Saturday 8 April 2023

Oceanic Phylloceratid Ammonites from the Middle Jurassic Neuquén Basin of Argentina.

The Neuquén Basin is a sedimentary basin to the east of the Andes in western Argentina, covering about 120 000 km². Here a series of deposits record repeated marine transgressions from the Jurassic well into the Tertiary, and is a significant oil reserve as well as a famous fossil-producing region. During the Middle Jurassic the basin was cut off from the Palaeopacific Ocean by a volcanic arc following the line of the modern Andes. In theory, this enclosed basin should have been devoid of open ocean organisms, such as Phylloceratid Ammonites, however, members of this group are occasionally reported from Middle Jurassic deposits within the Neuquén Basin, suggesting that a connection to to the open ocean was at least sporadically present.

In a paper published in the journal Revue de Paléobiologie, Genève on 15 January 2023, Bernard Jolly of Beaugency in France, Horacio Parent of the Laboratorio de Paleontología, at the Universidad Nacional de Rosario, and Alberto Garrido of the Museo Provincial de Ciencias Naturales 'Prof. Dr. Juan A. Olsacher', and the Centro de Investigación en Geociencias de la Patagonia of the Universidad Nacional del Comahue, describe Phylloceratid Ammonites belonging to three species from an exposure of the Middle Jurassic Los Molles Formation in Picún Leufú Department of Neuquén Province, Argentina. 

The outcrop from which the specimens were collected lies about 7 km to the west of the point where National Road-40 crosses the Arroyo Picún Leufú stream. The rocks here are of Aalenian Age (between 174.1 and 170.3 million years old), with most of the sequence being covered, but several layers of black shale and fine, calcarious sandstone exposed.

(A) The Neuquén Basin with indication of the study area. (B) Geological map of the study area with indication of the studied section. Jolly et al. (2023).

The first three specimens described are juvenile phragmocones (chambered sections of the shell, where the Animal lived before reaching maturity) assigned to the species Phylloceras perplanum, which has previously been recorded from Hungary and France. These are compressed and very involute Ammonites, with smooth, slightly rounded sides. The Argentinian specimens are very close to the French and Hungarian specimens in both form and dimensions.

The next three specimens are three more juvenile phragmocones, referred to the genus Calliphyloceras, and considered to show affinities to the species Calliphyloceras achtalense, which has previously been recorded from Georgia (eastern Europe) and France. These are compressed specimens with constrictions perfectly matching specimens of Calliphyloceras achtalense from what would have been the ancient Tethys Ocean, although slightly smaller and with with a narrower umbilicus. The European/Tethyan specimens are also slightly younger, dating from the Bajocian (170.3 to 168.3 million years ago).

The final specimen described is an entire conche (outer shell) assigned to the genus Costiphylloceras, and assigned to a new species, Costiphylloceras limayense, in reference to the Rio Limay, into which the Picún Leufú flows. The new species is erected on the basis of the occurrence of pronounced ribs on the upper third of the flanks, particularly on the ventral side. 

Phylloceratid Ammonites from the lower Aalenian of Picún Leufú; Los Molles Formation. (A)-(C): Phylloceras perplanum; juvenile phragmocones; (A) MOZ-PI-6640; (B) MOZ-PI-6641; (C) MOZ-PI-6642). (D)-(F) Calliphylloceras aff. achtalense; phragmocones; (D) MOZ-PI-6637; (E) MOZ-PI-6638; (F) MOZ-PI-6643). (G) Costiphylloceras limayense; phragmocone (MOZ-PI-6639). Jolly et al. (2023).

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