A new Middle Devonian ammonoid assemblage from Argentina and its implications for the recognition of the Taghanic Crisis interval in SW Gondwana

Abstract During the Early to Middle Devonian, southwestern Gondwana was characterized by a highly endemic marine fauna defining the Malvinoxhosan (= Malvinokaffric) Realm, a major austral paleobiogeographic domain encompassing South America (except Colombia and Venezuela), Antarctica, and South Africa. Several clades that were diverse elsewhere during this interval were extremely rare or absent in this realm, including conodonts, graptolites, and stromatoporoids. The scarcity of biostratigraphic marker groups has historically complicated stratigraphic correlations across Malvinoxhosan basins. Ammonoids are among these rare groups, previously reported only from a few localities in Argentina and Bolivia. Here, we report a new ammonoid assemblage from the Middle Devonian of the northernmost Precordillera Basin (Sierra de Las Minitas, La Rioja Province, Argentina). 15 specimens, housed in the Museo de Antropología y Ciencias Naturales de la Universidad Nacional de La Rioja, allowed the identification of three species: Epitornoceras baldisi (Leanza), previously known from the Chigua Formation type locality, and two new species assigned to Trimaenioceras Bockwinkel and Korn: Trimaenioceras ivanae n. sp. and Trimaenioceras alegria n. sp. Epitornoceras and Trimaenioceras are known to co-occur exclusively within the uppermost middle Givetian ammonoid biozone ( Pharciceras aff. P. amplexum zone), based on data from Morocco. The Argentine fossil-bearing strata are therefore assigned to this interval, providing one of the most precise biostratigraphic correlations for the Devonian of SW Gondwana. This biozone corresponds to the upper part of the Taghanic Crisis interval (end-Middle Devonian), suggesting an incursion of ammonoids into the cold-water basins of the Malvinoxhosan Realm during that time. UUID: http://zoobank.org/2e549c39-c6cc-402d-867a-d297ce3ed364