Early orthoceratoid cephalopods from the Argentine Precordillera (Lower-Middle Ordovician)

Abstract

The Early and Middle Ordovician Orthocerida and Lituitida of Precordilleran Argentina are described, and their systematics and paleogeographic significance are revised. These cephalopods show a strong affinity to coeval faunas of North China, suggesting a location of the Precordillera at middle latitudes in the Southern Hemisphere east of the North China block and relatively close to the Gondwanan margin during the early Middle Ordovician. The descriptive terminology of characters of the septal necks, the position and shape of the siphuncule, and the shape of the connecting ring is improved. The distribution of these characters support an emendation of the Baltoceratidae, Sactorthoceratidae, and Proteoceratidae.Brauliocerasn. gen. (Sactorthoceratidae) andPalorthocerasn. gen. (Orthoceratidae) are erected. The new speciesBraulioceras sanjuanense, Eosomichelinoceras baldisii, Gangshanoceras villicumense, and Rhynchorthoceras minorare proposed.Palorthocerasn. gen. from the Lower OrdovicianOepikodus evaeZone represents the earliest known orthocerid.