The Middle Miocene freshwater mollusk fauna of Lake Gacko (SE Bosnia and Herzegovina): taxonomic revision and paleoenvironmental analysis

Abstract The early Middle Miocene Lake Gacko was part of the Dinaride Lake System (DLS) and gave rise to a poorly known freshwater mollusk fauna. This was subject to malacological studies from the late 19th century onwards. Herein, we provide the first thorough taxonomic survey of Lake Gacko including revisions of several taxa. A total amount of 1,077 specimens was obtained from 17 samples, comprising at least 11 freshwater and 2 terrestrial gastropod species and 6 bivalve species. So far, none of the described taxa has been documented outside the Dinarides and the DLS, respectively. The fauna shows variable overlap with other paleolakes of the DLS, proofing once more the complex biogeographic patchwork of this system. The current paleoenvironmental reconstructions are tested and refined by application of a statistical analysis. This confirms the partition of the sedimentary history of Lake Gacko into three major depositional phases responding to astronomically forced climate changes. The low diverse mollusk assemblage in the initial phase, with abundant pulmonate and rissooid gastropods, signals a more arid climate with lowered lake level. During the second interval, the fauna becomes more diverse with common rissooid and melanopsid gastropods. It represents a more humid interval with enhanced precipitation and increased lake level, entailing the installation of a perennial lacustrine environment. The final phase is a return to arid conditions with the same elements as in the initial pulmonate‐rissooid assemblage. The sphaeriid Pisidium vukovici n. sp. is introduced as new species. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Remarks. The morphological variability of this species is enormous. Shell shape ranges from low valvatoid, as is the case for the (lost) holotype and the syntypes, to higher coiled, drop-shaped forms, including all kind of intermediates. Likewise, the expression of the opercular ridge and the thickening of the peristome vary greatly, partly depending on ontogenetic stage. Concerning shell shape, this species is similar to 'Lithoglyphus' tripaloi Brusina, 1884(Bania tripaloi after Neubauer et al. 2012. This form has less convex whorls and no stepped spire, lacks the apertural thickening, the opercular ridge and the basal emargination. Some specimens from units A and D, erroneously determined as Bania prototypica (Brusina, 1874) by Mandic et al. (2011: fig. 5.14), fully correspond to the present description of B. valvatoides. B. prototypica, as well as 'Amnicola' torbariana (Brusina, 1874) and 'Amnicola' stosiciana (Brusina, 1874), differ from B. valvatoides in the higher spire and the more distinctly stepped whorl outline with highly incised sutures. All forms share the characteristic protoconch sculpture and are therefore assigned to the genus Bania (Neubauer et al. 2012). The subspecies B. stosiciana crassa Brusina, 1897, originally described from Gacko, fully corresponds to the slender specimens studied here and is thus considered synonymous. Krstić et al. (2009) identified specimens from drill cores as "Pseudoamnicola (Staja) šoštarićiana Brusina, 1874". With regard to the denoted authority, this attribution obviously is a wrong spelling of Pseudamnicola [= Bania] stosiciana (Brusina, 1874). The specimens fully match with the present description. Detailed discussions of this genus and attended nomenclatorial problems as well as its phylogenetic relations are provided by Kadolsky (1993Kadolsky ( , 1998 and Neubauer et al. (2012). Occurrence. Gacko Basin (Vrbica, Gračanica). Sinj Basin (Ruduša in Sinj), Drniš Basin (Miočić) (Brusina 1884 Diagnosis. Small, slender, elongated ovoid shell. Protoconch conical and smooth. Whorls well rounded to stepped, slowly increasing in dimensions; separated by deep sutures. Last whorl large, but not inflated. Aperture narrow, egg-shaped, with posterior angulation. Columella fairly straight and everted over the slit-like umbilicus (compiled from Zilch 1959Zilch -1960Neubert 1998).

Figure 9C
Material. 3 spire fragments from sample 0907/Moll3. Description. Extremely slender lymnaeid. Most complete specimen comprising 4 whorls, exposing highly convex whorl outline producing stepped spire. Protoconch smooth, passing into teleoconch without distinct rim. Aperture unknown. Strong growth lines covering shell. Remarks. The distinctly stepped whorl outline and the slender shape suggest a classification as Galba. The slim habitus reminds of Omphiscola Rafinesque, 1819, which has convex but not stepped whorls. No comparable taxon is recorded from the DLS, but the insufficient material and preservation, do not allow establishing a new species.

