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Introduction to the special issue “The drowning swamp of Gračanica (Bosnia-Herzegovina)—a diversity hotspot from the middle Miocene in the Bugojno Basin”

The middle Miocene was a key period in Earth’s history as climate changed from one of the warmest phases of the Cenozoic Era, the Miocene Climatic Optimum (MCO; ~ 16.8–14.7 Ma, Holbourn et al. 2014), to colder conditions, which resulted in the establishment of permanent ice sheets on Antarctica. This climate change had a profound impact on terrestrial ecosystems worldwide.

The fossils and sedimentary succession documented in the Gračanica coal mine in central Bosnia and Herzegovina provide an insight into the terrestrial and limnic fauna and flora and the palaeoenvironment at this site during the early middle Miocene, somewhere between 14.8 and 13.8 Ma, just around the fading MCO.

The Gračanica site is situated about 10 km SSE from the city of Bugojno (WGS84 coordinates: 43.997662° N, 17.518516° E) in central Bosnia and Herzegovina in the so-called Bugojno Basin (Fig. 1). This basin is one in a series of intra-montane Dinarides basins, which are filled with Neogene lacustrine deposits (Mandic et al. 2016b). These basins are the geological witnesses of the former Dinarides Lake System (DLS), which was a widespread and long-lived cluster of lakes that tectonically developed on the Dinarides-Anatolian Island. During the early and middle Miocene, the Dinarides-Anatolian Island was not only a major biogeographic barrier between the Paratethys and the proto-Mediterranean Sea but also a part of an important migration corridor—the Gomphotherium land bridge—especially for terrestrial mammals on their way from Africa and/or Asia Minor to Europe (and reversely).

Fig. 1
figure 1

a Geographic position of the Bugojno basin and the locality Gračanica in Bosnia and Herzegovina; b palaeogeographic map with Dinarides-Anatolian Island; c geological section of the outcropping deposits at the Gračanica coal mine with palaeoenvironmental interpretation

The investigation of the Dinarides Lake System dates back to the pioneering works of Neumayr (1869, 1880, 1883) and Brusina (1874, 1884, 1897, 1902), who recognised it as a unique evolutionary hotspot within the continental biota of southeastern Europe and described an immense number of previously unknown mollusc species. The mollusc fauna of the Bugojno Basin, and in particular of the Gračanica coal pit, was mentioned first by Katzer (1903, 1918), whereas its taxonomic content was enumerated first in the course of a geological exploration by Muftić and Behlilović (1966). The numerous mammal remains found in the coal series of the Gračanica mine were attributed to five proboscidean and suoid species by Malez and Slišković (1964, 1965, 1976).

Since the launch of the Austrian FWF scientific project “Mollusk Evolution of the Neogene Dinaride Lake System” in 2006, the Dinarides became the focus of numerous investigations mostly conducted by PhD students at the Universities of Utrecht, Zagreb and Graz, supervised by W. Krijgsman, L. Matenco, D. Pavelić, H. Hrvatović, M. Harzhauser and O. Mandic, among others. These studies provided an independent and integrative stratigraphic framework, as well as the palaeoenvironmental, depositional and tectonic insight to many of the larger Dinarides basins. Up to recent times, the following Miocene basins were investigated in detail (from SW to NE)—Pag (Jimenez-Moreno et al. 2009), Sinj (Jimenez-Moreno et al. 2008; Mandic et al. 2009; de Leeuw et al. 2010; Neubauer et al. 2011; Šegvić et al. 2014), Livno-Tomislavgrad (de Leeuw et al. 2011), Gacko (Mandic et al. 2011, Neubauer et al. 2013) and Sarajevo (Andrić et al. 2017; Sant et al. 2018). Furthermore, numerous investigations on the general aspects of the Dinarides late orogeny and palaeogeographic evolution (Mandic et al. 2012; de Leeuw et al. 2012; van Unen et al. 2019a, b) and the taxonomy, evolution and environmental significance of endemic Miocene limnic molluscs (Harzhauser and Mandic 2008, 2010; Harzhauser et al. 2012; Neubauer et al. 2013, 2014, 2015, 2016) were conducted. Despite such an intensive survey of the latest Dinarides evolution, the Bugojno Basin did not receive much attention, up to the present, due to its unclear stratigraphic position. A detailed survey of its mammal record, initiated to solve the latter dating problem, is now documented through this special issue.

