Pycnomerus agtsteinicus Bukejs, Alekseev & McKellar, 2019

Bukejs, Andris, Alekseev, Vitalii I., Cooper, David M. L., King, Gavin A. & Mckellar, Ryan C., 2019, A new fossil species of Pycnomerus Erichson (Coleoptera: Zopheridae) from Baltic amber, and a replacement name for a Recent North American congener, Zootaxa 4550 (4), pp. 565-572 : 566-571

Pycnomerus agtsteinicus Bukejs, Alekseev & McKellar sp. nov.

( Figs 1–3 )

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Type material. Holotype: collection number 273-2 [ CCHH], adult, female (submentum without setose pit). Complete beetle with partially exposed hind wings, included in small, transparent yellow amber piece embedded in block of GTS-polyester resin with dimensions 13×10× 4.5 mm. Syninclusions consist of few stellate Fagaceae trichomes, and few small pieces of organic material.

Type strata. Baltic amber, mid-Eocene to Upper Eocene .

Type locality. Baltic Sea coast, Yantarny settlement (formerly Palmnicken), Kaliningrad region, Russia .

Etymology. The specific epithet refers to the amber source and is derived from the Upper German word for amber, “der Agtstein”.

Differential diagnosis. Pycnomerus agtsteinicus sp. nov. differs from P. simukovi Alekseev , described from Baltic amber, in the following morphological characters: (1) anterior portion of head with oblong pits (rounded pits in P. simukovi ); (2) pronotum more transverse, 1.2× as wide as long (0.9× as wide as long in P. simukovi ); (3) antennomere 11 transverse, rounded apically, and almost as wide as antennomere 10 (oblong, tapered, and distinctly narrower than antennomere 10 in P. simukovi ); (4) scutellar shield more transverse, 2.6× as wide as long (1.5× as wide as long in P. simukovi ); (5) sparser punctation of head and pronotum; (6) pronotum behind anterior angles more widely rounded (pronotal sides more parallel and almost straight within anterior one-third of length in P. simukovi ); and (7) ventrite 5 longer than ventrite 4 (ventrite 5 shorter than ventrite 4 in P. simukovi ).

Differences between P. agtsteinicus sp. nov. and subfossils of extinct P. rimatara and P. prebblei include the following combination of characters: (1) anterior portion of head with oblong pits; (2) pronotum more transverse; (3) flattened pronotal disc; (4) pronotal lateral margins without crenulation; and (5) habitus distinctly wider.

The newly described fossil species can be easily distinguished from the modern European natives Pycnomerus italicus and P. sulcicollis based on its non-reduced, 11-segmented antennae; and lack of deep, paired, longitudinal impressions on pronotal disc. It can be distinguished from P. terebrans and P. inexpectus based upon reduced anterior pronotal angles, distinctly 2-segmented antennal club, and elytral apices that are not flattened. Additionally, the new species differs from P. inexpectus in possessing pronotal disc without shallow impressions, and from P. terebrans in possessing pronotal sides that are not explanate.

Description. Body length 3.1 mm, maximum width 1.0 mm; body shape elongated, parallel-sided, dorsoventrally flattened; body shiny, unicolor, ferruginous (as preserved), glabrous.

Head prognathous, retracted into prothorax up to posterior margins of eyes, nearly as long as wide, with indistinct subantennal grooves; covered with small and dense punctation; distance between punctures about 1.0– 3.0× diameter of one puncture, punctures slightly smaller and denser in anterior portion of head, and interspaces glabrous; anterior portion of head with pair of oblong pits dorsally. Clypeus with widely and shallowly concave anterior margin. Compound eyes large, oval, slightly convex, entire, apparently without interfacetal setae, with distinct facets; each facet about as large as one head puncture (in posterior portion of head). Distance between compound eyes dorsally nearly equal to 5× transverse diameter of one eye. Mandibles bidentate apically. Submentum without setose pit. Antennae with 11 antennomeres, clavate, with distinct 2-segmented club, stout, short, reaching middle of pronotum, covered with fine, sparse, erect to semierect setation; antennomeres transverse; scape about 1.1× as wide and 0.8× as long as pedicel; antennomeres 3–9 subequal in size and shape; antennomere 9 about 2.7× as wide as long; antennomere 10 about 2× as wide as long, and 1.6× as wide as antennomere 9; antennomere 11 about 1.2× as wide as long, with widely rounded apex, and slightly narrower than antennomere 10.

