Litargus (Litargosomus) dantiscensis Alekseev, Kupryjanowicz et Bukejs, 2020

Alekseev, Vitalii I., Kupryjanowicz, Janusz, Kairišs, Kristaps & Bukejs, Andris, 2020, The first described fossil species of Litargus Erichson (Coleoptera: Mycetophagidae) from Eocene Baltic amber examined with X-ray microtomography, and new records of Crowsonium succinium Abdullah, 1964, Zootaxa 4768 (3), pp. 405-414 : 406-409

Litargus (Litargosomus) dantiscensis Alekseev, Kupryjanowicz et Bukejs sp. nov.

( Figs. 1–12 )

Type material. Holotype: No. UCPUwB 260. Adult, female (tarsi 4-4-4, apex of female terminalia exposed). The beetle inclusion is included in a small, transparent amber piece, with dimensions of 10×5× 2 mm without any further fixation. The complete beetle with partially exposed metathoracic wings is in good preservation state, but poorly visible using traditional light microscopy: sufficient parts of the ventral side are covered by milky or opaque amber, and meso- and metathorax are additionally hidden by gas microvesicles, making conclusions on the structure and punctation of the ventral side problematic. Syninclusions consist of minute particles of detritus.

Type strata. Baltic amber from Eocene amber-bearing blue Earth layers, mostly Bartonian age is interpreted for the extinct central European resin-producing forests ( Bukejs et al. 2019).

Type locality. Kąty Rybackie in the Vistula Spit, Pomeranian Voivodeship, Poland, along the Baltic Sea coast .

Description. Total body length 2.61 mm; elytral length 1.77 mm, and combined width of elytra at humeri 1.26 mm; pronotal length 0.53 mm, pronotal maximum width 1.18 mm; head length 0.3 mm, and head maximum width (including eyes) 0.6 mm. Body elongate and ovoid in dorsal view, flattened; unicolourous, light brown; dorsum punctured, and finely microreticulated, with dimorphic pubescence.

Head hypognathous, short, and flattened; head distinctly narrower than maximum pronotal width, not abruptly constricted behind eyes; punctured (clypeus with few, fine punctures; frons densely covered with fine punctation, distance between punctures equal to 0.5–1.5× diameter of one puncture; vertex impunctate). Pubescence on head uniform, moderately long, sparse, semierect. Compound eyes prominent, coarsely facetted. Frontoclypeal suture fine, distinct, trisinuate. Clypeus with apical margin convex. Labrum apparent, with slightly rounded apical margin. Mandibulae bidentate apically. Last maxillary palpomere subcylindrical, about 2× as long as previous palpomere, slightly obliquely truncated apically. Antennae rather short (reaching middle of pronotum), 11-segmented, clavate, with distinct 3-segmented club; scape cylindrical, apparently shorter than pedicle; pedicel cylindrical, elongate, length 1.8× width; antennomere 3 elongate, length about 2.3× width; antennomeres 4–8 subequal in length and sharp, cylindrical, nearly as long as wide; antennomere 9 trapezoidal, as long as wide, dilated apically, 2× as wide as antennomere 8; antennomere 10 slightly transverse, dilated apically; antennomere 11 elongate, length 1.3× width, pointed apically. Relative length ratios of antennomeres 1–11 equal to 6:9:8:5:5:5:5:5:8:7:10.

Pronotum transverse, width 2.2× length, margined laterally, widest posteriorly, with two very shallow posterior basal impressions. Pronotal vestiture dimorphic: (1) moderately long, sparse, semierect, directed posteriad, dark setae; and (2) fine, short, semierect hairs. Pronotal punctation moderately dense, fine, with approximate distance between punctures equal to 1.5–3.0× diameter of one puncture; discal punctures obliterated. Anterior pronotal angles obtuse, prominent; posterior pronotal angles acute, embracing elytral humeri. Posterior pronotal margin weakly trisinuate, slightly wider than elytral base; lateral margins almost straight, slightly rounded in posterior portion; anterior margin convex medially. Prohypomera impressed posterolaterally. Prosternal process rather narrow, width about 0.6× transverse diameter of procoxa, flat, rounded apically. Procoxal cavities open.

Scutellar shield transverse, width about 2.0× length, oval. Elytra rather short, length 1.4× combined elytral width; with maximum width in anterior one-third of elytral length; apices rounded. Elytral punctation distinct, moderately dense, fine, forming irregular rows (more or less distinct on elytral disc); distance between punctures in rows equal to 1.5–2.0× diameter of one puncture; intervals flat, with additional, finer punctures; distance between rows about 3.0–4.0× diameter of one puncture. Elytral pubescence dimorphic, consisting of: (1) moderately long, sparse, semierect, directed posteriad, dark setae forming 11 regular rows on each elytron: ten complete rows (reaching apical part of elytron), plus one incomplete row that is about half as long (second row counting from suture); (2) fine, short, semierect hairs. Metathoracic wings fully developed. Elytral epipleura wide and distinctly excavated anteriorly.