This special issue presents the palaeontological results of the investigations of the fauna and flora found in the locality of Gračanica. Additional chapters dealing with the geology and depositional environment (Mandic et al. in prep.) and an integrative bio- and magnetostratigraphic dating of the Gračanica section (Mandic et al. in prep.) as well as a mammalian faunal analysis (NN) are foreseen to be published soon in a subsequent issue of this journal.

The site Gračanica is a still active coal pit. The exploitation of the lignites started in 1938 as underground mining, but from 1977 on changed into an open-cast mining. However, traces of the former underground mining occasionally come to light in the quarry, when historic mining tunnels (Fig. 2d) are cut by the ongoing excavations.

Fig. 2
figure 2

Impressions of the Gračanica coal mine and the lower, lignite-rich half (unit 1) of the Gračanica section. a Panoramic photo of the mine; b road sign board in Gračanica indicating the mine; c exemplary detail of the alternating sequence of dark lignites and light limestones; d collapsing historic mining tunnel; e large proboscidean long bone in a lignite layer, showing the mellow preservation of bone tissue, with Charlene Gaillard (then JM) for scale; f panoramic photo showing how the vertical and lateral facies change. All photos by U. Göhlich in 2017

The outcropping deposits in the Gračanica open cast mine is about 40 m in thickness (Fig. 1c); the lower half of this section (~ 20 m height; Fig. 1c, unit 1) contains rich intercalations of dark lignites, which are exploited by the mining company “Gračanica” (Fig. 2b).

The site has long been known for its fossils of large mammals and molluscs, but in the course of this project and investigation, non-mammalian vertebrates, arthropods, siliceous microfossils and flora also contributed importantly to our understanding of this site.

As far as we know, all mammalian fossils as well as a fossil of a crocodilian (Vasilyan 2020, this issue) found in Gračanica come from the lignite-bearing lower half of the section (Fig. 1c, unit 1); it is interpreted as deriving from a swampy environment. The upper half of the section (~ 20 m height; Fig. 1c, units 2 + 3) is made up of light-coloured, laminated marls, clays and silt-, sand-, and limestones (without any lignites) and represents an open lake environment with changing lake levels (Fig. 4e, f). These lake deposits yielded fossils of fishes, reptiles, amphibians, but also insects, a crab, algae cysts, diatoms and sponge spicules. The fossil molluscs, ostracods, a macroflorule and a rich pollen record were found throughout the complete geological succession (Fig. 1c). The entire section reflects a transgressive lacustrine succession and shows vertical and lateral facies changes (Mandic et al. 2016a); therefore, the ongoing mining continuously changes the aspect of the section (Figs. 2f, 4a, 4d).

Geological and palaeontological fieldwork at the Gračanica deposits was initiated more than a decade ago by O. Mandic (NHMW) in the frame of the Austrian FWF Project (P18519-B17) on the mollusc evolution in the Dinarides Lake System. The team including O. Mandic, M. Mandic, de Leeuw and W. Krijgsman (both University of Utrecht) logged two geological sections in the Gračanica quarry in 2008 and 2009 and conducted palaeomagnetic sampling and geophysical (magnetic susceptibility, natural gamma ray) logging throughout the sections. Furthermore, O. Mandic collected fossil molluscs during field seasons and visits in 2007–2009, 2016 and 2017; he took sediment samples throughout the entire section in 2009 for pollen and ostracod analyses. In 2016, the excursion of the RCMNS workshop “Lake-Basin-Evolution” in Zagreb/Croatia visited Gračanica (Mandic et al. 2016a, b). At that occasion, A. Pisera took samples for analyses of siliceous microfossils (diatoms, cysts of golden alga, green microalga, sponge spicules). Already in 2007, O. Mandic and a team of small mammal specialists (W. Wessels, H. de Brujn (both University Utrecht), G. Daxner-Höck (NHMW), Z. Marković (NHM Belgrade)) had visited the Gračanica mine to conduct screen washings for small mammals, but the yield was very scanty. In 2017, a small joint digging crew from the Jurassica Museum in Porrentruy (Switzerland) (D. Vasilyan, R. Renaud, Ch. Gaillard) and the Museum of Natural History in Vienna (Austria) (O. Mandic, U. Göhlich) (Fig. 3) got permission to conduct a short digging campaign in the mine. In the frame of this campaign, mostly fossils from the upper half of the section were collected and further screen washing samples of about 70 kg were taken from five different marly layers intercalated between the lignites; unfortunately, they did not produce any further small mammal fossils, but only small molluscs and gyrogonites. The almost missing small mammal record as well as the missing or very bad preservation of bones (Fig. 2e) in general is probably a result of humid acid from the lignites, which affects and dissolves boney tissue, more so on tiny objects. In 2019, researchers from the Jurassica Museum in Porrentruy returned to the coal mine for a very short visit and collected a few more fossils.

