The late Givetian (Middle Devonian) ammonoid Epitornoceras of Argentina (southwestern Gondwana).

ALLAIRE N.; RUSTÁN J.J.; KORN, D. ; VACCARI N.E.; EZPELETA, M.; BALSEIRO, D.

GEODIVERSITAS

PUBLICATIONS SCIENTIFIQUES DU MUSEUM

Lugar: Paris; Año: 2024

1280-9659

During the Early to Middle Devonian, southwestern Gondwana was characterized by circumpolar marine environments with striking endemisms and nearly absence of typical Paleozoic groups (mainly conodonts, graptolites and stromatoporoids), which defined the Malvinoxhosan (=Malvinokaffric) Realm. Ammonoids constitute one of these very rare clades with just few records from early Eifelian to early Frasnian from Bolivia and Argentina. We revised the taxonomy of the only one Devonian ammonoid species from Argentina, Tornoceras baldisi Leanza, 1968, based on type material and new specimens from the type area (Chigua Formation, Precordillera, western Argentina).We also report the first record of iridescent color pattern in Paleozoic ammonoids worldwide. We reinterpreted the stratigraphic polarity of the sedimentary succession, and ammonoid fossils occur in the uppermost interval of the formation. Tornoceras baldisi is herein assigned to the genus Epitornoceras, and consequently a late Givetian age is proposed for the bearing levels. These occurrences better constrain the age of the upper part of the Chigua Formation (Chavela Member), which is now considered younger than the Belén and Huamampampa formations from Bolivia and possibly coeval to the upper part of the Iquiri Formation. Although the ammonoids indicate a biogeographic dilution of the Malvinoxhosan signature during the late Middle Devonian, the realm is still recognizable on the basis of endemic trilobites as typical indicators.