BACTRITOID CEPHALOPODS FROM SOUTHWESTERN GONDWANA: NEW INSIGHTS BASED ON LOWER DEVONIAN RECORDS OF ARGENTINA

CICHOWOLSKI, M.; RUSTÁN, JUAN JOSÉ

Mar del Plata

Congreso; Reunión de Comunicaciones de la 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 conchis small, globular to ovate, and the more common adult animal presented atypical slender conch with small and ventral siphuncle, and a suture line witha small v-shaped ventral lobe. The bactritoids are common elements in the Middle and Upper Devonian of the world, except in SW Gondwana, where they are scarce and very little known. Although the excellent stratigraphic records that exist for the Malvinokaffric region, we just know the report of Emsian bactritids from South Africa. Otherwise, until present, bactritids were unknown from South America, with the only mention of a Pennsylvanian Bactrites from Perú. New Early Devonian collections from the Talacasto Formation at San Juan and La Rioja Provinces allowed us to recognize with certainty the Family Bactritidae Hyatt, along several orthoceratoids, specially the genus Bactrites Sandberger. The lowest records belong to the lower pelitic interval of theunit, classically assigned to the late Lochkovian, and continue to the upper sandy interval, considered as Emsian. Besides of the enrichment of the cephalopod records of the Malvinokaffric Realm, our new evidence would include the oldest forms of the groups, considering that until moment the first unquestionable records came from the Emsian of Morocco. The preliminary Argentine evidences are particularly interesting due to its possible role regarding discussions about the origin of bactritoids from orthoceratoids inthe Early Devonian.