Fig. 3
figure 3

Impressions of the field and digging campaign in 2017. a Digging crew in 2017 (from left to right): Davit Vasilyan (JM), Ursula Göhlich (NHMW), Oleg Mandic (NHMW), Renaud Roch (JM) and Charlene Gaillard (then JM); b U. Göhlich splitting fine laminated marls and siltstones in the upper half of the Gračanica section; c D. Vasilyan, R. Roch and Ch. Gaillard documenting and packing fossils; d R. Roch splitting slabs in the upper half of the Gračanica section; e D. Vasilyan climbing on the limestone bed in the upper half of the lignite succession; f O. Mandic measuring magnetic susceptibility (with a SM-20 by Gf Instruments) in the upper part of the lignite-rich lower half of the section; g and h U. Göhlich taking screen washing samples of marls interlayered in the middle part of the lignite rich; i mine geologist Hamdija Puljarga and surveyor Branko Grabovac guiding Oleg Mandic in the coal pit; j and k hay piles (j) and cabbage fields (k) near Gračanica, typical for this region. All photos (except f and i) by U. Göhlich in 2017; photos f and i by Medina Mandic in 2009

Fossils, especially of fossil mammals, are known from this site since the beginning of the exploitation. Most of the large mammal fossils were and are found by the miners as chance findings during the mining activities. Therefore, their exact finding positions within the section are unknown. However, the preservation of the mammalian fossils and the often still attached residue of sediment indicate that they all come from the lignite-rich lower half of the section. This is confirmed by the miners and by occasional findings of fossils during the research field seasons.

The age of the Gračanica deposits is early middle Miocene (late Langhian), but its exact dating is still ambiguous. Initially, a scanty small mammal fauna indicated correlation with the early to middle Miocene and the Mammal Neogene zones MN4–MN6 (Wessels et al. 2020, this issue). The large mammal fauna correlates best with the Neogene Mammal units MN5 and/or early MN6 (Aiglstorfer and Mayda 2020, this issue; Bastl et al. 2020, this issue; Becker and Tissier 2020, this issue; Becker et al. 2020, this issue; Coombs and Göhlich 2020, this issue; Göhlich 2020, this issue; Stefen 2020, this issue). Biostratigraphic and biochronological datings based on molluscs pointed to an absolute age of the deposits slightly younger than 15.0 Ma (Mandic et al. 2020, this issue) correlating MN5 (Hilgen et al. 2012). The youngest of all biostratigraphic datings for the Gračanica fauna is postulated to be between 14.0 and 13.7 Ma by van der Made (2020, this issue), based on the suid fauna, correlating MN6 (Hilgen et al. 2012). Correlation of the pollen palaeoclimate proxy with orbital parameters and insolation points to an age of the Gračanica section between 14.80 and 14.55 Ma (Jiménez-Moreno and Mandic 2020, this issue), correlating late MN5. Ongoing magnetostratigraphic analyses (Mandic et al. in prep) also suggest a date somewhere between 14.8 and 13.8 Ma, correlating with late MN5 or early MN6, respectively. The entire Gračanica section (ca. 40 m in height) is estimated to have been deposited during a time span of 200 kyr (Jiménez-Moreno and Mandic 2020, this issue).

