Lower Devonian cephalopods from Precordillera, Western Argentina

CICHOWOLSKI, MARCELA; RUSTÁN, JUAN JOSÉ

Mendoza

Congreso; 4th International Palaeontological Congress; 2014

International Paleontological Association

We report for the first time a cephalopod assemblage from the Lower Devonian of Argentina. Fossils come from the Talacasto Formation, which is extensively exposed in the Central Argentine Precordillera in San Juan Province. This unit represents a muddy shelf depositional system developed during a highstand, embracing the early Lochkovian-late Emsian. Sixty cephalopod specimens were recovered from several levels throughout the succession and preliminary analyzed. Many of them are very poorly preserved, and then were classified as indeterminate forms of Orthocerida and Oncocerida. In addition, the recognition of Bactritida represents the first mention in Argentina of this phylogenetically important group. Based on a few specimens studied by means of medial polished sections, Pseudorthocerida was also recognized in spite of having no preserved apices. The type of endosiphuncular deposits, furthermore, suggests they can be assigned to Pseudorthoceratidae. These new data are relevant in paleobiogeographic discussions related to the Malvinokaffric Realm, a marine Early-Middle Devonian southern circumpolar region (including basins from southern South America, South Africa and Antarctica) characterized by a high endemism level and absence of some typical Paleozoic groups. Cephalopods were interpreted to be absent from the Malvinokaffric Lower Devonian, despite the fact that ?Michelinoceras? was reported from Bolivia, and scarce specimens assigned to the taxonomic wastebasket ?Orthoceras? and ?Spyroceras? were cited from the Brazilian Paraná Basin. Our new evidences constitute the best-known record of Early Devonian Malvinokaffric cephalopods, demonstrating that they were undoubtedly present in the southwestern marine margin of Gondwana during Early Devonian times, being relatively diverse and abundant.