Cephalopods from the triassic-jurassic boundary interval in west central Argentina

Abstract

The section of Arroyo Alumbre, in the Rio Atuel area, Malargue province, west central Argentina, consists of a c. 300 m thick continuous succession of uppermost Triassic and basal Jurassic strata, ascribed to the Arroyo Malo Formation. The lower half of the sequence contains a succession of levels with different taxa, from the base, Rhabdoceras suessi Hauer, ?Peripleurites sp., Aulacoceras cf. carlottense Whiteaves, and Choristoceras marshi Hauer. Other levels higher up in the section yielded Psiloceras tilmanni Lange, Psiloceras cf. primocostatum, P. cf. pressum Hillebrandt, Psiloceras cf. polymorphum Guex, and Psiloceras rectocostatum Hillebrandt. All these taxa are described herein. Choristoceras marshi is indicative of the Upper Rhaetian Marshi Zone, and the taxa occurring below indicate that the lowermost part of the section is Norian-Lower Rhaetian, whereas the taxa of the overlying beds are indicative of the Tilmanni, Primocostatum and Rectocostatum zones of the South American Hettangian zonation. The Triassic-Jurassic boundary is located between the last occurrence of Choristoceras marshi and the first occurrence of Psiloceras tilmanni. This fauna is compared and correlated with those recorded at the Triassic-Jurassic boundary in Chile, Peru and North America.