those of S. jaegeri holotype (Demoulin 1968: 252, figs 18a, c, Staniczek and Godunko 2012: 74, fig. 11c). Styliger plate angulate, deeply incised with three prominent projections; medial projection markedly broad; basal segment of forceps basally (a) markedly narrower than adjoining apical part of plate (b) (a/b = 0.57); forceps 4-segmented; segment 2 longest, segment 4 approximately 2.47 to 2.65 times as long as wide; length ratio of segment 3 to segment 4 approximately 0.9:1. Penis lobes elongated, medially incised, triangular, with relatively inconspicuous incision between lateral and medial penis sclerites. Surface details of penis lobes not visible.

Paracercus vestigial, 5-segmented.

Some minor differences regarding the proportions of the fore/hind wings and forceps segments between the holotype of S. jaegeri and the specimens described above are present. However, we attribute these specimens to S. jaegeri due to the presence of 1+2 intercalaries in the cubital field, pointed setae along outer margin of foretibia, and the shape of plate and penis lobes.

Holotype. Male imago in Baltic amber (Eocene), SMNS BB–2377. Well preserved specimen in clear amber, well visible in dorsoventral aspect. Each of both forewings bent upwards approximately at half length (left forewing partly twisted); left hind wing twisted; genitalia visible in ventral and, partly, lateral view.

For measurements see Table 1.

Colouration pale, dorsal side slightly darker than ventral side. Brownish diffuse maculation on thorax and abdomen preserved. Ocelli well preserved. Eyes large, medially contiguous; brownish maculation on eye surface partly preserved; antennae longer than head (Figs 5A–C, E, 6A–B).

Thorax with traces of brownish pigmentation on dorsal and ventral sides; “Verlumung” mainly on lateral sides near wing bases. Lateroparapsidal suture elongate, typical for Siphloplecton; no conspecific pigmentation around lateroparapsidal suture; mesonotal suture medially bulged (Fig. 6A); furcasternal protuberances of mesothorax contiguous (Fig. 6B); lateral aspect of thorax hardly visible.

Wings translucent, hyaline, not pigmented, venation well visible; pterostigma translucent, without pigmentation, with simple veins. Cubital field of forewing well visible on right wing only (twisted on left wing): one distinct pair of intercalary veins (iCu2 and iCu3) situated close to CuP and connected with CuP and CuA; one long intercalary vein (iCu1) connected with CuA and also with iCu2 by crossveins; additional short intercalaries present, ending at hind margin of wing (one of them between iCu1 and iCu2, basally joining both veins) (Fig. 5C). Hind wings with triads RS, MA and MP, 0.31× forewing length; costal process bluntly pointed (Fig. 5D).

Legs damaged; both right fore- and hind legs lost; tarsus of left foreleg only partly preserved (for details see Table 1). Tibia of left foreleg paler than femur and tarsus. Left foretibia with strong, sharply pointed setae along outer margin. Tibia of left middle and hind legs black; preserved part of first tarsomere of left middle leg black. Tibia of middle and hind legs with trace of tibiopatellar suture; first tarsomere of middle and hind legs fused with tibia. Tarsal claws lost (Fig. 5A–B).

Abdominal segments completely preserved, sterna visibly paler than terga; genitalia dark.

Styliger plate angulate, mediocaudally deeply incised; these incisions with pronounced, broad, triangular projections; medial projection not large, blunt apically (Figs 5F, 7). Basal segment of forceps basally relatively wide; forceps 4-segmented, segment 4 approximately 3.30 times longer than wide; length ratio of segment 3 to segment 4 approximately 1:1. Penis lobes distinctly elongated, well separated from each other apically, ellipsoidal; medial sclerite rounded apically; lateral and medial sclerite markedly separated on outer side with inconspicuous incision between sclerites; features of ventral surface of penis lobes not visible (Figs 5F, 7).

Paracercus vestigial; cerci partly lost.

The new species can be placed within the jaegeri species group based on the characteristic shape of the penis lobes, the arrangement of cubital intercalaries of the forewing and the sharply pointed setae along the outer margin of the foretibia. Siphloplecton landolti sp. nov. can be separated from the closely related S. jaegeri by (1) shape of styliger with relatively small and apically rounded medial projection; (2) relatively wide base of basal forceps segment compared to adjoining apical part of plate; (3) proportions of last forceps segments; (4) ellipsoidal shape of penis lobes with medial sclerite rounded at tip.

The body measurements of S. landolti sp. nov. are comparable to other representatives of the S. jaegeri species group, but the ratio of hind/forewing length is lower in comparison with S. jaegeri itself.

Following our tradition of naming new fossil species of Siphloplecton after Swiss ephemeropterists, this species is named after Peter Landolt, Lausanne, to honour his contributions to the knowledge of Swiss mayflies.

Holotype. Female imago in Baltic amber (Eocene), SMNS BB–2626. Well preserved specimen in clear amber, well visible in dorsoventral aspect. Right foreleg lost; hind wings (especially left wing) twisted. Head and thorax covered by “Verlumung”. Additionally, piece of amber with numerous cracks, thus details of thoracic terga invisible, and thorax only partly visible from ventral side. For measurements see Table 1.

