The ammonite biohorizons of the Lambertiknollen Bed of SW Germany as a faunistic response to climate change in the latest Callovian (Middle Jurassic)

The moderately rich ammonite assemblage of the distractum Biohorizon is described from the Lambertiknollen Bed, a set of phosphatic concretions layers in the upper part of the Callovian Ornatenton Formation of SW Germany. The distractum Biohorizon is dominated by the highly variable Subboreal species Quenstedtoceras lamberti (Sowerby, 1819), the index ammonite of the Middle Jurassic Lamberti Zone. In addition, there are several species of Submediterranean oppeliids besides Submediterranean aspidoceratids, peltoceratids, perisphinctids, and Subboreal kosmoceratids. Tropical faunal elements such as lytoceratids and phylloceratids did not reach the area during this time interval. After a short hiatus indicated by an intercalated belemnite breccia, another layer with phosphatic concretions follows. It contains mostly reworked and fragmented ammonites of the newly introduced aff. zudacharicum Biohorizon, which is the youngest biohorizon of the Lamberti Zone in the Swabian Jurassic. Despite the presence of Submediterranean taxa, the ammonitic assemblages have a rather Subboreal character. This Subboreal character of the ammonite assemblages as well as the great abundance of phosphatic concretions, clay sedimentation and glauconite formation points to rather cold seawater temperatures at least at the seafloor probably caused by upwelling. A comparison of the two succeeding ammonite assemblages points to an increased cooling trend towards the Callovian-Oxfordian boundary.