A new Berriasian species of Andiceras Krantz from the Neuquén Basin, Mendoza, Argentina

Abstract

Since Krantz (1926) raised the genus Andiceras from Late Tithonian-Early Berriasian sediments from two localities of the Neuquén Basin, west-central Argentina, new records of this genus outside this country are dubious and scarce. From a thin interval of the last portion of Vaca Muerta Formation (Lower Tithonian-Lower Valanginian), in the Real de las Coloradas section, Mendoza, 27 well preserved specimens of a new species assignable to Andiceras had been recovered. Andiceras sp. nov. has a medium sized, strongly evolute, discoidal shell, with almost parallel flat flanks, venter gently convex and whorl section remarkably ompressed, higher than wide, with its maximum whorl breadth around the umbilical border. The very shallow rounded umbilical wall tends to be slightly more pronounced on outer whorls. The ornamentation is characterized by low relief, gently rursiradiate fine ribs on umbilical wall,bended forward at the umbilical angle and rectiradiated over the flanks. Most of the primary ribs bifurcate near the middle of the flanks, while some remain simple and in a few cases some virgatotome branching does occur. Ribs may be interrupted on inner whorls of internal moulds but are only depressed where shell is preserved. On outer whorls ribbing is continuous and straight in the venter. Portions of body chamber preserved do not denote any change in ornamentation. Suture line appears to be well indented and characterized by narrow saddles and a deep trifid lateral lobe. All the specimens appear between levels yielding Parodontoceras callistoides and Groebericeras bifrons, of the Substeueroceras koeneni and Argentiniceras noduliferum assemblage biozones respectively. The former one has raditionally been considered of Late Tithonian age (Leanza 1981, Riccardi 1988), but based on new findings and correlations with the Tethys now it is proposed to embrace the Tithonian-Berriasian boundary (Leanza 1996, Riccardi 2008). Therefore Andiceras sp. nov. would be of early Berriasian age in the classic scheme, but middle Berriasian in the new one. A systematic review of the perisphinctids of Vaca Muerta Formation is being currently carried out, thus we expect to refine the Andean Biozonation scheme in the nearby future.