Male in Baltic amber; size of piece approximately 12 × 5 × 5 mm (Fig. 1), with collection label data “GZG.BST.16192 / (alte Nr. G. 645) / Coleoptera: Carabidae / Bembidium / Geologisch-Paläontologisches Institut und Museum / Göttingen” (front side) and “Species „b“ / Nr.2” (back side), in Geoscience Museum, University of Göttingen, Germany (GMG). This amber piece is part of the former Königsberg Collection which is currently preserved at the GMG. This collection includes pieces of Baltic amber collected in the area of the Curonian Spit up until the first half of the 20^th century.

Figures 1–3. 

Trechus exhibitorius sp. n., light microscopic images of the holotype. 1 general view of the fossil with contours of the amber piece 2 left lateral aspect 3 dorsal aspect.

The amber piece is markedly darkened, and its surface shows several fine cracks, very probably as a result of its extended exposure to air. The beetle body is partly covered by milky coating and thus its right ventral surface and the mouth parts cannot be investigated by light microscopy (Figs 2, 3). The specimen is slightly shrunken with parts of its exoskeleton, which is thus dissociated from the inclusion wall. The latter represents the negative imprint of the beetle body and could be imaged together with the detached parts of the beetle exoskeleton using micro-CT, e.g., the detached margins of the pronotum (Fig. 4). Parts of the exoskeleton are broken along sutures and impressions, e.g., pronotal median longitudinal impression (Fig. 4), gula, occipital impression, mentum/submentum (if not naturally so), prosternum/proepisternum (Figs 5, 6, 7). The aedeagus is partly moved out off the abdomen and attached in this position by the last abdominal segment; the endophallus with the claw-like apex of the copulatory piece is partly inflated (Fig. 12). At its base, the median lobe is completely disrupted from the sclerites of segment IX (genital ring); latter is lost including parts of the median lobe basal bulb and the left paramere (Fig. 13).

Figures 4–6. 

Trechus exhibitorius sp. n., volume rendering of the holotype, anterior portion. 4 dorsal aspect 5 ventral aspect 6 right lateral aspect.

Figures 7–12. 

Trechus exhibitorius sp. n., holotype, volume rendering of selected body parts. 7 head, ventral aspect 8 head, anterior part, dorsal aspect 9 mandibles, view from dorsal (the dorsal surface of the right mandible is cut together with the labrum in order to show the mandibular dentition; the contour of the apical portion of the right mandible is indicated by a dotted line) 10 elytra, caudal aspect 11 elytral epipleuron and left external part of metathorax 12 elytra and abdomen, right lateral aspect. Abbreviations: aed = aedeagus; cl = clypeus; hs = humeral series; lbr = labrum; M = phoretic mite; meps = metepisternum; ms = medial series; mt = mentum; pas = preapical seta; rst = recurrent stria; sas = subapical series; smt = submentum; ta, tb, tm = apical, basal, medial denticle of the right mandible.

Figures 13–14. 

Trechus exhibitorius sp. n., holotype, volume rendering of selected body parts. 13 frontal view inside the abdomen showing the aedeagus which is fixed between the last abdominal segments 14 right front leg with protarsi i-v. Abbreviations: B = air bubble; cpa = apex of the longer copulatory piece; mla = median lobe apex; mlb = basal bulb of median lobe; pm = parameres; st = sternites; tg = tergites.

One Acari specimen (phoretic mite of Parasitidae?) attached on the right elytron of T. exhibitorius, stellate hairs, and numerous dirt particles.

Body length: 4.4 mm.

Colour: The whole body surface appears blackish brown, very shiny; variation in colouration of the different parts of the beetle body is not recognizable.

Microsculpture: Surface of head and elytra, disc of pronotum with shallowly engraved slightly transverse meshes, clypeus with slightly engraved almost isodiametric meshes, base and laterobasal furrows of pronotum with moderately engraved almost isodiametric meshes (magnification ×100).

