The topmost Hauterivian aptychi collection from the Pseudothurmannia Beds of the Mraznica Formation consists of three subspecies of Lamellaptychus angulocostatus (Peters) (L. a. angulocostatus, L. a. angulicostatus, L. a. radiatus), accompanied by rare L. filicostatus Stefanov. These forms represent the stratigraphically youngest lamellaptychi association at all. The decline of the lamellaptychi-bearing ammonites is interpreted as an effect of contemporaneous changes in both paleoceanographic (sea current system) and paleobiogeographic (”Urgonian” carbonate platform development) patterns, as well as in the composition of marine microplankton associations, traceable in the area studied.