A series of 19 articles in this special issue introduces taxonomic studies on all faunal and floral groups found in the fossil record so far; their analyses provide a picture of the then biocenoses and depositional palaeoenvironment and habitat at the Gračanica site during a short period in the early middle Miocene.

In the frame of the present investigations, 29 mammalian taxa, a minimum of 6 ectothermic vertebrate taxa, 19 mollusc taxa, 15 arthropod taxa and more than 63 plant taxa are identified. Furthermore, a variety of siliceous microfossils (e.g. sponge spicules, diatoms and chrysophyte cysts) of at least 17 different species are documented. Most of the described vertebrate taxa are only sparsely represented by a few specimens. The Gračanica fauna is species-rich (specious), but poor in individuals. A complete faunal list is given below.

Gračanica is type locality for some newly established taxa: a new rhino species (Plesiaceratherium balkanicum n. sp.) (Becker and Tissier 2020, this issue) and four new species of molluscs (Bania goehlichae n. sp., Prososthenia krijgsmani n. sp., Paradrobacia hrvatovici n. sp. and Illyricocongeria forcakovici n. sp.) (Mandic et al. 2020, this issue) are described from this site.

The preservation of the mammalian fossils from Gračanica argues for their autochthonous origin. Some transport can be assumed only in the littoral lacustrine facies as indicated by the sedimentary structures (Fig. 4d) and the mollusc composition.

Fig. 4
figure 4

a Overview of the sedimentary succession in the Gračanica mine, showing a dark, lignite-bearing lower interval, overlain by the light limnic limestone and marl in the upper part of the outcrop; b tree log in the lowermost part of the section; c laminated dark, coaly layer with compacted, horizontally oriented mollusc shell fragments (white); d foresets in the upper, transitional part of the coal-bearing succession showing progradation of the deltaic facies to the left; e synsedimentary slumping structure in the deeper water limnic facies of the Gračanica section (Fig. 1c, unit 2); f laminated marl in the upper half of the Gračanica section representing the deeper water limnic facies. All photos by U. Göhlich in 2017

Large mammal fossils presented here are housed in the collections of the Natural History Museum Vienna (NHMW, Austria), in the Jurassica Museum, Porrentruy (JM, Switzerland) and minor also in the Bavarian State Collection of Paleontology and Geology in Munich (SNSM-BSPG, Germany). Small mammals are stored in the Natural History Museum in Belgrade (PMB). Furthermore, all molluscs and the gyrogonites are kept in the NHMW; the ectothermic vertebrates, the crab, the insects, and the macroflora are housed in the JM; the siliceous microfossils (sponges, diatoms and chrysophyte cysts) are stored in the Institute of Paleobiology, Polish Academy of Sciences, Warszawa; ostracods are reposited in the Croatian Geological Survey, Zagreb, Croatia; the pollen and algae are kept in the Department of Stratigraphy and Paleontology, University of Granada, Spain.

This special issue comprises the following contributions:

Wessels et al. (2020) found small mammal remains in three outcrops of the open pit lignite mine Gračanica near Bugojno (Bosnia and Herzegovina). Despite the low number of specimens, two Democricetodon species, which differ in size, are identified. The co-occurrence of two Democricetodon species is common in many late early and middle Miocene European small-mammal assemblages. The presence of both D. mutilus and D. gracilis allows a correlation to MN4–MN6 (Palaeobiodiversity and Palaeoenvironments 100(2) https://doi.org/10.1007/s12549-018-0366-8).

Stefen (2020) describes five castorid teeth from the early middle Miocene locality Gračanica in Bosnia and Herzegovina, probably originating from lower lacustrine lignite-rich deposits. These teeth from the upper jaw are similar in preservation, wear stage and size and may represent one single individual. Due to their size and morphology (mainly state of hypsodonty and closing hypostria in P4), they are referred to Stenefiber depereti. Since a semiaquatic lifestyle is assumed for Steneofiber, it is most probable that the teeth are of autochthonous origin (Palaeobiodiversity and Palaeoenvironments 100(2) https://doi.org/10.1007/s12549-018-0365-9).

