We describe early Tremadocian (Kainella meridionalis Biozone) cephalopods from the Cordillera Oriental, Jujuy, NW Argentina. They consist of numerous small specimens collected at the Quebrada de Arenal, Trancas section, near the town of Tilcara, in the Alfarcito Member of the Santa Rosita Formation. All but three specimens were assigned to a new species of Ellesmeroceras (Family Ellesmeroceratidae), E. humahuacaensis sp. nov., based on its slightly endogastric curvature, the characteristics of the siphuncle and chambers dimensions. Micro CT scanning of one specimen aided in the description of the apex and facilitated the construction of a 3D model of the species. A single, similar specimen was assigned to Ellesmeroceras sp. pending the availability of additional material. Two specimens differ from the rest, being exogastric with a lower angle of expansion. They are tentatively assigned to Bassleroceras sp. This material indicates that Cambrian and early Tremadocian cephalopods are not as different as previously thought. “Diversification” and “extinction” events during the late Cambrian may be attributed to taxonomic “over-splitting” and taphonomic and/or sampling biases, respectively. These specimens are currently the oldest recorded in the Central Andean Basin and of West Gondwana, and probably represent the first migration of cephalopods into the region, when the water column was still poorly colonized. During the middle Tremadocian, subsequent immigrations and originations of several cephalopod orders accounted for a rise in diversity and expansion into new niches during this interval. Some of these taxa persisted into the middle Floian, at which time, a second increase in diversity is recorded. Ellesmeroceras humahuacaensis sp. nov. is interpreted as a sub-vertical nektobenthic organism.

Key words: Cephalopoda, Ellesmeroceratidae, Ellesmeroceras , stem cephalopods, Tremadocian, Ordovician, Santa Rosita Formation, Cordillera Oriental.

Marcela Cichowolski [mcicho@gl.fcen.uba.ar; ORCID: https://orcid.org/0000-0002-9079-6350] and Daniel A. Morón Alfonso [paleokarzis@gmail.com; ORCID: https://orcid.org/0000-0002-8865-8370], Instituto de Estudios Andinos “Don Pablo Groeber” (IDEAN), Universidad de Buenos Aires-CONICET, Intendente Güiraldes 2160, Ciudad Universitaria- Pabellón II C1428EGA, CABA, Argentina. N. Emilio Vaccari [evaccari@unc.edu.ar; ORCID: https://orcid.org/0000-0002-2965-1566] and Beatriz G. Waisfeld [bwaisfeld@unc.edu.ar; ORCID: https://orcid.org/0000-0001-9474-6855], Centro de Investigaciones de Ciencias de la Tierra (CICTERRA), Universidad Nacional de Córdoba-CONICET, Av. Vélez Sarsfield 1699, X5016GCB, Córdoba, Argentina. Alexander Pohle [alexander.pohle@rub.de; ORCID: https://orcid.org/0000-0001-6979-1048], Institute of Geology, Mineralogy, and Geophysics, Ruhr University Bochum, Universitätsstrasse 150, 44801 Bochum, Germany. Romain Vaucher [romain.vaucher88@gmail.com; ORCID: https://orcid.org/0000-0003-3051-4128], Department of Earth Sciences, University of Geneva, Rue des Maraîchers 13, 1205 Geneva, Switzerland; Institute of Earth Sciences (ISTE), University of Lausanne, Geopolis, CH-1015 Lausanne, Switzerland.