Rediscovery of a classic Middle Triassic fossil site of the Balaton Highland (Transdanubian Range, Hungary): cephalopods, brachiopods and vertebrate remains from the Akol Hill at Barnag

Abstract

Recent collecting of the classic Middle Anisian fossil site of the Akol Hill at Barnag (Nagyvázsony Plateau, Balaton Highland) yielded a plenty of new palaeontological data.The Middle Anisian platform carbonate succession (Tagyon Formation) is overlain by 1-2 metres thick reddish, nodular, crinoidal limestone (Vászoly Formation) that contains a very rich fossil assemblage with ammonoids, nautiloids and brachiopods. Some vertebrate fossils were also found. Besides fish teeth, remains of marine reptiles are also present in the fauna. Four vertebrae, a piece of a rib, as well as a tooth crown fragment have also been found.Based on the geological buildup and the new biostratigraphic data the active stage of the Barnag Platform was much shorter than that of the other two Middle Anisian platforms of the Balaton Highland (Tagyon Platform, Kádárta Platform). The ammonoid fauna of the lowermost thin limestone bed above the Tagyon Formation contains elements reworked from the Balatonicus Zone. The overlying some beds contain a very rich lower Illyrian ammonoid fauna which is more diverse than the equivalent faunas of the Tagyon Platform and the Kádárta Platform. The ammonites of the Balatonicus Zone in the basal part of the Vászoly Formation prove that the drowning of the Barnag Platform occured even during the Pelsonian, earlier than in the case of the Tagyon and the Kádárta Platforms where the oldest basin sediments are mid-Illyrian (Camunum Subzone of the Trinodosus Zone) above the drowning surface.The Akol Hill locality also provided the first known occurrence of ichthyosaurs like Cymbospondylus, predators that probably dwelled mostly open marine environments but also might have been well adapted for hunting in shallow environments like submarine highs.