The Floian eothinoceratid radiation in northwestern Argentina (southern Central Andean Basin)

CICHOWOLSKI, M. & MARENGO, L.

Dijon, Francia

Simposio; 8th International Symposium Cephalopods – Present and Past; 2010

University of Burgundy & CNRS Dijon - France

The Central Andean Basin is the southern continuation of the extended Andean beltthat also comprises the Cordillera Oriental of Peru, as well as the Cordillera Oriental andCentral of Bolivia. The southern part of this basin is exceptionally exposed in the CordilleraOriental in northwestern Argentina. Siliciclastic successions of the Cordillera Oriental ofArgentina and Bolivia were deposited along the western Gondwanan margin of SouthAmerica.The oldest known cephalopods in the Central Andean Basin, and particularly in NWArgentina, belong to the family Eothinoceratidae. Their remains come from the MiddleTremadocian of the Sierra de Mojotoro (Cordillera Oriental), and were identified asSaloceras cf. sericeum Evans, due to similarities with the Avalonian species.The diversity and abundance of the Tremadocian record of cephalopods from theNW Argentina are very low, although during the Floian an important radiation existed,corresponding to the great diversification occurring globally for this group of mollusks. Thefamily Eothinoceratidae would have been importantly represented in this radiation at thewestern margin of Gondwana. Even though until recently its presence in this region passedalmost occult, now it is noticed that they are not only the oldest record of nautiloids in thebasin, but also various taxa exist in Floian strata. New records from NW Argentina areadded to the already described species from Bolivia (corresponding to the generaEothinoceras and Margaritoceras). Several specimens were collected from differentlocalities in the Cordillera Oriental. Margaritoceras is also present in Argentina, and newtaxa have to be described. The concentrations of eothinoceratids are very common in theAcoite Formation, along with specimens of the family Protocycloceratidae.Other Ordovician cephalopods of Argentina come from the Precordillera Basin,where the family Eothinoceratidae seems to be absent. This is important from apaleogeographic point of view, since the Precordillera is considered to have been part ofLaurentia up to Cambrian times, when it detached during the Ordovician crossing theIapetus Ocean until its collision against the South American margin of Gondwana. Duringthe Floian this microcontinent has probably been located at low paleolatitudes, at the timewhen the San Juan Limestone (main unit with cephalopods) was deposited.