Bastl et al. (2020) investigate the fossil carnivores from Gračanica and detect the caniform families Amphicyonidae (Amphicyon giganteus), Ursidae (Hemicyon goeriachensis, Ursavus brevirhinus) and Mustelidae (indet.) and the feliform family Percrocutidae (Percrocuta miocenica). This fauna comprises omnivore, mesocarnivore and hypercarnivore taxa. Noteworthy is the lack of any Felidae, but this might be an effect of the general scantiness of carnivoran fossils in the site. The authors point to similarities of the Gračanica fauna with the carnivore faunas of the localities Prebreza (Serbia), Mordoğan, Çandır, Paşalar and Inönü (all in Turkey), which are of comparable middle Miocene age. The occurrence of H. goeriachensis in Gračanica is the southernmost, while that of P. miocenica is the northernmost record in Europe; for A. giganteus, it is the first evidence outside of central Europe (except for Namibia). The coeval occurrence of A. giganteus and H. goeriachensis in the deposits of Gračanica suggests a possible assignment to MN6, even if MN5 cannot be ruled out (Palaeobiodiversity and Palaeoenvironments 100(2) https://doi.org/10.1007/s12549-018-0353-0).

Van der Made (2020) documents three taxa of suoids in Gračanica and discusses extensively their taxonomic problematics: the palaeochoerid Choeromorus lemuroides (previously Taucanamo sansaniense) and the suids Bunolistriodon latidens (Listriodontinae) and Conohyus simorrensis (Tetra-conodontinae). These species are identified to belong to three anagenetic lineages, known from Western Europe to Anatolia, which are described in detail. Based on these lineages, biostratigraphic correlations are proposed for an area extending from Western Europe to Anatolia. Based on the suoids, van der Made concludes the age of the fauna to be between 14.0 and 13.7 Ma, correlating MN6, which is the youngest of all proposed biostratigraphic datings for the Gračanica fauna (Palaeobiodiversity and Palaeoenvironments 100(2) https://doi.org/10.1007/s12549-020-00420-9).

Aiglstorfer and Mayda (2020) present the ruminant fauna from Gračanica. It comprises the tragulid Dorcatherium vindebonense, an undeterminable palaeomerycid (Palaeo-merycidae gen. et sp. indet.), the bovids ?Tethytragus sp. and Eotragus ?clavatus and the giraffe Giraffokeryx sp.; for the latter, it is the westernmost occurrence to date. They interpret the ruminant faunal assemblage to show Asian as well as European affinities and point to the role of the Balkan Peninsula as a migration corridor between the two continents (Palaeobiodiversity and Palaeoenvironments 100(2) https://doi.org/10.1007/s12549-018-0354-z).

Coombs and Göhlich (2020) identify the very few dental chalicothere remains to represent most probably Anisodon grande, a chalicotheriine. Although no postcranials are found in Gračanica, chalicotheriines are known to be characterised by their gorilla-like body plan with a strong disproportion of long forelimbs and shorter hindlimbs. Given the age designation of the fauna as MN5 or earliest MN6, the Gračanica chalicothere is among one of the earliest known occurrences of this subfamily and of the genus Anisodon in Europe (Palaeobiodiversity and Palaeoenvironments 100(2) https://doi.org/10.1007/s12549-018-0357-9).

Becker et al. (2020) describe two species of anchitheres from the lignites of Gračanica: the medium-sized Anchitherium hippoides and the larger-sized A. ezquerrae. Both are recorded for the first time in southeastern Europe. Although the genus Anchitherium is broadly cosmopolitan, the record of A. hippoides was, so far, mostly restricted to Western and Central Europe, whereas A. ezquerrae was only known from the Iberian Peninsula (Palaeobiodiversity and Palaeoenvironments 100(2) https://doi.org/10.1007/s12549-020-00419-2).

Becker and Tissier (2020) confirm the co-occurrence of four rhino species in Gračanica, one of which is a new species: Plesiaceratherium balkanicum n. sp., Lartetotherium sansaniense and Hispanotherium matritense are three “small-sized”, brachyodont rhinoceroses (body mass less than 1 ton), whereas the heavy-weighted Brachypotherium brachypus reached more than 2 tons. This rhinocerotid assemblage is interpreted to inhabit a mesic woodland with diverse habitats from swampy forest to drier and more open environment (Palaeobiodiversity and Palaeoenvironments 100(2) https://doi.org/10.1007/s12549-018-0352-1).

