Bactritoid cephalopods from southwestern gondwana: new insights based on Lower Devonian records of Argentina

CICHOWOLSKI, MARCELA; RUSTÁN, JUAN JOSÉ

Congreso; Reunión Anual APA; 2015

Asociación Paleontológica Argentina

The bactritoids, considered the ancestors of the ammonoids, are characterized by orthoconic to cyrthoconic conchs of very simple morphology. The embryonic conch is small, globular to ovate, and usually adult specimens presented a typical slender conch with small and ventral siphuncle, and a suture line with a narrow v-shaped ventral lobe. The bactritoids are common elements in the Middle and Upper Devonian all along the world, with the exception of SW Gondwana, where they are scarce and poorly known. Although the excellent stratigraphic records known for the Lower Devonian of the Malvinokaffric region, there is only a report of Emsian bactritids from South Africa. Additionally, bactritids were known from South America only for an old report about a Pennsylvanian Bactrites from Perú. New Early Devonian collections from the Talacasto Formation at San Juan and La Rioja Provinces (west-central Argentina) allowed us to recognize the Family Bactritidae Hyatt, specially the genus Bactrites Sandberger, along several orthoceratoids. The oldest records belong to the lower pelitic interval of this unit, classically assigned to the late Lochkovian, and continue to the upper sandy interval, considered as Emsian. Our findings enhance the diversity and abundance of cephalopods in the Malvinokaffric Realm, where this group of mollusks was thought to be scarce. Additionally, they could constitute a key record regarding the existing discussion about the origin of bactritoids in the Early Devonian, as they would correspond to the oldest records of the group, considering that previously the first unquestionable records came from the Emsian of Morocco.