MORPHOMETRY IN EARLY JUVENILE AMMONOIDS, ENABLED BY GRINDING TOMOGRAPHY: A KEY TO THE PATTERNS OBSERVED IN THEIR EXTINCTION AT THE K/PG BOUNDARY?

Ammonoids were bad active swimmers, but at the same time distributed widely, occasionally even globally. They were always suspected to disperse during their earliest ontogenetic stage. It would thus be an obvious conclusion that there are morphologies in this juvenile stage which are related to the geographic dispersal of a species.

Grinding tomography was applied to ca. 80 planispiral specimens from various levels between the lowest Cenomanian to the highest Maastrichtian, from different localities in different palaeobiogeographic realms and with differing preservation. After grinding, the datasets are combined to scaled virtual 3-dimensional models of the fossils. These allow for morphometry of internal characteristics at a resolution not reached before. Data can even be taken down to the ammonitella, i.e. the hatchling’s shell which is preserved in the inner whorls. These morphometric data can be plotted against the systematic taxa, palaeobiogeographic distribution and the geological timescale. This combination of morphometric, stratigraphic and palaeobiogeographic data indeed shows a relation between the morphology of the innermost whorls and the palaeobiogeographic distribution. The combination of these findings with high-resolution data from the K/Pg boundary allows for a new interpretation of extinction patterns observed in the Ammonoids.

This talk presents the results of a scientific project which was dedicated to these questions. Financial support by the German Science Foundation (DFG grant IF61/4-1) is gratefully acknowledged.