Pronotum transverse, 1.2× as wide as long, widest in anterior one-fourth of pronotal length; pronotal disc flat; covered with small, sparse punctures. Lateral margins of pronotum apparently smooth, almost straight, slightly converging posterior to widest point, becoming rounded posterolaterally, with narrow bordering; anterior margin or pronotum slightly concave, apparently without bordering, unmodified; posterior margin convex, with narrow bordering. Anterior angles acute, slightly protruding; posterior angles widely rounded.

Scutellar shield small, elongated oval, transverse, about 2.6× as wide as long.

Elytra elongated oval, parallel-sided, flattened, about 2.1× as long as wide, with distinct humeri. Each elytron with 10 deep striae; striae distinct throughout entire length of elytron; interstrial intervals distinctly wider than stria, and convex, with sparse, minute punctures; scutellary striole absent. Macropterous.

Pro- and metaventrite with large, sparse, rounded punctures (similar to punctures of ventrite 1, and larger than pronotal punctation); hypomeron without antennal cavities; prosternal intercoxal process wide, distinctly wider than diameter of procoxa, expanded in apical portion.

Legs short, robust, with sparse and fine punctation. Procoxal cavities externally closed. Procoxae globose; meso- and metacoxae oval, transverse; all coxae widely separated by more than one transverse diameter of coxa. Femora clavate; tibiae dilated apically, almost straight, slightly longer than femora, with two apical spines at inner margin. Tarsi nearly as long as tibia, without dense setae ventrally, with few semierect setae; tarsal formula 4-4-4. Relative length ratios of metatarsomeres 1–4 equal to 5:3:3:7. Claws thin, free, simple.

Abdomen with five visible ventrites; abdominal sutures distinct throughout length; ventrites 1–3 more solidly fused together than ventrites 4–5; ventrites 1–4 with large, sparse punctures, and distance between punctures about 0.5–3.0× diameter of one puncture; ventrite 5 without preapical groove, covered with large and distinctly denser punctation (distance between punctures less than diameter of one puncture), and with small punctures at posterior margin. Intermetacoxal apophysis (intercoxal process of ventrite 1) widely rounded. Relative length ratios of ventrites 1–5 equal to 9:7:7:5:6 (medially).

Remarks. Several morphological characters mentioned in the comparison of the newly described species with P. simukovi , can vary sexually or individually in some well-studied extant species of Pycnomerus . In addition, the original description of P. simukovi from Baltic amber was based on a specimen that is possibly a little distorted and of unknown sex (the area of the submentum was not discernible, and the presence or absence of a setose pit there is unclear). Consequently, the description of P. simukovi was provided with heavily schematized drawings. Both of the specimens under consideration (i.e., the specimen described here as P. agtsteinicus sp. nov., and the specimen previously described as P. simukovi ) possess a similar set of general morphological characters, and are without a doubt, morphologically close species that clearly differ from Recent European congeners. It is possible that the new species could be the female of the previously described Eocene species, and that the differences described herein could be interpreted as sexual characters or intraspecific variation. Despite the general morphological similarity, between the two fossil species we suggest that the new species is distinct, and that it can be discerned based upon the full combination of aforementioned characters. However, it is worth noting that assessing reliable specific differences in Eocene Pycnomerus species would require further study of additional fossil material from this deposit. The detailed study of the Recent Pycnomerus fauna from the tropics and temperate areas of the Southern Hemisphere, as well as the world-wide revision of the genus, will be necessary elements of future studies required for a more precise systematic placement of the extinct Eocene Pycnomerus species within this large genus.

Figures

FIGURE 1. Pycnomerus agtsteinicus sp. nov., holotype 273-2 [CCHH]: A, B—dorsal habitus photomicrograph, and corresponding SR X-ray micro-CT rendering; C, D—ventral habitus photomicrograph, and corresponding SR X-ray micro-CT rendering. Scale bars represent 0.5 mm.

FIGURE 2. Pycnomerus agtsteinicus sp. nov., holotype 273-2 [CCHH]: A, B—lateral left habitus photomicrograph, and corresponding SR X-ray micro-CT rendering; C, D—lateral right habitus photomicrograph, and corresponding SR X-ray micro-CT rendering. Scale bars represent 1 mm.

FIGURE 3. Pycnomerus agtsteinicus sp. nov., holotype 273-2 [CCHH]: A, B—frontal habitus photomicrograph, and corresponding SR X-ray micro-CT rendering; C—caudal habitus SR X-ray micro-CT rendering; D—left antenna, ventral view; E—left antenna, dorsal view; F—left protarsus, ventral view; G—left middle leg, ventral view. Scale bars represent 0.5 mm (A–C), 0.25 mm (D–G).