Miocene siliceous microfossils from the open cast coal mine Gra\u010danica (Bugojno paleolake, Bosnia and Herzegovina) and their significance: a preliminary report

Abstract

The Langhian (middle Miocene) marls of lacustrine succession cropping out in the open cast coal mine Gračanica (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 alga Botryococcus sp. were often observed in one sample. The most common spicules are spinose oxeas that strongly resemble the extant species Ochridaspongia rotunda Arndt, 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 genus Ephydatia Lamouroux, 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 being Staurosirella leptostauron (Ehrenberg) D.M.Williams and Round 1988, Epithemia sp., Ellerbeckia sp. and Encyonema sp. Much less common are Staurosirella pinnata (Ehrenberg) D.M.Williams and Round 1988 and two undetermined species of Fragilaria as well as Eunotia. Eleven chrysophyte cyst morphotypes were uncovered. Alkaline and oligotrophic conditions in the paleolake are suggested by the presence of the representative of the genus Ochridaspongia Arndt, 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 of Botryococcus sp. suggests rather oligotrophic condition, indicating that conditions and nutrient levels were variable. The first fossil occurrence of the sponge genus Ochridaspongia indicates that this genus originated not in the Lake Ohrid, but much earlier during the early middle Miocene in the Dinarides Lake System.