Earliest Records of the GenusSpiticerasUhlig in the Neuquén Basin, Argentina: Systematic and Biostratigraphic Implications

Abstract

The earliest records of the genus Spiticeras Uhlig in Western Gondwana occur in the Upper Jurassic—Lower Cretaceous deposits of the Vaca Muerta Formation in the Neuquen Basin, Argentina. Those records involve two species, Spiticeras acutum Gerth and Spiticeras hauthali Gerth, which were firstly described from Arroyo Durazno, Mendoza. A systematic revision and reconsideration of the biostratigraphic distribution of both species was performed based on the study of type material and new bed-by-bed collections in the type locality and other southern Mendoza sections. S. acutum and S. hauthali display a succession of ornamentation stages that, together with other morphological characters, sustain their assignation to the genus Spiticeras. Addidionally, macroconchiate and microconchiate specimens were identified in both taxa. The record of S. acutum from the lowermost beds assigned to the Late Tithonian—Early Berrisian Substeueroceras koeneni Assemblage Biozone substantiates the downwards extension of the known range of the species in the Neuquen Basin. Given that the basal portion of the S. koeneni Biozone can be correlated with the Late Tithonian Standard “Durangites” Zone and that these early spiticeratins records have been found to be associated with a Late Tithonian secondary nannofossil bioevent (Raghodiscus asper (Stradner) first occurrence) in two of the studied sections, a Late Tithonian age is suggested for the earliest records of Spiticeras in Gondwana.