Morphological fluctuations of ammonoid assemblages from the Muschelkalk (Middle Triassic) of the Germanic Basin—indicators of their ecology, extinctions, and immigrations

Abstract

For a stratophenetic analysis of Middle Triassic ammonoids from the German Muschelkalk (Anisian, Ladinian), whorl expansion rates, whorl width indexes, umbilical width indexes, maximal conch sizes, body chamber lengths, the orientations of the aperture, and a number of sculpture parameters of approximately 500 specimens were identified. 274 of these data sets, sorted according to their stratigraphic age, were evaluated in scatter plots as well as canonical discriminant function analyses. Several of the diagrams that were produced in these analyses reflect more or less steady changes in conch morphology through geological time, except for some intervals with abrupt and rather drastic transformations. These morphological discontinuities are synchronous with immigrations into the Germanic Basin of crinoid and brachiopod taxa. This discovery indicates disturbances in the endemic evolution of the ammonoids caused by such 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. It also demonstrates that the evolution of ‘Germanic’ ceratites happened mainly within the Germanic Basin but partially probably also within the Tethyan Sea. The ecology of the ceratites from this Basin is discussed. Accordingly, they were stenohaline, good backward swimmers with a good manoeuvrability, and able to achieve neutral buoyancy.