Early Cretaceous gastropods from the Neuquén Basin, west-central Argentina

CATALDO, CECILIA SOLEDAD; LAZO, DARÍO GUSTAVO

Plymouth

Simposio; 8th International Symposium on the Cretaceous System; 2009

Despite being relatively abundant in many lithostratigraphic units in association with bivalves and ammonites, the gastropod fauna of the Neuquén Basin remains rather poorly studied in comparison to other mollusc groups. Remarkably, Early Cretaceous gastropods of Argentina have not been studied thoroughly since Weaver (1931). The only further contribution to this subject was made by Damborenea et al. (1979). The present work is based on a comprehensive study of 224 specimens of Lower Cretaceous gastropods housed at the Area de Paleontología, Universidad de Buenos Aires (CPBA). It consists in a taxonomic study, an analysis of stratigraphic distribution within the framework of a detailed ammonite biozonation, a paleobiogeographical analysis of the recorded taxa and their possible affinities with faunas from other parts of the world, a taphonomic study of the collected specimens and the recognition and interpretation of the paleoecological parameters of the species involved. The study material comes from Pilmatué Member strata exposed in ten sections of the Agrio Formation measured in the following localities of the Neuquén province: Agua de la Mula, Bajada Vieja, Pichaihue, Salado Sur, Cerro Marucho, Cerro Birrete, Cerro Los Bueyes, Cerro Lotena, Casa Nuestra y Loncopué. The bearing beds are of Late Valanginian to Early Hauterivian age according to detailed ammonite zonations (e.g., Aguirre-Urreta et al., 2007). Four Late Valanginian to Early Hauterivian species were identified: Pleurotomaria gerthi Weaver, 1931, Tylostoma jaworskii Weaver, 1931, Protohemichenopus neuquensis Camacho, 1953 and Harpagodes cf. H. desori (Pictet y Campiche). This latter species is left in open nomenclature as the poor state of preservation of the only specimen available precludes a more accurate taxonomic determination. Pleurotomaria gerthi (Vetigastropoda, Pleurotomarioidea, Plurotomariidae), Tylostoma jaworskii (Caenogastropoda, Tylostomidae) and Harpagodes cf. H. desori (Caenogastropoda, Stromboidea, Strombidae) were epibenthic mobile browsers that inhabited loose to firm grounds in shoreface and transition zone, as interpreted from a lithological-environmental characterisation of the bearing beds coupled with paleoecological analysis. Protohemichenopus neuquensis (Caenogastropoda, Stromboidea)  was a substrate generalist, inhabiting shoreface, transition zone and upper offshore. A shallow infaunal life habit is inferred for this species. Extant representatives of this family live mostly buried with the apertural plane facing downwards and build tubes in sand for suspension-feeding. Of these four species, three are endemic forms of the Neuquén Basin. Only Harpagodes cf. H. desori has been recorded in Early Cretaceous beds of other parts of the world: France, Switzerland, Mexico, Austria, Crimea (Ukraine), Northern Caucasus, Bulgaria and Trinidad and Tobago. Genus Protohemichenopus is also endemic to the basin. As is also seen between bivalves and ammonites, the gastropod fauna of the Neuquén Basin  shows closer affinities with Tethyan faunas, which supports the proposition of the existence, during the Early Jurassic-Early Cretaceous interval, of open marine connections between this Southern Pacific depocentre and other oceanic water masses. Revision of former collections in addition to further field collecting will yield new insights into the general understanding of this group in southern South America, thus improving the results outcoming from the current, newly started line of investigation.