Remarks. Gyraulus geminus
Orygoceras Brusina, 1882 Type species. Orygoceras dentaliforme Brusina, 1882, Miocene, Dalmatia/SE Croatia. Diagnosis. Shell small, dentaliform, asymmetrical and more or less curved. Protoconch planispiral bearing faint spiral striae. Teleoconch surface with faint growth lines, occasionally with ring-like sculpture or spiral lines. Aperture oblique and elliptical to semilunar (modified after Brusina 1882Brusina , 1902. Figure 10D 1882 Orygoceras dentaliforme Brusina, p. 42, pl. 11, figs 9-15. 2011 Orygoceras dentaliforme. Material. 4 fragments from samples 0804/039, 0804/039 coal and 0804/039b. Description. Shell uncoiled and dentaliform. Protoconch consisting of about half a whorl bearing faint striae. Teleoconch mainly smooth apart from fine growth lines. In some specimens distinct rings may occur in various ontogenetic stages. Shell elliptical to semilunar in cross-section. Aperture not preserved in any specimen. Remarks. The development of sculpture is assumed to range within intraspecific variability as no clear separation of distinct morphotype-groups can be performed. Discussion on the validity of the many species introduced by Brusina (1882Brusina ( , 1902 is provided by Neubauer et al. (2012). Planorbarius Duméril, 1806 Type species. Helix cornea Linnaeus, 1758, Recent, Europe; by subsequent designation (Froriep 1806). Diagnosis. Shell moderately to very large, thick-shelled, disciform. Shell on both sides immersed, more on apical side. Whorls high, evenly rounded, increasing regularly in diameter, covered with growth lines and spiral striae. Aperture round or elliptical, basal margin protruding (compiled from Zilch 1959Zilch -1960Brown 1994). Material. 5 spire fragments from samples 0804/039b, 0907/Moll4 and 0703/024; additional fragments < 1 mm found in 0804/039 coal. Description. Spire completely immersed with highly convex initial whorls and strongly incised sutures. Protoconch consisting of c. 1 whorl and measuring 550 µm. Initial cap making up c. 50 µm; covered with short and faint irregular threads. Termination indicated by onset of spiral rows of distinct circular pits (c. 5 µm). Between them fragile irregular ridges may be formed in axial direction. Onset of teleoconch indicated by formation of growth lines. Whorls exposing distinct shoulder to moderately curved flank. Strong irregularly shaped, spiral grooves covering whole teleoconch surface. They are confined to bundles of 4-5 grooves each, separated by slightly raised smooth bands (Fig. 11F). Where they cross more pronounced growth lines a weakly undulated pattern is formed. Aperture only marginally preserved; seemingly not conspicuously thickened or everted. Remarks. The identification of the fragments is based on the characteristic protoconch features and the spiral teleoconch sculpture, which are typical elements for Planorbarius. A determination on species level is not possible from the available material.

Orygoceras dentaliforme Brusina, 1882
Subfamily Ancylinae Rafinesque, 1815 Ferrissia Walker, 1903 Type species. Ancylus rivularis Say, 1817, Recent, Eastern North America. Diagnosis. Shell thin-walled, low ancyliform, with acute apex pointing to the right. Protoconch with radial striae and small pit in its center. Surface smooth or covered with radial striae (modified after Walker 1903). Holotype. Type specimen is lost; no neotype defined yet. Type locality. Gacko-Avtovac, Gacko Basin, SE Bosnia and Herzegovina; Middle Miocene. Material. 25 specimens and several spire fragments from samples 0804/039, 0804/039 coal, 0804/039b, 0907/Moll3, 0708/GA14 and Gacko 3. Description. Small and narrow ancylid shell with broadest diameter anterior. Largest specimen found attaining 6 mm in length, 3.5 mm in width and 2 mm in height. Apex round and blunt, slightly inclined to the right, measuring c. 400 µm. Initial part attaining about half of total protoconch diameter, smooth with small, circular and shallow pit in its center. It is surrounded by a collar-like array of thin radial threads, which are terminated distally by a distinct rim. Growth lines already starting during this interval and forming more or less strong concentrical rings. Inner surface covered with highly granular pattern in early ontogeny that gradually fades out towards shell margins. In some specimens radial threads may occur in late ontogeny, but only on the inner surface.