Göhlich (2020) presents the early middle Miocene proboscidean fauna—deinotheres and gomphotheres—from Gračanica. Probably four taxa are documented in the site, exclusively represented by fossil teeth: the deinothere Prodeinotherium bavaricum and the gomphotheres Gomphotherium angustidens, cf. Gomphotherium subtapiroideum and cf. Protanancus sp. The tentative systematic identification of some gomphothere taxa is due to the scantiness and fragmentary condition of the studied material and by the fact that several of the teeth represent milk teeth, which are more difficult to determine. The occurrence of Protanacus is noteworthy, because this taxon is extremely rarely documented in southeastern Europe so far; it is generally known from Africa and Asia (Palaeobiodiversity and Palaeoenvironments 100(2) https://doi.org/10.1007/s12549-020-00436-1).

Vasilyan (2020) studied the fauna of ectothermic vertebrates and provide preliminary results on fish, amphibian and reptile fauna from the Gračanica locality, Bugojno palaeolake. The aquatic and semiaquatic fauna comprise killifishes (Cyprinodontiformes indet.), a barb (Barbini indet. (aff. Barbus)), a crocodile newt (Chelotriton sp.), a painted frog (Latonia sp.) and a crocodile (Alligatoroidea indet., ?Diplocynodon). The crocodile remains are confined to the lower, lignite-rich part of the section corresponding to the swamp deposit (unit 1). The fishes and amphibians come from the upper lacustrine unit 2. A Barbus-relative (Barbini indet. (aff. Barbus)) possibly represents one of the oldest fossil records of the group found in Europe. The crocodile newt (Chelotriton sp.) represents the first record of this group from the area. For the first time, fossil tadpoles of the genus Latonia are described, which moreover, are the first record of tadpole for both the family Discoglossidae and clade of discoglossoid frogs. The fossils, coming from the lacustrine interval, are referred to the deeper water facies. Taphonomic observations of the fossil remains suggest a relatively short phase of decay of the animal bodies and their (probably quick) sedimentation in the deeper parts of the lake, with poor oxygen content. The rich killifish material (ca. 90 skeletons), order Cyprinodontiformes, will be studied in more detail and published separately in the near future (Palaeobiodiversity and Palaeoenvironments 100(2) https://doi.org/10.1007/s12549-019-00381-8).

Xafis et al. (2020) investigate the palaeodietary preferences of large mammals present in Gračanica. A multiproxy approach by employing dental microwear and dental mesowear analysis provides new perspectives on the terrestrial palaeoecology of the Dinarides Lake System. The analysis of the herbivores reveals the presence of browsing, “dirty browsing” and mixed-feeding taxa, with seasonal fruit, or even grass intake; pure grazers are completely absent. Additionally, the analysis of the carnivores suggests the presence of hyaena- and cheetah-like hypercarnivores, as well as generalists. The palaeodietary traits of the fossil mammals suggest a closed canopy-like environment, which is supported by the fossil plant assemblage. Palaeopalynological data confirms the omnipresence of fleshy fruit-bearing plants and herbaceous taxa, as well as grasses, which justifies the seasonal fruit browsing, the common “dirty browsing” and the occasional grazing behaviour visualised for some of the fossil mammals from Gračanica (Palaeobiodiversity and Palaeoenvironments 100(2) https://doi.org/ 10.1007/s12549-020-00435-2).

Butzmann et al. (2020) present a species-poor florule from the early middle Miocene lacustrine marls from the Gračanica coal mine. The florule comprises angiosperm leaves (Lauraceae, Myricaceae, “Rhamnuswarthae), Glyptostrobus europaeus twigs, a frond fragment of Pronephrium stiriacum and a seed assignable to the Vitaceae. The composition of the florule is suggestive of a swampy lowland environment; climatic conditions were humid subtropical (Cfa climate). Lignite seams containing abundant fossil wood (including large logs) characterise the deposits underlying the marls; the interburden between the lignite has yielded abundant plant chaff, including shoot fragments of Equisetum, and the charophyte gyrogonites Lychnothamnus duplicicarinatus and Chara molassica notate (Palaeobiodiversity and Palaeoenvironments 100(2) https://doi.org/10.1007/s12549-018-0356-x).

