Time contrasting palaeobiogeographies among Hauterivian–Lower Aptian rudist bivalves from the Mediterranean Tethys, their climatic control and palaeoecological implications

Abstract

From the Mediterranean Tethys, the Hauterivian−Barremian rudist genera Lovetchenia, Agriopleura, Pachytraga and Offneria simplex, show contrasting palaeobiogeographies through time. Lovetchenia is a western European form in the Hauterivian whereas it is an Eastern European and Apulian form in the late Barremian and early Aptian. Agriopleura is also a western European form in the late Hauterivian and Barremian p.p., whereas in the early Aptian this genus is only found on the Southern Tethyan margin. Pachytraga documented from the Hauterivian of both the European and Southern Tethyan margins, disappeared during the Barremian and re-occupied Western Europe and some parts of Africa during the early Aptian. Offneria simplex, a Barremian form from the Caribbean Province, is recorded in the uppermost Lower Barremian of SE France. The regional disappearance of these taxa is not linked to platform or rudist community demise, whereas community changes reflect the resulting modifications in biodiversity. There is a good correspondence between the regional disappearance of western European taxa, cooling events documented by oxygen isotope studies, and biological interchanges between the Boreal and Tethys realms. The regional disappearance of Lovetchenia and possibly Pachytraga are coeval with the Pseudothurmannia ohmi cooling episode whereas that of Agriopleura is coeval with the Ancyloceras vandenheckii cool episode. By contrast the intrusion of Offneria simplex in Western Europe coincides with the Coronites darsi warm peak. Thermal shifts between cool and warm episodes fluctuate from 3 to 5 °C, and the thermal threshold assumed to control the demise of western European taxa is in the range of 18 to 20 °C, a value similar to those controlling the geographic distribution of modern coral reefs, or the boundary between tropical and subtropical seas. The southward displacement of the rudist biomes also accounts for the migration of climatic belts and a decreasing of 3 to 5 °C in palaeotemperatures. Spatial contractions of Lovetchenia and Agriopleura biomes, and the O. simplex intrusion, suggest that these rudists were more sensitive than others to the thermal regime. This observation shows that rather than considering rudists in general as “tropical”, some of them may have been both tropical and subtropical and others tropical. For Pachytraga other factors than temperature, possibly oceanographic changes, may have been responsible for the overall Mediterranean disappearance of this genus in the Barremian.