Paratype. Female imago in Baltic amber (Eocene), MNHK, MP/1626. Partly damaged specimen, visible in ventral aspect. View on body hampered by resin influxes, numerous cracks, and considerable “Verlumung”, so thoracic sutures are hardly visible. Head lost. Pronotum damaged, its structure invisible. Distal part of mesonotum, metanotum and abdominal segments I−IV dorsally covered by plant tissue. Right forewing twisted along its length; left forewing lacks its basal part and distal end. Right foreleg, right middle leg, left middle tarsus, and both hind legs lost. Cerci only partly preserved. For measurements see Table 1.

General colour of body pale, yellow to yellowish-brown. Ventral side of body slightly darker than dorsal side. Ocelli well preserved. Eyes medially separated, but approximated (Fig. 8A). Distance between eyes 0.22× of head width. Antennae complete, slightly longer than head, pale.

Thorax ventrally yellowish coloured by “Verlumung”, mainly in posterior part. Furcasternal protuberances of mesothorax contiguous (Fig. 8B); lateral aspect of thorax not visible.

Wings translucent, hyaline, not pigmented. Wing venation well visible only from ventral side; basal part of wings covered by numerous cracks from dorsal side. Cubital field of both forewings with one pair of intercalary veins (iCu2, iCu3) towards CuP and one additional vein (iCu1) near CuA (Figs 8C–D, 9). Right hind wing hardly visible, twisted; left hind wing completely preserved with three pairs of triads, 0.34× of right forewing length. Costal process bluntly pointed apically and small (Fig. 8E).

Legs relatively well preserved, except forelegs (right is lost; left incomplete). Measurements of leg segments in Table 1. Tibiae of middle and hind legs each with trace of tibiopatellar suture; first tarsomere of middle and hind legs fused with tibia (Fig. 8F). Tarsi with 5 tarsomeres; tarsal claws dissimilar: one hooked and one blunt (Fig. 8G).

Abdominal segments completely preserved, pale; terga VIII–X slightly darker than remaining terga. Abdominal sterna slightly covered by “Verlumung”. Subgenital plate relatively broad, 2.00× as wide as long, convex and rounded apically. Subanal plate not elongated, narrow, with pointed tip (Figs 8H, 9). Paracercus poorly visible, vestigial, with at least 4 visible segments; cerci dark, partly preserved.

Body colouration light, yellowish-brown to brown. Irregular brown, dirty brown to black maculation over body, especially on forewings and legs (Fig. 10A).

Details of mesonotum hardly visible; mesonotal suture typical for Siphloplecton, original lateroparapsidal suture colouration invisible due to resin influx. Furcasternal protuberances contiguous, distinctly brown; other parts of mesosternum paler.

Pterostigma with anastomosed crossveins. Cubital field of right forewing well preserved; iCu1 basally connected to CuA and iCu2 by crossveins, 2 additional CuA-iCu1 crossveins present. Pair of intercalaries iCu2 and iCu3 basally connected to each other; further crossveins connect to CuA and CuP (Figs 10B, 11A). Hind wings relatively long, approximately 0.40× of length of preserved right forewing; costal processes hardly visible. Hind wings with triads RS, MA and MP; costal process (visible only on right hind wing) small and blunt (Fig. 10C).

Left foreleg intensely brown to blackish distally; middle legs paler, yellow to yellowish-brown. Preserved foreclaw dissimilar (one claw hooked, one claw blunt).

Most of abdominal segments ventrally covered with “Verlumung”. Preserved sterna paler than terga. Subgenital plate not elongate, approximately 1.80× wide as long, widely rounded apically. Subanal plate not elongate, pointed at tip (Figs 10D, 11B). Paracercus 5-segmented.

We allocate Siphloplecton studemannae sp. nov. within the S. jaegeri species group based on the presence of three distinctive intercalaries (grouped in one pair and one additional intercalary vein) in the cubital field of both forewings. The same arrangement of intercalary veins was described and figured for the holotype of S. jaegeri by Staniczek and Godunko (2012: 73, 74, fig. 10b). Other diagnostic characters of this species group, namely the presence of sharply pointed setae at the outer margin of the foretibia, cannot be observed in the type specimens of S. studemannae sp. nov. So it cannot be excluded that this character is present in the females of this fossil species, similarly to the different occurrence of this character in the two sexes of extant species (Berner 1978).

The new species can be characterized by the presence of the following characters: (1) eyes separated, but close-set; (2) cubital field of forewings with one pair of intercalaries and one intercalary vein connected with CuA; (3) subgenital plate relatively broad with width/length ratio 1.80−2.00. This combination of characters definitely separates the described females of S. studemannae sp. nov. from all other previously known Recent and fossil taxa.

Siphloplecton studemannae sp. nov. is currently the only species of the genus Siphloplecton described from female specimens that can also be attributed to a certain species group. This is only possible due to the unique arrangement of cubital intercalaries, which is characteristic for the S. jaegeri species group. The previously described S. barabani and S. hageni, also only known from female specimens, do not provide any clear distinguishing characters that would allow their placement in one of the other fossil species groups defined for Siphloplecton, which can only be grouped based on similarities in the male genitalia.

Following our tradition of naming new fossil species of Siphloplecton after Swiss ephemeropterists, this species is named after Denise Studemann, Lausanne, to honour her contributions to the knowledge of Swiss mayflies.