Microscydmus

Jałoszyński, Paweł & Perkovsky, Evgeny, 2016, Diversity of Scydmaeninae (Coleoptera: Staphylinidae) in Upper Eocene Rovno amber, Zootaxa 4157 (1), pp. 1-85 : 66-68

Microscydmus sp. 1

( Figs 29 , 199–203 )

Material studied. Late Eocene of Europe , Rovno amber: sex unknown; inclusion in elongate, subtrapezoidal prism of amber 11.5 mm long ( Fig. 29 ), collection number UA-2216 ( SIZK).

Description. Body ( Figs 199–203 ) elongate, relatively stout and moderately convex, moderately light brown; BL 0.58 mm.

Head ( Figs 199–203 ) short and subtrapezoidal, broadest at eyes, HL 0.13 mm, HW 0.13 mm; vertex ( Fig. 202 ; vt) and frons ( Fig. 202 ; fr) confluent, weakly convex and together subtrapezoidal, strongly transverse; eyes large, semioval, strongly convex; tempora distinctly shorter than eyes. Punctures ( Fig. 202 ) on frons and vertex very fine but discernible, separated by spaces 2–3 × as wide as diameters of punctures; setae ( Fig. 202 ) sparse, short, nearly recumbent. Antennae ( Figs 199–203 ) distinctly shorter than half BL, AnL about 0.20 mm, scape and pedicel strongly elongate, shapes of most of flagellomeres difficult to assess, antennomeres IX–XI distinctly broader than VIII ( Fig. 202 ), IX and X slightly transverse, XI much shorter than IX–X combined, about 1.2 × as long as broad, slightly narrower than X and rounded at apex.

Pronotum ( Figs 199, 202 ) round, broadest near middle; PL 0.15 mm, PW 0.18 mm; anterior and posterior margins nearly straight, anterior and posterior pronotal corners broadly obtuse-angled and blunt; sides strongly rounded; base with short but distinct transverse antebasal groove ( Fig. 202 ; abg) and one large and shallow lateral antebasal pit ( Fig. 202 ; abp) at each side. Punctures on pronotal disc ( Fig. 202 ) very small but discernible, separated by spaces 3–4 × as wide as diameters of punctures; setae ( Fig. 202 ) short and sparse, nearly recumbent.

Elytra ( Figs 199–203 ) elongate oval, moderately convex, EL 0.30 mm, EW 0.20 mm, EI 1.50; punctures and setae similar to those on pronotum ( Fig. 202 ) but slightly denser.

Legs ( Figs 200–201, 203 ) moderately long, slender, unmodified.

Remarks. Diagnostic characters of Microscydmus were illustrated and discussed by Jałoszyński (2014d). These are typically very small beetles, rarely exceeding 1 mm, and often below 0.8 mm. The specimen from Rovno amber is extremely small (BL merely 0.58 mm) and has most of the important ventral structures obscured by turbid or darkened amber. However, besides the small body, the specimen has the general shape typical of Microscydmus , with a short head and round pronotum; its antennae have clubs composed of three distal antennomeres; the prothoracic hypomera have hypomeral ridges ( Fig. 203 ; hyr) and, above all, the pronotal base bears a short but deep median transverse antebasal groove ( Fig. 202 ; abg) flanked at each side by one lateral antebasal pit ( Fig. 202 ; abp), and each elytron has one small basal fovea ( Fig. 202 ; bef) located in a strikingly large and deep, nearly round basal impression. Especially the latter character is a hallmark of most species of Microscydmus . This specimen is consequently assigned to Microscydmus .

Even in extant species generic diagnostic characters are poorly visible and the scanning electron microscopy or examination of specimens in transparent mounts are unavoidable to confirm their generic placement. Among about 170 currently known species of Microscydmus , many have been misplaced, diagnoses having been based solely on the tiny body and general appearance (Jałoszyński, unpublished obs.). Besides the monstrous genus Euconnus , Microscydmus is the most challenging taxon among all Glandulariini , and a complex and heterogeneous conglomerate of several distinct genera hides behind this name. Aedeagi, extremely small, thin-walled and prone to distortion during preparation, additionally complicate the situation. External features can be used to define new species only in exceptional cases, e.g., when conspicuous modifications of exoskeletal structures have been found (see e.g., Jałoszyński (2009c)). The fossil described above is rather unremarkable and it must remain as Microscydmus sp.

Figures

FIGURES 21 – 34. Specimens of Rovno amber with inclusions of Scydmaeninae. K- 8972, Rovnoscydmus sp. specimen 8 (21); K- 3322, Rovnoscydmus sp. specimen 9 (22); K- 15548,? Rovnoscydmus sp. specimen 10 (23); K- 8971,? Rovnoscydmus sp. specimen 11 (24); K- 6045, Euconnus palaeogenus sp. n. (25); K- 9126, Euconnus sp. 1 (26); K- 24859,? Euconnus sp. 2 (27); K- 24302, Stenichnus proavus sp. n. (28); UA- 2216, Microscydmus sp. 1 (29); K- 6943,? Microscydmus sp. 2 (30); K- 3657 a,? Microscydmus sp. 3 (31); K- 6764,? Scydmoraphes sp. 1 (32); K- 6274, Glandulariini gen. & sp. indet. 1 (33); K- 7089, Glandulariini gen. & sp. indet. 2 (34).

FIGURES 199 – 203. Microscydmus sp. 1 (UA- 2216). Dorsal (199, 202), right lateral (200, 203) and left lateral (201) views. Abbreviations: abg, antebasal groove; abp, antebasal pit; bef, basal elytral fovea; fr, frons; hyr, hypomeral ridge; mcp, mesocoxal projection; nr, ' neck' region; vt, vertex.