Boreal ammonites from the Brno Carbonate Platform (Czechia): High-resolution biostratigraphy of the Middle–Upper Jurassic boundary

Abstract

This study provides a taxonomic and biostratigraphic evaluation of Boreal ammonites from the erosional relics of the Brno Carbonate Platform (Czech Republic) located on the northern shelf of the Tethys Ocean during the Jurassic. Several sections were studied in the Brno-Hády and Olomučany areas. Ammonite taxonomy and biostratigraphy of these typical localities of the Brno Carbonate Platform was presented more than 90 years ago, and it has not been updated since then. In our study, we present a new collection of mainly Boreal cardioceratid ammonites with short stratigraphic range covering an interval between the uppermost Callovian Lamberti Zone to the lower Oxfordian Cordatum Zone. The immigration of Boreal ammonites to the studied area corresponds to the maximum of the ‘Boreal Spread’ event, which was the greatest southward expansion in the evolutionary history of the ammonite family Cardioceratidae and a major bio-event at the Middle–Late Jurassic (Callovian–Oxfordian) transition. The high-resolution stratigraphy of Callovian-Oxfordian transitional succession is provided for the first time for the Brno Carbonate Platform. The occurrences of the latest Callovian–early Oxfordian Boreal ammonites in the Brno Carbonate Platform indicate a direct marine connection with the Boreal Province during the Lamberti–Cordatum zones range. We also provide new data on the palaeobiodiversity and palaeobiogeography of Boreal ammonites and some remarks on the palaeogeography of the northern margin of the Tethys Ocean. The most probable migration route for Boreal ammonites was the Moravian Gate, connecting the Brno Carbonate Platform and the Polish Basin.