TOWARDS A PALEOECOLOGICAL AND PALEOGEOGRAPHICAL MODEL FOR AMMONOIDS DURING DECCAN VOLCANISM

When for all sites preserving ammonoids from within the ultimate 0.5 myr of the Cretaceous, this is the time of extensive Deccan volcanism and just prior to the Chicxulub impact and their final extinction, paleodepth and sampling intensity are added to the generic occurrences, a distinctive twofold clustering becomes apparent between those sites where scaphitids and baculitids are present and those where these Ancyloceratina are absent. The first cluster can be further divided into four clusters, three of them based on the occurring scaphitid genus, plus a fourth from which only Baculitidae are listed. The first three all locate in the northern hemisphere, and because northern American sites only preserve Discoscaphites, Eurasian sites only Hoploscaphites and north African sites only Indoscaphites, a clear paleogeographic clustering can be observed. On top, in all sites of the first cluster, scaphitids and baculitids take 60 to 100% of collected specimens into account. In the second cluster, which is spread across both hemispheres, the faunal composition is completely different. Here, except for the globally distributed ‘paperclip shaped’ Diplomoceras, Ancyloceratina are totally absent, and Lytoceratina and Ammonitina are the most abundant elements.

Paleodepth clearly is the most important character responsible for this clustering, next to paleogeography. Our results also show that each cluster has a different (maximal) generic diversity, with the highest diversity being found at the deeper water sites of the first cluster, this is where the two ‘realms’ meet. Our results have major implications for those studying ammonoid biostratigraphy, diversity and extinction. Next to paleodepth and paleogeography, also collecting intensity is of major importance when a site’s generic diversity is assessed. The fourth most diverse site is that of the type Maastrichtian, definitely a shallow water site, but one with the longest record of intensive collecting (>150 years).