Middle Jurassic inoceramids from Argentina

Abstract

Extensive collections from stratigraphically controlled sections allow reassessment and systematic revision of Middle Jurassic inoceramids from Argentina. Two species are doubtfully referred to Parainoceramus Voronetz, including P.? westermanni n. sp. Five species of Retroceramus Koshelkina, including R. cf. R. marwicki (Speden), R. patagonicus (Philippi) and R. stehni n. sp., are present. The Bajocian–early Callovian species of Retroceramus show strong affinities with Aalenian–Bathonian inoceramids from the Northern Hemisphere, and also with the South Pacific forms of the galoi–haasti group of Heterian–Ohauan age, except for minor differences in outline, ornamentation, and the umbonal region.The time range of each species in Argentina was determined by associated ammonoids. There is strong morphological evidence, supported by the stratigraphic distribution of species, that Retroceramus cf. R. marwicki, R. patagonicus, and R. stehni belong to a single clade, which developed from the late Bajocian to the early Callovian in the Andean region.