Euroleptochromus tuberculatus Yin & Cai, 2018

Yin, Zi-Wei & Cai, Chen-Yang, 2018, A new fossil species of Euroleptochromus Jałoszyński (Coleoptera: Staphylinidae: Scydmaeninae) from Eocene Baltic amber, Zootaxa 4500 (1), pp. 146-150 : 147-148

Euroleptochromus tuberculatus Yin & Cai , sp. nov.

Type material. Holotype (SNUC-Paleo-0046), sex undetermined; originated from Baltic amber ( Russia, Oblast Kaliningrad); deposited in the Insect Collection of the Shanghai Normal University , Shanghai, China.

Diagnosis. Head lacking elongate postgenal process, postgena with cuticular tubercle bearing three thick bristles; elytral index (length/width) slightly less than 2.0; pronotum elongate, pronotal index (length/width) nearly 1.40; scape nearly 2.9 times as long as pedicel; profemur with five (3-1-1) ventral spines.

Description of holotype. Body ( Figs 1 , 2A ) slender, length (a sum of head, pronotal and elytral length which were measured separately) 3.20 mm, reddish-brown. Head ( Figs 1A , 2A–B ; he) strongly transverse, broadest at eyes and flattened, length from anterior clypeal margin to base 0.44 mm, width across eyes 0.55 mm; most part of head obscured by ‘milky’ film, only following structures can be seen: vertex strongly transverse, convex on sides and flattened at middle; occipital constriction much narrower than head; tempora much longer than eye in dorsal view, strongly rounded and strongly convergent towards distinct occipital constriction; postgena lacking long projection, cuticle slightly raised to form tubercle, with three long bristles ( Fig. 2B–C ; pgb); eyes ( Fig. 2C ; ce) large and strongly projecting from silhouette of head, finely faceted; punctures and setae on frons and vertex fine, inconspicuous; antennal insertions located anterior to eyes. Details of mouthparts not visible, except for strongly elongate maxillary palpus ( Figs 1A ; mp) which is much longer than head; palpomere II much longer than III but shorter than III and IV combined ( Fig 2B ; mp2–4), divided by angulate expansion located in basal 2/5 into two unequal parts, proximal part distinctly curved, distal part nearly straight and slightly broader, angulate expansion with two robust anterior bristles ( Fig. 2B–C; b ); palpomere III slender, nearly cylindrical in basal half and then gradually broadening distally; palpomere IV less than half length of palpomere III, suboval, about twice as long as wide. Antennae much shorter than body, length 1.56 mm, lengths of antennomeres I–XI ( Fig. 2B; a 1 –11): 0.38 mm (I), 0.13 mm (II), 0.11 mm (III), 0.13 mm (IV), 0.12 mm (V), 0.12 mm (VI), 0.12 mm (VII), 0.12 mm (VIII), 0.09 mm (IX), 0.09 mm (X), 0.15 mm (XI); relative lengths of antennomeres (shortest antennomere VIII and IX as 1): 4.2: 1.4: 1.2: 1.4: 1.3: 1.3: 1.3: 1.3: 1: 1: 1.7. Antennomeres sparsely covered with suberect setae of various lengths. Pronotum ( Fig. 1A ) elongate and broadest distinctly in front of middle, strongly narrowing towards base, length along midline 0.87 mm, maximum width 0.65 mm, pronotal index 1.38; disc convex and sparsely covered with shallow but distinct punctures, setae fine; detail of pronotal base vague, dorsal pits and groove putatively present. Prosternum and mesoventrite entirely covered by ‘milky’ film. Metaventrite ( Fig. 1B ) slightly impressed posteromedially; metacoxae ( Fig. 1B ; mtc) broadly separated. Elytra ( Figs 1A , 2A ) strongly convex, broadest near middle, length along suture 1.89 mm, maximum width 0.99 mm, elytral index 1.91; each elytron with four dorsal ( Fig. 1A; 1 –4) and two lateral rows of distinct, large punctures; humeral calli prominent, elongate; elytra sparsely covered with short suberect setae. Legs ( Figs 1B , 2A ) long and slender, protrochanter ( Fig. 2D ; ptr) suboval, one extremely long, thin apical seta can be clearly seen (not in figure); all femora strongly clavate; profemur ( Fig. 2D ; pfe) strongly broadened, with five (3-1-1) long ventral spines, first three spines closely placed; protibia ( Fig. 2A ) moderately curved; remaining legs unmodified. Sternite III long, sternites IV–VI distinctly shorter than sternite III and subequal in length, sternite VII slightly longer than VI, sternite VIII partly covered by ‘milky’ film, putatively longer than sternite III (as in E. sabathi and E. setifer ).

Type locality and horizon. Russia: Kaliningrad Oblast; middle to upper Eocene .

Etymology. The specific name refers to the presence of a postgenal tubercle of the new species which is species-characteristic.

Comments. The new species can be readily assigned to Euroleptochromus by the strongly transverse head, elongate maxillary palpus much longer than head, maxillary palpomere II with a sub-median protuberance bearing apical bristles, and maxillary palpomere IV only about twice as long as wide, all of which are shared with the other two congeners, i.e., E. sabathi and E. setifer . Interestingly, apart from the different proportions of the pronotum, elytra, and antennomeres, and different spination of the profemur, E. tuberculatus lacks an elongate postgenal projection (present in E. sabathi and E. setifer ), and instead has a moderately raised tubercle bearing three thick bristles. The discovery of E. tuberculatus demonstrates for the first time a notable variability of the postgenal structures within Euroleptochromus . This degree of variation, however, is not unfamiliar in their closely related genus Leptochromus . Extant members of Leptochromus are commonly found in the lowlands of Central and South America ( Lord et al. 2014). Their postgenae vary from being evenly convex without any trace of a tubercle, to having a slight to distinct cuticular convexity or an elongate projection (the latter as in E. sabathi and E. setifer ) ( O’Keefe 2002). This shared similarity between Euroleptochromus and Leptochromus suggests that the variability in the postgenal structures probably evolved before the common ancestor of these two genera.

Figures

FIGURE 1. Habitus of Euroleptochromus tuberculatus. A. Dorsal view. B. Ventral view. Abbreviations: 1–4—rows of punctures 1–4, ant—antenna, mp—maxillary palpus, mtc—metacoxa, s3–8—sternites III–VIII. Scale bar: 1.0 mm.

FIGURE 2. Details of Euroleptochromus tuberculatus. A. Lateral habitus. B. Head, right maxillary palpus, and right antenna. C. Lateral view of head, showing postgenal tubercle, bristles, and angulate expansion of maxillary palpomere II. D. Profemur, with five ventral spines. Abbreviations: a1–11—antennomeres I–XI, b—bristle, ce—compound eye, mp2–4—maxillary palpi II–IV, pfe—profemur, pgb—postgenal bristle, ptr—protrochanter. Scale bar: 0.5 mm in A–B, 0.2 mm in C–D.