Abrasion surfaces on internal moulds of ammonites as palaeobathymetric indicators

Abstract

Some abrasion surfaces displayed by internal moulds of ammonites, such as annular abrasion furrows formed on the venter surface and ellipsoidal abrasion facets preferentially developed on the last third part of the preserved whorl, are diagnostic criteria of taphonomic reworking. These taphonomic criteria indicate that the internal moulds have been exhumed and moved before their final burial. We explain the origin of these abrasion surfaces by the action of directional (non oscillatory) currents in very shallow water, most probably in intertidal environments. The distribution of ammonite moulds displaying such preservational features is correlated with the changes in the facies and the trend of the sedimentary sequences. Taphonomic analysis of Jurassic ammonites of the Iberian Range underlines the importance and usefulness of taphonomic gradients in sequence stratigraphy and palaeogeographic reconstructions.