Remarks. The lack of radial ribs on the outer teleoconch surface separates this species from Ferrissia wittmanni (Schlickum, 1964)  Diagnosis. Small egg-shaped to almost cylindrical shells with 4-7 whorls and obtuse apex. Aperture 30-40% of total height, thickened and everted, usually bearing 1 parietal, 1-2 palatal and 1 columellar tooth. Parietal and columellar tooth continue as lamellae on the columella. Umbilicus slit-like (compiled from Zilch 1959Zilch -1960Strauch 1977). Figures 12A-B, D Material. 1 complete specimen and few fragments from samples 0804/039 and 0804/039 coal. Description. Small elliptical shell with up to 4 whorls. Protoconch fine-punctate, domeshaped, measuring 300 µm. Whorls roughly cylindrical in profile with maximum convexity in adapical half. This results in a slightly stepped outline. Last whorl attaining about two thirds of total shell height, forming moderately convex base. Narrow umbilicus developed. Aperture displaying typical angle between columellar and parietal region, widely convex basal margin and roughly straight palatal lip. Margin broad, thickened and everted and throughout covered with numerous small granules (Fig.  12D). Columellar and palatal lamellae forming small rounded triangular tubercles; parietal lamella quite strong. Distinct prosocline growth lines covering shell, occasionally forming regular riblets.

Vertigo sp. Figures 12C, E
Material. A single fragment bearing the aperture from sample 0804/039b. Description. Only last whorl with aperture preserved. Parietalis strong, accompanied by weaker, thin and slightly inclined angularis; columellaris broad but rather weak; very weak basalis formed below; palatalis inferioris inclined, displaying most prominent tooth; palatalis superioris thin and similarly inclined but weaker expressed. Narrow umbilicus developed. Remarks. The generic identification is based on the arrangement of the teeth and the indented palatal margin. The dentition with the strong parietalis and infrapalatalis is reminiscent of Vertigo callosa (Reuss, 1849) Milojević, p. 357, figs 3-4. 1978Congeria friči. -Kochansky-Devidé & Slišković, p. 60, 94, pl. 11, figs 1-7, textfig. 8. 2011 Lectotype. Single left valve embedded by interior side into sediment (NHMS MB1683/7), originally illustrated by Brusina (1904: pl . 3, fig. 7m) and designated as lectotype by Kochansky-Devidé & Slišković (1978: pl. 11, fig. 1). 12 paralectotypes are additionally available from the same locality. Type locality. Field at the hill-slope east of Šipovo, Bosnia and Herzegovina; Middle Miocene. Material. 1 specimen from the historical collection of the NHMW of Gacko and about 20 originally articulated but largely fragmented individuals from 4 rock samples collected in the Mytilopsis shell-bed unit of the open-cast mine Gračanica . Description. Shell fragile, thin-walled, less inflated, equivalve, inequilateral with pointed, prosogyr beak in anterior-most position. Long as high, usually about 40 mm in length, with maximal length of 52 mm in available material. Circular to sub-triangular in outline, with wing-like, upwards pointed dorsal margin and rounded posterior and ventral margins. Hinge plate straightened, meeting ventral margin at about 110°. Byssal notch broad and shallow concave. Posterior sinus occasionally present; up to three fine transversal threads can arise posteriorly striking in its direction. Anteriorly, sharp transversal ridge present within the first 15 to 20 mm of growth. Additionally to fine growth lines, broad, concentric rugae might be present. Exceptionally well preserved specimens show a nacreous, shiny interior surface