Head: Moderately large and transverse; length 0.94 mm. Mandibles moderately slender, the right one tridentate, with all denticles combined and subequally distributed, but with the apical one much more protruding than the second (Figs 7–9). Labrum with apical margin markedly concave, with three pairs of setae. Clypeus with two pairs of setae in normal position. Shape of apical segments of maxillary and labial palpi as in Trechus sensu stricto. Mentum and submentum completely divided by a distinct suture (artefact? Fig. 7). Eyes (Fig. 6) rather small, flat; tempora about half as long as eyes, convex, moderately wrinkled to the neck, smooth. Frons and supraorbital area smooth, markedly convex, with supraorbital furrows deep, complete, uniformly bent on disc (Fig. 4); two supraorbital setae present and in normal position for Trechus. Antennae moderately slender, with three antennomeres extending beyond the pronotal base; scapus robust, 1.15 times longer and 1.3 times broader than pedicellus, 1.1 times longer than third antennomere; third and fourth antennomeres of the same length.

Prothorax: Pronotum rather large, transverse (width/length = 1.47), length 0.84 mm, 1.53 times broader than head, broadest somewhat before middle, with sides evenly rounded in anterior 2/3 and straight before laterobasal angles; latter small, almost rectangular, not protruded laterally. Basal margin 1.22 times broader than apical margin. Disk markedly convex, smooth. Anterior margin straight in middle, lateroapical angles slightly protruded, rounded. Posterior margin not beaded, slightly convex in middle, markedly incised towards outer quarters, not shifted anteriorly at basolateral angles. Median longitudinal impression distinct, not deepened near base, disappearing near apex; anterior transverse impression very fine, smooth; posterior transverse impression linear, deep on sides, shallower in middle, smooth; laterobasal foveae indistinct, smooth. Lateral groove moderately narrow throughout, very slightly widened towards base, smooth. Both lateral and laterobasal setae present, with the lateral seta located near apical third of pronotum. Proepisternum glabrous, smooth.

Pterothorax: Elytra markedly convex on disc, in dorsal view narrow ovate, length 2.58 mm, length/width = 1.50, widest near their mid-length, moderately wider than pronotum (width of elytra/width of pronotum = 1.30), glabrous beside normal setation. Humeral angles fully rounded, basal groove absent. Parascutellar stria moderately long, parascutellar seta present. Striae finely punctate apart from the apical fifth of elytra; first stria complete, outer striae disappearing near apex; three inner striae deeply impressed with intervals convex, fourth stria finer, fifth stria very fine, sixth and seventh striae absent, eighth stria finely impressed from level of the medial setae of the umbilical series and deeply impressed from level of the preapical group. Each elytron with two discal setae in third stria, and with preapical seta, located in the apical cross of the second and third striae, slightly closer to the suture than to the apical margin of elytra (Fig. 10). Recurrent stria rather short, extending towards the reduced fifth stria anteriorly, hardly bowed, with its front end situated at level of the preapical seta (Fig. 10). Setae of umbilicate humeral series close to the elytral margin, with four setae almost equidistant from each other; setae of the medial group of the umbilicate series far removed from the preapical group (Figs 6, 12). Metepisternum short, glabrous and smooth, with outer margin 1.3 times longer than anterior margin (Fig. 11).

Abdomen: Abdominal sternites V–VII each with one (male) pair of setae near apical margin; surfaces smooth, without hairs or micropunctures.

Legs: Moderately robust, all femora unmodified; protibiae straight and moderately dilated towards apex (structures on protibial surface are not visible using light microscopy due to milky coating, and could not be imaged using micro-CT). Basal two protarsomeres moderately dilated (Fig. 14).

Male genitalia (Figs 13, 15, Movie 1): Median lobe in apical half fully closed on dorsal side, in lateral view evenly bent throughout, with basal bulb of average size for Trechus (bulb is destroyed on its left side including left paramere missing); apical lamella moderately long, simple, conically tapering. Endophallus armed with two copulatory pieces, one is long rod-shaped and twisted, angularly curved and pointed apically, the other is less distinct, mainly due to its ambiguous distal portion, but with proximal portion semi-circular, very similar in shape and size to that of the first piece.

