New species and stratigraphic data on Lower Bajocian (Middle Jurassic) lytoceratids (Ammonoidea) from Lókút, Bakony Mts, Hungary

摘要

From exceptionally rich and mostly well-preserved Lower Bajocian ammonite assemblages, eight lytoceratid species are described and discussed in detail. They belong to four genera (Lytoceras, Megalytoceras, Alocolytoceras and Nannolytoceras) and include some stratigraphically important forms. Thus the material yielded the hitherto known youngest (Lower Bajocian Otoites sauzei Zone) representative of genus Alocolytoceras (A. isztimeri n. sp.). On the basis of here-identified forms, the range of Lytoceras subfrancisci could be extended up into the Otoites sauzei Zone. A new species (Nannolytoceras gibbosum n. sp.) from the Witchellia laeviuscula/Otoites sauzei zonal boundary interval suggests that the origin of Nannolytoceras could have been earlier than previously recorded. The faunal compositions of the assemblages reflect a typical Tethyan character, with lytoceratids representing only the third most numerous ammonite group behind Phylloceratina and Ammonitina.