BIOGEOGRAPHY OF SANTONIAN MAASTRICHTIAN ANTARCTIC AMMONOIDS: PALEOCLIMATIC, PALEOENVIRONMENTAL AND PALEOGEOGRAPHICAL CONTROLS

E. OLIVERO

Mendoza

Congreso; 4th International Palaeontological Congress; 2014

IPA- CONICET Mendoza

A distinctive biogeographic pattern of Late Cretaceous ammonoids from the James Ross Basin (Antarctica) is the progressive Campanian/Maastrichtian dominance and endemism of kossmaticeratid ammonites. This pattern has been explained in terms of progressive geographical isolation of Antarctica during the breakup of Gondwana. However, new geological, paleontological and paleoclimatic data from the James Ross Basin strongly suggests that, in addition, lowering sea-water temperatures and expansion of the shelf during the Santonian-Maastrichtian were the dominant controls in the distribution of the Antarctic mollusk faunas. The biogeography of the Antarctic Late Cretaceous ammonoids is characterized by a Santonian-early Campanian cosmopolitan/Indopacific fauna, followed by a late Campanian-early Maastrichtian radiation of endemic kossmaticeratids, and then by a late Maastrichtian restriction of this group. Accompanying trends are the last occurrence in the Campanian of several mollusk groups that are well represented in the Maastrichtian elsewhere in the world, such as inoceramid and most trigoniid bivalves, and the nostoceratid, scaphitid, and baculitid ammonoids. The progressive geographical isolation of Antarctica alone does not explain satisfactorily the early extinction of relatively warmer taxa and the general absence in Antarctica of Turonian-Coniacian kossmaticeratids. The Campanian/ Maastrichtian dominance of the Kossmaticeratidae, concomitant with an Austral temperature decline in the sea water, supports the idea that kossmaticeratids were stenothermal ammonites, which flourished in Antarctica when the water masses had their preferred temperature and were displaced when a certain threshold temperature was reached in the late Maastrichtian. By the late Campanian, the last Antarctic inoceramids have a distinctive shell structure that probably reflects thermal stress. In Tierra del Fuego, deep-marine inoceramids disappear by the early Maastrichtian, concomitant with a marked change from anoxic-dysoxic to well-oxygenated bottom conditions. The local extinctions of relatively warmer taxa closely match temperature cooling trends in the southern oceans. The oxygenation event at the inoceramid extinction level in Tierra del Fuego probably reflects cooling and enhanced bottom ventilation, promoted by circulation of deep Antarctic waters. In the James Ross Basin, Turonian-Coniacian sedimentation took place mostly in deep-marine settings. The development of a shelf area started in the Santonian and stretched during the Campanian- Maastrichtian when the shelf extended for about 150 km into the Weddell Sea. Turonian-Coniacian kossmaticeratids are recorded in shelf deposits from Indopacific areas around Antarctica. They are particularly well-known from India, Madagascar and South Africa; thus, their general absence in the James Ross Basin may reflect the lack of suitable shelf habitats in Antarctica at that time.