Figure 15. 

Trechus exhibitorius sp. n., holotype, volume rendering of the aedeagal median lobe with endophallus. a right lateral aspect b dorsal aspect c left lateral aspect d, same as c, but with contours of the two copulatory pieces indicated by coloured lines.

Figure 16. 

Trechus exhibitorius sp. n., reconstruction of the external shape in dorsal view.

Movie 1. 

Trechus exhibitorius sp. n., holotype, volume rendering of the aedeagal median lobe with endophallus. YouTube link: https://youtu.be/u7hzlyrxOTA

Species epithet is derived from the Latin term “exhibitorius” = exhibitor. This name was given under the impression that the new species presents itself in a rather vulgar way.

Some characters of the pronotal structure, namely the missing laterobasal foveae coupled with rectilinear lateral portions of the basal transverse impression seem to be most useful for correct interpretation of the systematic position of the species in question. These two character states are usually correlated, although in some extant groups, the sublinear basal transverse impression may combine with the moderately impressed laterobasal foveae, the latter being separated from the lateral groove by clearly convex portions of the basal slope of the pronotal disc (e.g. T. balkaricus Belousov, 1990 and T. fusculus Motschulsky, 1850 with numerous related Caucasian and Turkish species). The boundary between the basal foveae completely missing and barely detectable is rather subjective and is often difficult to be recognized even in extant species. However, the combination of these two characters in the amber species makes a rather reliable basis for further considerations. In this respect, T. exhibitorius sp. n. resembles members of the obtusiusculus species group from the Balcans, southern Alps and Carpathians (some Caucasian species, such as T. fischtensis Reitter, 1888, seem to be also closely related to this group), the montanellus and liopleurus species group sensu lato (Belousov, 1987), the former from the Alps, Beskids and Sudetes, the latter from the Caucasus and Turkey. We deliberately neglected the two other groups with a similar structure of the pronotum: the tingitanus and quadristriatus (incuding T. obtusus Ericson, 1837 and relied taxa) species groups since the tingitanus group differs drastically by having deeper and strongly punctured exterior elytral striae while the quadristriatus group has a clearly convex pronotal basal margin with lateral portions oblique.

In the following, we will discuss our findings with the three remaining species groups. Members of the obtusiusculus species group and related Caucasian taxa, all differ readily from T. exhibitorius sp. n. in the endophallus armature consisting of a simple, anisotopous, scapus-like plate and therefore do not seem to be directly related to the species in question. The situation becomes much more interesting with regard to the montanellus and liopleurus groups. Jeannel (1927) considered both groups to be of Asian origin. This assumption seems to be confirmed with the description of T. mordkovitschi Shilenkov, 1982 and especially T. shilenkovi Belousov & Kabak, 1992, both from Siberia which are very similar to members of these two groups. According to Jeannel (1927) the differences between the montanellus and liopleurus groups mainly concern the degree of development of the external elytral striae and therefore, the conjunction of the apical recurrent stria with the fifth elytral stria which is clear in members of the montanellus species group and indistinct, with the apical striole abruptly interrupted anteriorly and external stria much more reduced, in members of the liopleurus species group. Though this observation is correct for many species of the groups under consideration, it is not present in all species. The Caucasian liopleurus species group is very heterogeneous and, doubtless, deserves to be split in a number of more natural species subgroups. Within each of these subgroups, we can see similar evolutionary patterns resulting in appearance of large-sized wingless species with parallel-sided habitus, shallow external elytral striae, narrow and parallel internal interspaces and massive pronotum characterized by the basal transverse impression subrectilinear and sharply engraved as well as the basolateral foveae nearly missing. On the other hand, some members of the liopleurus group have an ovate habitus, similar to that of members of the montanellus group, external elytral striae well-impressed, with clear conjunction of stria 5 with the apical recurrent stria and the pronotum with distinct basolateral foveae and the basal transverse impression which is, though sharply engraved, clearly bent laterally. Among these species, T. gagrensis Jeannel, 1927 is especially of high interest, since it is, doubtless, most closely related to T. liopleurus Chaudoir, 1850. However, the pronotal structure similar to that of T. exhibitorius sp. n. is observed also in some species related to T. balkaricus within the maculicornis species group as defined by Jeannel (1960) and some species of the fusculus (= bradycelloides) group (Jeannel 1960; Pawłowski 1979), despite the fact that most of species assigned to these groups have the pronotum with distinct laterobasal foveae and the basal transverse impression rather shallow and clearly bent. To summarize, we can infer that the pronotal characters listed above might have evolved independently and might be associated with a similar mode of life; their phylogenetic importance should be cautiously considered.