with slightly thickened but smooth shell margin. Remarks. The species is closely related to M. aletici and M. drvarensis due to its size, circular, dorsally flabellate outline, fragile shell and fine transversal ridge at the proximal shell exterior surface. However, it clearly differs from M. aletici by the straightened anterodorsal margin missing the anterior beak-like prolongation of the hinge line (Neubauer et al. 2012). M. drvarensis is distinctly stronger inflated, has a longer transversal ridge, an acute umbonal angle and is not exceeding 40 mm in length. Based on this features we regard shells erroneously identified as M. drvarensis by Stojčić in Milojević (1966) Raić & Papeš (1977) has not been confirmed by Kochansky-Devidé & Slišković (1981), who found there only the similar M. aletici. Figures 14A-C 1892 Congeria Jadrovi Brusina, p. 195. 1897Congeria Jadrovi. -Brusina, p. 30, pl. 17, figs 12-14. 1902Congeria Jadrovi. -Brusina, pl. 21, figs 2-5. 1978Congeria jadrovi. -Kochansky-Devidé & Slišković, p. 37, 87, pl. 1, figs 26-39. 2011 Holotype. NHMZ 3150-796/1. Type locality. Trnovača, Sinj Basin, SE Croatia; Middle Miocene. Material. 8 shells from samples 0907/Moll4 and 0907/Moll4A. Description. Shell elongated triangular with weak and wide anterior byssal notch and rounded ventral end. Prodissoconch smooth, cap-like, regularly circular, measuring 250 µm. A distinct rim and beginning growth lines signal its termination. Broad and slightly curved keel developed, reaching from umbo to ventral margin and forming convex crest in dorsoventral view. Hinge plate small and curved bearing posterior apophysis. Thin interior ligamental band present, originating posteriorly from umbo. Remarks. The similarly shaped Mytilopsis neumayri (Andrusov, 1897) from the Middle to Late Miocene of the Pannonian Basin System is larger and has a lower keel; its anterior external surface field is strongly reduced. Mytilopsis sp. Figures 14D-F Material. 3 hinge fragments from samples 0804/039 coal (2 LV) and 0804/039b (1 RV). Description. Shell modioliform, inflated, with smooth exterior surface except for fine growth lines. Umbo prosogyr but not terminal. Prodissoconch smooth, circular, measuring around 275 µm. Anterior margin rounded; dorsal and ventral margins straightened, meeting anteriorly at rectangular to acute angle. Septum narrow, posteriorly elongated, bearing transversally oriented apophysis; anteriorly bends 45° downwards, grading there into a narrow ledge, oriented subparallel to anterior margin; anteriorly shallow, crescent depression present. Remarks. The presence of an anterior shell portion in the dreissenids of the DLS was reported by Kochansky-Devidé & Slišković (1978, 1981) from a few species only, including M. aletici and M. scaphula. Each of these species is characterized by the presence of a transversal dorsal ridge at the proximal exterior shell surface, which is, however, not developed in this species. With exception of the relatively larger and posteriorly shifted apophysis, the available fragments are reminiscent of specimens from the Lower Miocene Freshwater Molasse (Kirchberg-Formation, S Germany), attributed to M. amygdaloides (Dunker, 1848) by Nuttall (1990) and Kowalke & Reichenbacher (2005). However, these specimens differ notably from the type material of M. amygdaloides (Dunker 1848: pl. 21, figs 8-9) from the Freshwater Molasse near Ulm (S Germany). The holotype (single right valve) shows a terminal beak position and thus a consequently different shell outline. Therefore, the identification of the Kirchberg-Fm. specimens as M. amygdaloides seems questionable.

