Palaeoecology, fluctuations in conch morphology, palaeobiogeography, extinctions and immigrations of Triassic cephalopods from the Germanic Basin

Among the few cephalopod genera from the Muschelkalk, specimens of the ammonoid genera Cerat-ites and Paraceratites outnumber representatives of other genera with respect to diversity, morpho-logical disparity, as well as abundance by far. During the late Anisian and Ladinian, the species of Ceratites and Paraceratites underwent continuous morphological transformations that are reflected in a fairly large number of species and subspecies as well as successive biozones, respectively.

For a stratophenetic analysis of Middle Triassic ammonoids from the German Muschelkalk, data sets of numerous specimens of Ceratites and Paraceratites, were evaluated in scattergraphs as well as canonical discriminant function analyses. Several of the diagrams, which were produced in these analyses, reflect more or less steady changes in conch morphology through geological time, except for some intervals with abrupt and drastic transformations. These morphological discontinuities are synchronous with immigrations of crinoid and brachiopod taxa. This indicates disturbances in the endemic evolution of the ammonoids caused by extinctions and immigrations. At a small scale, this case study demonstrates that a rising sea-level may have boosted the faunal exchange between an open marine and a restricted epicontinental basin, causing a minor regional increase in biodiversity.