Miocene siliceous microfossils from the open cast coal mine Gracanica (Bugojno paleolake, Bosnia and Herzegovina) and their significance: a preliminary report

摘要

The Langhian (middle Miocene) marls of lacustrine succession cropping out in the open cast coal mine Gracanica (Bugojno basin, central Bosnia and Herzegovina) yielded a rich siliceous microfauna. The most common are sponge spicules (megascleres), less common are diatoms and chrysophyte cysts. Cell wall remains of a green algaBotryococcussp. were often observed in one sample. The most common spicules are spinose oxeas that strongly resemble the extant speciesOchridaspongia rotundaArndt, 1937 (family Malawispongiidae Manconi and Pronzato, 2002) that is endemic to Lake Ohrid, and we believe that the fossil material belongs to the same genus. Other, much less common spicules, birotules and thick smooth strongyles were attributed to the genusEphydatiaLamouroux, 1816 (family Spongillidae Gray, 1867) and most probably Potamolepidae Brien, 1967 respectively. All together, they belong to 5-6 different taxa. The diatoms are represented by 11 species, the most common beingStaurosirella leptostauron(Ehrenberg) D.M.Williams and Round 1988,Epithemiasp.,Ellerbeckiasp. andEncyonemasp. Much less common areStaurosirella pinnata(Ehrenberg) D.M.Williams and Round 1988 and two undetermined species ofFragilariaas well asEunotia. Eleven chrysophyte cyst morphotypes were uncovered. Alkaline and oligotrophic conditions in the paleolake are suggested by the presence of the representative of the genusOchridaspongiaArndt, 1937 that thrives in such environments. The most common diatom species further suggest that the water of this paleolake was shallow, high in mineral content, alkaline, with a high pH and moderately to highly productive. Common occurrence ofBotryococcussp. suggests rather oligotrophic condition, indicating that conditions and nutrient levels were variable. The first fossil occurrence of the sponge genusOchridaspongiaindicates that this genus originated not in the Lake Ohrid, but much earlier during the early middle Miocene in the Dinarides Lake System.