New Fossils of Nautilus-Like Cephalopod Unearthed in Argentina | Sci.News


Paleontologists have redescribed Epitornoceras baldisi, an enigmatic species of ammonoid cephalopod from the Devonian period of Argentina, using new fossils from the Chigua Formation.

An artistic reconstruction of the ammonoid Epitornoceras baldisi during the Middle Devonian. Image credit: H. Santiago Druetta / CICTERRA / Universidad Nacional de Córdoba.

An artistic reconstruction of the ammonoid Epitornoceras baldisi during the Middle Devonian. Image credit: H. Santiago Druetta / CICTERRA / Universidad Nacional de Córdoba.

Epitornoceras baldisi lived in marine environments of the supercontinent Gondwana during the Late Givetian age of the Middle Devonian period, around 383 million years ago.

“During the Early to Middle Devonian, southwestern Gondwana was characterized by circumpolar marine environments with striking phases of increased endemism and near absence of typical Paleozoic groups (mainly conodonts, graptolites and stromatoporoids),” said Dr. Ninon Allaire from the Universidad Nacional de Córdoba and colleagues.

“One of these clades is the Ammonoida. They are extremely rare: only a few occurrences were reported in Bolivia and Argentina in deposits ranging from early Eifelian to early Frasnian (393 to 383 million years ago).”

Epitornoceras baldisi was first described in 1968 as a member of the cephalopod genus Tornoceras.

The team’s new results suggest the species should be reassigned to another genus, Epitornoceras.

This is the first South American record of this genus, which otherwise is recorded in North America, North Africa and Europe.

Epitornoceras is present in North Africa, and in the Rhenish-Bohemian and Appalachian basins,” the paleontologists said.

“Its diversification and the acquisition of its wide geographic distribution took place after the late middle Givetian Taghanic biocrisis.”

“Hence, the occurrence of Epitornoceras during the Late Givetian in southernmost western Gondwana is not surprising, as there is a well-recorded pattern of immigrants from tropical regions in circumpolar basins after the Taghanic biocrisis.”

New specimens of Epitornoceras baldisi from the type area of the Chigua Formation, San Juan province, Argentina. Image credit: Allaire et al., doi: 10.5252/geodiversitas2024v46a7.

New specimens of Epitornoceras baldisi from the type area of the Chigua Formation, San Juan province, Argentina. Image credit: Allaire et al., doi: 10.5252/geodiversitas2024v46a7.

In the study, the authors examined ten new specimens of Epitornoceras baldisi collected from the uppermost part of the Chigua Formation, San Juan province, Argentina.

The study area is located in the Argentine Precordillera, an intraplate fold-and-thrust belt located at the southern end of the Central Andes foreland basin.

Epitornoceras is a cosmopolitan genus, which diversified after the Taghanic biocrisis of the Late Middle Givetian, according to Appalachian, Moroccan and European records,” the researchers said.

“Its presence in southernmost western Gondwana suggests immigration from tropical regions to the cold Malvinoxhosan Realm during the Late Givetian.”

“In spite of the record of cosmopolitan ammonoids in the Argentine Precordillera, the co-occurring trilobite family Calmoniidae along with additional endemic genera like Belenopyge, support that the Malvinoxhosan (Malvinokaffric) realm is still identifiable during the Late Givetian in southernmost western Gondwana as a major biogeographic unit, on the basis of their typical diagnostic elements.”

The team’s paper was published in the journal Geodiversitas.

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N. Allaire et al. 2024. The Late Givetian (Middle Devonian) ammonoid Epitornoceras Frech, 1902 from Argentina (southwestern Gondwana). Geodiversitas 46 (7): 327-341; doi: 10.5252/geodiversitas2024v46a7



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