Jimémez-Moreno and Mandic (2020) provide a palynological analysis throughout the sedimentary sequence in the Gračanica open cast mine, which detects significant changes in the environment and lake sedimentation during the late phase of the Miocene climatic optimum (MCO). Statistical analysis and sedimentary rates suggest that eccentricity- and precession-dominated orbital-scale variability is recorded in the studied core. The warmest conditions are registered at the base of the studied section that could be correlated with an eccentricity maximum at the end of the MCO. From then on, a cooling trend is recorded until the top of the sedimentary sequence, with coldest maxima, tentatively correlated with a minimum in eccentricity and insolation. Furthermore, observed changes in the vegetation and corresponding variations of the lake level are interpreted as smaller-scale cyclical climatic events (i.e. warm-dry vs. cold-humid) that possibly are related to precession cyclicity. The palynological study of the Gračanica section suggests that sedimentation in this Bugojno palaeolake basin lasted for about 200 kyr and was strongly conditioned by climate at the beginning of the middle Miocene climate transition. Correlation of the pollen palaeoclimate proxy with orbital parameters and insolation points to an age of the Gračanica section between 14.80 and 14.55 Ma (Palaeobiodiversity and Palaeoenvironments 100(2) https://doi.org/10.1007/s12549-020-00416-5).

Pisera et al. (2020) analyse the siliceous microfauna of the Bugojno palaeolake, consisting of predominantly sponge spicules (megascleres) and subordinately of diatoms and chrysophyte cysts. The sponge spicules represent 5–6 different taxa; the most common spicules are those of a fossil species of the extant genus Ochridaspongia (Malawispongiidae), whereas those referred to the genus Ephydatia (Spongillidae) and to the Potamolepidae are less common. The presence of Ochridaspongia is not only the first fossil occurrence of this sponge genus, but also suggests alkaline and oligotrophic conditions in the palaeolake. The diatoms are represented by 11 species (e.g. Staurosirella leptostauron, S. pinnata, Epithemia sp., Ellerbeckia sp. and Encyonema sp., Fragilaria sp. and Eunotia sp.), the most common of which indicate a shallowness of the palaeolake, with alkaline water high in mineral content and high pH and moderately to highly productive. Furthermore, 11 chrysophyte cyst morphotypes are identified. In one sample, the green alga Botryococcus sp. is common and suggests rather oligotrophic condition, indicating that conditions and nutrient levels were variable (Palaeobiodiversity and Palaeoenvironments 100(2) https://doi.org/10.1007/s12549-019-00378-3).

Mandic et al. (2020) investigated changes in the mollusc composition along the 40-m-long Gračanica section. The authors present a very first taxonomic documentation of a mollusc fauna from the Bugojno Basin, comprising 17 gastropod and 2 bivalve species, including limnic as well as terrestrial dwellers. New to science became dreissenid bivalve Illyricocongeria forcakovici nov. sp., and the hydrobiid gastropods Prososthenia krijgsmani nov. sp. and Bania goehlichae nov. sp. The palaeoecological analysis based on quantified samples showed distinct shifts in mollusc composition and abundance. Hence, the terrestrial snails were found restricted to the lower, coal-bearing interval of the section. Furthermore, the shallow limnic gastropod representatives like melanopsids, neritids and hydrobiids were abundant in deltaic and littoral settings, as indicated by the accompanied sedimentary features. Finally, the dreissenid bivalves were frequent in littoral and sublittoral environments. The Lake Bugojno fauna shows only a low to moderate level of similarity to other Miocene records in the Dinarides, probably reflecting its slightly younger stratigraphic position (Palaeobiodiversity and Palaeoenvironments 100(2) https://doi.org/10.1007/s12549-020-00423-6).

Harzhauser et al. (2020) introduced a new helicid species Paradrobacia hrvatovici n. sp. restricted in the Gračanica section to the coal-bearing depositional interval. This terrestrial pulmonate gastropod genus was up to now known by only three species that lived in the middle Miocene of Austria, the late Miocene of Greece and the Pliocene of Germany. Consistent with the new record from Gračanica, all latter species were found in lignites or lignite-bearing marls, which is why the genus is interpreted as an indicator of moist woodland environments (Palaeobiodiversity and Palaeoenvironments 100(2) https://doi.org/10.1007/s12549-018-0344-1).