Legs slender, rather short, tetramerous. Procoxae hemispherical; mesocoxae oval, transverse; metacoxae elongate-oval, strongly transverse. Tibiae straight, dilated apically, slightly flattened, with two bipectinate spurs apically, and row of sparse setae on outer edge. Tarsal formula 4-4-4; protarsi long, nearly as long as protibia; meso- and metatarsi distinctly longer than meso- and metatibia, respectively; metatarsomere 1 longest, longer than metatarsomeres 2–3 combined. Tarsal claws simple, equal in size.

Abdomen with five visible ventrites, ventrite 5 with widely rounded apical margin. Relative length ratios of abdominal ventrites 1–5 equal to 21:14:10:10:13 (medially). Female terminalia exposed.

Remark. Due to gas vesicles attached to the ventral side of thorax, which obscured light microscopy and were not possible to remove through segmentation in the X-ray micro-CT dataset, the mesocoxae were impossible to examine. The character state “mesepimera reaching mesocoxal cavities” is inferred for the fossil, but not observed.

Derivatio nominis. The specific epithet “ dantiscensis ” is toponymic and refers to the region of the specimen’s origin. It is the adjective formed from Dantiscum, the Medieval Latin name of present-day Gdańsk ( Poland).

Differential diagnosis. The new fossil species belongs to the genus Litargus based on the following morphological characters: antennae with distinct 3-segmented club; pronotum widest posteriorly, slightly wider than combined elytral bases; posterior pronotal angles embracing elytral humeri; protibiae with two apical, bipectinate spurs; apical maxillary palpomere distinctly longer than preapical one and not obliquely truncate; elytra punctures not forming distinct, regular rows. The combination of the following features allows the assignment of Litargus dantiscensis sp. nov. to the subgenus Litargosomus Motschulsky : elytral length 1.4× maximum combined elytral width; maximum elytral width occurs within anterior one-third of elytral length; terminal antennomere not strongly oblique, and distinctly shorter than antennomeres 9–10 combined; elytral epipleura wide and distinctly excavated.

Litargus dantiscensis sp. nov. differs from the single described Baltic amber mycetophagid beetle, Crowsonium succinium Abdullah, 1964 , in having the antennal club distinct (loose in Crowsonium ); the pronotum widest posteriorly (distinctly narrowed towards posterior margin in Crowsonium ) and more transverse (width less than 2× length in Crowsonium ); lateral margins of pronotum and elytra not bordered by spine-like, decumbent hairs; elytral punctation not forming distinct rows (elytral punctures arranged in ten or more longitudinal rows in Crowsonium ); and reduced total body length (1.98–2.04 mm in Crowsonium by original description).

The new fossil species can be distinguished from Recent representatives of the subgenus Litargosomus in possessing: a uniform, light body color (without species-specific elytral patterns of spots and strips present in extant species); a pronotal length/width ratio equal to 0.45; elytral length/width ratio equal to 1.41; and the elytral pubescence arranged into distinct, regular rows of setae.

Assumed biology. Litargus dantiscensis sp. nov. probably occurred under bark and on tree trunks, and acted as a fungus feeder. Similar to its extant relatives, it may have been associated with a range of fungi, such as representatives of Sordariomycetes and Leotiomycetes (Ascomycota), and/or several bracket fungi (Basidiomycota) ( Nikitsky 1993; Milléo et al. 2011). The perceived ‘rarity’ of Litargus inclusions in amber seems to be primarily a matter of small size and collecting bias, but there may also be a connection to its habitat preference. Some of the wood-inhabiting fungi listed above are more common on deciduous trees, as opposed to the gymnosperms that have been proposed as the source for Baltic amber.

Figures

FIGURES 1–3. Photomicrographs of Litargus (Litargosomus) dantiscensis sp. nov., holotype, No UCPUwB 260: 1—habitus in dorsal view; 2—habitus in ventral view; 3—habitus in left lateral view (3). Scale bars represent 1.0 mm.

FIGURES 4–5. Photomicrographs of Litargus (Litargosomus) dantiscensis sp. nov., holotype, No UCPUwB 260: 4—forebody in dorsal view; 5—details of elytra in ventral view showing punctation and setation. Abbreviations: a2–a11—antennomeres 2–11, respectively; ap—apical maxillary palpomere; cl—clypeus; e—compound eye; fcs—frontoclypeal suture; lab—labrum; bpi—basal pronotal impression. Scale bars represent 0.25 mm.

FIGURES 6–9. Micro-CT rendering of Litargus (Litargosomus) dantiscensis sp. nov., holotype, No UCPUwB 260, habitus: 6—dorsal view; 7—ventral view; 8—frontal view; 9—caudal view. Scale bar represents 1.0 mm.

FIGURES 10–12. Micro-CT rendering of Litargus (Litargosomus) dantiscensis sp. nov., holotype, No UCPUwB 260, habitus: 10—left lateral view; 11—right lateral view; 12—ventral reconstruction without legs. Scale bar represents 1.0 mm.