Middle Palaezoic cephalopods from southwestern Gondwana and their palaeobiogeographic meaning

CICHOWOLSKI, MARCELA; RUSTÁN, JUAN JOSÉ; URIZ, NORBERTO

Fez

Congreso; 10th INTERNATIONAL SYMPOSIUM ?CEPHALOPODS ? PRESENT AND PAST?; 2018

Some groups of non-ammonoid cephalopods were considered exclusive of warm waters from low paleolatitudinal regions, typically including the Orders Ascocerida,, Discosorida and the Family Lamellorthoceratidae. However, recent findings from South America? have demonstrated that a reassessment of their geographic distribution is needed. New data mainly come from the Hirnantian (?)?early Llandovery from the Paraná Basin in Paraguay (the Eusebio Ayala and Vargas Peña formations), where ascocerid specimens were found. Palaeogeographic reconstructions indicate that fossil localities, characterized by a scarce and little diverse fauna, were located at 60°S by the latest Ordovician. On the other hand, Early Devonian rocks of the Argentine Precordillera revealed several specimens of the genus Arthrophyllum (Lamellorthoceratidae). Also in these rocks, we identified a relatively diverse and abundant cephalopod fauna composed by bactritids, pseudorthocerids, oncocerids and orthocerids. Thus, this new information contrasts with previous models of the Malvinokaffric Realm, an austral circumpolar major paleobiogeographic unit for the Devonian, allegued as virtually devoided of cephalopod records. Based on these considerations, we think that supposedly restricted distribution of some cephalopods during Silurian and Devonian might be a bias artifact due to the uneven availability of information from northern and southern world hemispheres. Future work will strengthen these preliminary conclusions.