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A conspicuous renewal in the ammonite faunas of the Mediterranean Tethys occurred in the latest Hauterivian. This faunal turnover took place following a stepwise pattern. The first step occurred at the boundary between the Pseudothurmannia ohmi Subzone and the Pseudothurmannia mortilleti Subzone, coinciding with the base of the so-called Faraoni Level. This is a Corgrich interval that has been recognised in several basins of the Mediterranean Tethys and seems to be the expression of a shortlived oxygen-deficient event. It correlates with a well-documented second-order peak transgression. The oxygen depletion preferentially affected the deep nektic ammonites, which would explain the extinctions within this group around the Faraoni Level. On the contrary, an increase in the trophic resources in the photic zone favoured the diversification of epipelagic ammonites. Concurrently, an abrupt change took place at this level in the nannoconid assemblage composition. A minor second event, located at the base of the Pseudothurmannia picteti Subzone, was marked by the replacement of a few planktic ammonite species by closely related forms, and the structure of the ammonite assemblage was not substantially altered. The coincidence of this event with a further restructuring of the calcareous nannofossil assemblage suggests that some changes had to occur in the planktic ecosystem during the sea-level highstand subsequent to the peak transgression. The third and last stage of the renewal process started in the upper part of the P. picteti Subzone, coinciding with a drastic sea-level fall. It is characterised by the extinction of many of the species that had appeared in the two previous events, resulting in an extensive modification of the assemblage structure. The regression would probably cause a drop in the primary productivity and, consequently, an improvement in the oxygenation level of the sea bottom. This would explain the extinction of several planktic ammonite species and the appearance of new nektic and nektobenthic lineages. D 2005 Elsevier B.V. All rights reserved. |
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