THE DEVONIAN AMMONOIDS FROM ARGENTINA

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

Salta

Congreso; RCAPA 2022; 2022

During the Early to Middle Devonian, southwestern Gondwana was characterised by a marine fauna with a high proportion of endemism that defined the Malvinoxhosan (=Malvinokaffric) Realm. Moreover, many clades that were diverse elsewhere during this time interval, were either very scarce or absent from this major biogeographic unit. The ammonoids constitute one of these very rare clades: only a few occurrences limited to the Middle Devonian of western South America have been documented, including a number of genera recognized in the early Eifelian to early Frasnian from Bolivia (Tornoceras, Sporadoceras, Agoniatites and Mimotornoceras) and a single species reported as Tornoceras baldisi from Argentina by Leanza, in 1968. Here, we reinvestigate the taxonomy of the Devonian ammonoids from Argentina based on new findings from the type area of the Chigua Formation (Chinguillos Group) in the San Juan Province (western Precordillera), where the two type specimens of Tornoceras baldisi come from. The age of the unit has been previously constrained to the late Emsian to Givetian based on palynological data. A total of eight new wellpreserved specimens in nodules provide new insights into the morphology of these Argentinian tornoceratids. Furthermore, this new material includes the first ammonoid specimen exhibiting an iridescent colour pattern discovered in Palaeozoic deposits worldwide. Until now, such a pattern was only known from Mesozoic ammonoids. Based on the conch geometry and the shape of the suture lines, the type material of Tornoceras baldisi and the new found specimens are assigned to the same species of the genus Epitornoceras. Our revision allows a better constraint on the age of the ammonoid-bearing levels of the Chigua Formation. Based on the stratigraphic range of Epitornoceras in Morocco, Germany and North America, a late Givetian age can be attributed to the Argentinian ammonoid species. The stratigraphically constrained occurrences of Mimotornoceras in the early Eifelian of Bolivia and Epitornoceras in the Givetian of Argentina further suggest several independent immigration events from tropical regions to the Malvinoxhosan Realm during the Middle Devonian when the realm still recorded an evident biogeographic identity characterised by the high diversity of endemic trilobites