Occurrence. Gacko Basin (Gračanica
Family Sphaeriidae Deshayes, 1854Deshayes, (1820 Pisidium Pfeiffer, 1821 Type species. Tellina amnica Müller, 1774, Recent, Palearctic andNorth America. Diagnosis. Elongate, inequilateral shell. Umbo pointing posteriorly, often protruding, sometimes with terminating growth rim. Ligament immersed. Right valve with one cardinal tooth and two anterior and two posterior teeth. Left valve with two cardinal teeth and one anterior and one posterior tooth (modified after Pfeiffer 1821; Zeissler 1971). Brusina, 1884Figures 14J-L 1884Pisidium Bellardii Brusina, p. 48. 1897 Holotype. Single right valve illustrated in Brusina (1897)  Description. Elliptical, inequilateral, anteriorly elongated shell. Umbo narrow, elongated and suppressed; no pronounced beak. In lateral view, shell highly curved, nearly semilunar. Fine and regular growth lines covering surface. Hinge plate short and narrow with weak emarginations formed by lateral teeth. On right valve outer lateral teeth broad and low; inner ones pointy and more distinct. Cardinal tooth (C 3; dental nomenclature after Herrington 1962) forming sharp and simple crest with proximal thickening; it is inclined and slightly curved. Ligament pit short, narrow, lanceolate and well demarcated. On left valve lateral teeth both small but quite distinct, in lateral view forming pointy, triangular cusps. C 4 short and sharp, inclined and slightly angled, distally touching margin of hinge plate; in lateral view exposing M-like profile with two acute tips, one on each side. Lower cardinal tooth (C 2) straight and parallel to hinge plate; in lateral view forming rectangular, sharp crest; roughly same length as C 4. Remarks. This taxon has not been described so far properly. Clessin in Brusina (1884) only provided a comparison with the extant P. fossarinum Clessin in Westerlund, 1873(= P. casertanum Poli, 1791. Despite similar shell shape this form differs in its strongly angled C 2 and C 3. Occurrence. Gacko Basin (Vrbica, Gračanica). Sinj Basin (Ruduša in Sinj, Trnovača), Drniš Basin (Miočić) (Brusina 1884(Brusina , 1897. Type locality. Open-cast coal-mine "Gračanica" near Gacko, Minepit B. Type horizon. Unit A of the Section Gračanica in Mandic et al. (2011), footwall coal II lithostratigraphic unit of the Gacko Basin, Langhian, Middle Miocene (~15.83 Ma). Diagnosis. Shell subtrigonal. Hinge plate massive; prominent lateral teeth. Description. Subtriangular shell with regularly semicircular dorsal margin. Umbo low and broad, measuring 750 µm. In lateral view, shell roughly semilunar. Outer surface covered with faint, regular growth lines. Hinge plate prominent and broad. Ligament pit short and lanceolate. Cardinals subcentral, oblique, dipping anteriorly; C 2 broadened, blunt; C 3 prominent, crescent, convex-up, sharp, surrounded by two shallow cavities; C 4 sharp and straightened. Laterals prominent and long with bulbous, sharp cusps. On right valve anterior teeth massive, forming distinct emargination; posterior teeth somewhat weaker. In both cases inner teeth stronger than outer ones. Remarks. This taxon can be clearly distinguished from Pisidium bellardii Brusina, 1884 by its subtrigonal shell shape and very prominent hinge plate. P. hybonatum Brusina, 1897 is somewhat broader in outline and has a less prominent hinge plate. This species, originally described from Pliocene deposits of the southern Pannonian Basin, was additionally reported by Jovanović (1935) from the close-by Bihać Basin. Shells from the possibly coeval Austrian Aflenz Basin, determined as Pisidium cf. casertanum Poli, 1791by Harzhauser et al. (2012, are broader and expose a narrower, delicate hinge plate. Likewise, P. amnicum (Müller, 1774) (= P. priscum Hörnes, 1862;non Eichwald, 1830) from the Middle to Late Miocene of the Central and Eastern Paratethys and the Upper Freshwater Molasse is broader and has narrower and less distinct lateral teeth (cf. Schneider & Prieto 2011). The about coeval P. bakonyensis Kókay, 2006 from Hungary is quite similar in shape but exposes an elevated umbo and smaller lateral teeth. In shape and hinge characteristics it is similar to the Recent P. supinum Schmidt, 1851, but does not develop such a high umbo with the typical fold. Extant P. tenuilineatum Stelfox, 1918 has stronger posterior teeth and a transversally oriented C 3.

Pisidium vukovici
Superorder Palaeoheterodonta Newell, 1965 Order Unionida Stoliczka, 1871 Family Unionidae Rafinesque, 1820 Unio Philipsson, 1788 Type species. Mya pictorum Linnaeus, 1758, Recent, Europe. Diagnosis. Shell inequilateral, oval to elongated, rounded in front and pointed or biangulate behind, with a more or less developed posterior ridge, often becoming slightly arcuate when old. Beaks only moderately inflated, generally sculptured with coarse ridges, which run parallel with the growth lines, or are somewhat doubly looped, sometimes broken and showing fine radiating lines behind. Surface smooth, slightly concentrically ridged or pustulous. Hinge plate narrow; two pseudocardinals and two laterals in the left valve and one pseudocardinal and one lateral in the right, with rarely a vestige of a second lateral. Cavity of the beaks not deep or compressed (compiled from Simpson 1900).