Fortunately, the fossil in question is the first extinct Trechus species with the aedeagus and endophallus armature well preserved. As to the aedeagus shape, its apical portion, slightly attenuated downwards, is very similar to that of both known species of the montanellus species group, T. shilenkovi from the Altai mountains and members of the alpigradus subgroup within the liopleurus group, especially T. arnoldii Belousov, 1987 from the West Caucasus. The observed structure of the endophallus armature is challenging to interpret, especially bearing in mind that it was fixed half-way inflated. However, we can clearly see one piece which is long and twisted, angularly curved and pointed apically. A large, branch-like and twisted copulatory piece is rather common in the endophallus armature of many Trechus species groups. In addition to the T. sylvicola K. Daniel & J. Daniel, 1898 from the montanellus group listed above, some Turkish species of the fusculus group are worth to be mentioned, for example: T. michaeli Pawlowski 1978 and T. ziganensis Jeanne, 1976. Though both of these species have the pronotum with rather distinct laterobasal foveae and less sharp transverse basal impression, some related species have the pronotum structure quite similar to that of T. exhibitorius sp. n. The second piece of the endophallus armature of this species is less distinct, mainly due to its ambiguous distal portion. However, its proximal portion is clearly detectable, semi-circular, and very similar in shape and size to that of the first piece (Fig. 15d). This fact deserves careful consideration. Examination of the inflated endophallus in Trechus species shows that in some Trechus lineages, including those addressed by this paper, the most primitive structure of the endophallus armature consists of two branch-like similar pieces attached to the proximal end of the endophallus (becoming the distal end when inflated) (unpublished data I.B.). Their basal portions seem to be semi-circular in projection, but are in fact semi-spherical to ensure their firm position on the convex surface of the inflated endophallus (such a condition is typical of members of the maculicornis species group). Their evolution resembles a development of the right piece coupled with progressive reduction of the left one. In this case, the basal portion of the left piece becomes either much smaller or is incorporated (at least, topologically) in the basal portion of the right piece. This evolution of the endophallus armature seemed to take place independently in various Trechus lineages. If the above interpretation of the endophallus armature in T. exhibitorius sp. n. is correct, this species combines a rather primitive structure of the endophallus armature and aedeagus (including an average size of the aedeagus vs. hypertrophied in strongly evolved species) with some specialized external characters commonly found in mesophylous Trechus. It should likely be placed near the basal portion of the phylogenetic stem branching into the montanellus, liopleurus, fusculus and, probably, osmanilis and maculicornis species groups. However, monophyly of the above mentioned species groups needs to be tested using molecular methods.

Two additional Trechus fossils were described from the Eocene Baltic amber: T. balticus and T. eoanophthalmus (Schmidt and Faille 2015, Schmidt et al. 2016). Trechus exhibitorius sp. n. is easily distinguished from the blind T. eoanophthalmus by the fully developed eyes and the much slender body. T. balticus is distinctly smaller (3.6 mm versus 4.4 mm in T. exhibitorius sp. n.), has a stouter body with broader head and shorter elytra, larger and deeper impressed micro-meshes on disc of head and more markedly reduced external elytral striae.