Hajek-Tadesse (2020) investigated ostracod contents in sieving residues of 13 sediment bulk-samples regularly distributed along the 40-m-thick succession of the coal-mine Gračanica. The coal-bearing lower part of the succession allowed detection of 11 species. In contrast, the upper, marl-dominated part of the section did not allow determinations at the species level, because of a high-degree of fragmentation of ostracod shells. Furthermore, the topmost sample from a sandy layer was barren of fossils. In the samples of the lowermost third of the section, the pioneering species, such as Fabaeformiscandona cf. pokornyi and Vestalenula cylindrica, show increased abundances and indicate a very shallow, temporary environment, with low water energy, partially of palustrine type, with possible seasonally fluctuating water salinities, within a warm subtropical climate. The transitional interval from the coal- to the marl-dominated part of the section shows dominant Cypridopsis cf. biplanata indicating a well-oxygenated body of shallow, richly vegetated, open water. Finally, the upper part of the section is composed of different but not closely detectable candonid genera indicating a deeper and stable open lake environment (Palaeobiodiversity and Palaeoenvironments 100(2) https://doi.org/10.1007/s12549-019-00403-5).

Hyžný (2020) presents a single specimen of a freshwater crab (decapod) from the limnic sediments of the former Lake Bugojno, found in the late Langhian site of Gračanica. The freshwater brachyuran is identified as Potamon sp. The Gračanica specimen is supposed to be the oldest Potamon occurrence from southern Europe. The articulated female individual with preserved pereiopods suggests no postmortem transport and indicates the autochthonous deposition in the riparian zone of the lake. Peculiar preservation of cuticular disks confirms that these structures have preservation potential. If found isolated, these disks may confirm the presence of decapods in studied assemblages and can be considered an additional tool for identification of otherwise rare freshwater crab fossils (Palaeobiodiversity and Palaeoenvironments 100(2) https://doi.org/10.1007/s12549-019-00374-7).

Wedmann and Skartveit (2020) describe two fossil March flies (Insecta, Diptera, Bibionidae) from the open lake deposits of the early middle Miocene Gračanica mine near Bugojno. These two fossils represent the first insects documented from the Dinarides Lake System. They are identified as Plecia sp. and Bibio sp. The specimens seem not to be conspecific with any known fossil bibionids, but the incomplete preservation does not allow descriptions of new species. The habitus and colouration of the Bibio specimen are very similar to those of the recent species Bibio hortulanus. The presence of the genus Plecia is considered to indicate a warm climate, while Bibio points more to a temperate climate (Palaeobiodiversity and Palaeoenvironments 100(2) https://doi.org/10.1007/s12549-018-00369-w).

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Acknowledgements

We thank all of the authors and coauthors for their contributions to this special issue. Our thanks go to the authorities of the Gračanica coal mine (ZD Rudnik uglja "GRAČANICA" d.o.o.- Gornji Vakuf-Uskoplje, JP Elektroprivreda BiH d.d. - Sarajevo) for permission to work in the pit. We are particularly grateful to Dževad Forčaković, Hamdija Puljarga and Branko Grabovac from the mine’s exploration department for guiding us in the field and for making our work in the coal pit as pleasant as possible. We thank Hazim Hrvatović (Federal Geological Survey Sarajevo) for his continued support of our studies in Bosnia and Herzegovina. Susanna Gartner (Vienna), volunteer at the NHMW, is thanked for picking out microfossils (e.g. gyrogonites, small gastropods) from the screen-washing residues. Finally, our special thanks go to the managing editor Sinje Weber (SMF, Frankfurt) for all her invested work, her professional assistance, and her endless patience. Thank you, Sinje!

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Göhlich, U.B., Mandic, O. Introduction to the special issue “The drowning swamp of Gračanica (Bosnia-Herzegovina)—a diversity hotspot from the middle Miocene in the Bugojno Basin”. Palaeobio Palaeoenv 100, 281–293 (2020). https://doi.org/10.1007/s12549-020-00437-0

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