A Neuquensaurus australis dominated bone-bed from Cinco Saltos (Río Negro, Argentina): First reporto on fossil biostratinomy.

PAOLO CITTON; MARTINA CARATELLI ; VIRGINIA ZURRIAGUZ; IGNACIO A. CERDA; AGUSTÍN MARTINELLI; JONATAN KALUZA

Congreso; Reunión de Comunicaciones de la Asociación Paleontológica Argentina; 2023

Neuquensaurus australis is a small-bodied, saltasaurine titanosaur sauropod reported from different sites within the Anacleto Formation (Campanian, Upper Cretaceous) of Cinco Saltos area (Río Negro province, Argentina). One of these sites includes a multitaxic monodominant bone-bed that yielded dozens of skeletal elements including cranial, axial, and appendicular bones. The assemblage is presently dominated by Neuquensaurus australis, which is represented at least by three individuals (a juvenile and two sub-adults), plus another titanosaur taxon and other, currently undetermined, vertebrate remains. Here we discuss some biostratinomic aspects of the assemblage, based on preliminary analyses of part of the material, and a first possible interpretation about the type of accumulation. Skeletal remains were collected few metres below the unconformable contact between the Anacleto and Allen formations, from a fine- to medium-grained, 60 cm thick massive sandstone layer barren of microfossils. The bone-bearing deposit lays on an erosional surface developed on medium-grained, cross-bedded, yellowish sands, also barren of microfossils. These deposits are included in a succession of mainly dark-red and green mudstones with carbonate nodules and reddish laminated siltstones, intercalated with light-yellow to light-grey, tabular and through cross-stratified sandstones, and subordinate conglomerates. Accumulation of fossil bones likely occurred close to a channelized area within a wide floodplain furrowed by low to medium-energy flowing streams. In general, fossil bones referred to Neuquensaurus australis, excluding few exceptions currently under consideration, are complete, including fragile anatomical parts like neural spines of cervical vertebrae. Material so far analysed shows slight weathering on those bone surfaces that were directly in contact with the palaeosurface. Longitudinal cracking and subordinate mosaic cracking affect long bones diaphyses and flat surfaces of bones (e.g. vertebrae). Occasionally, cortical tissue on edges appears imploded by the collapse of underlying pneumatised tissue. Preliminary data suggest that skeletal remains suffered an overall short exposure time. A short ablation time can also be inferred, considering that Voorhies’s categories of different susceptibility to transport are identified in the same place. Material not referable to Neuquensaurus australis, considered until now as isolated findings, is instead affected by different weathering stages, among which flaking, suggesting a longer time of subaerial exposure. Observed uniformity in biostratinomic features of Neuquensaurus australis, coupled with the occurrence of isolated elements of other taxa with higher weathering, would characterize the bone-bed as a mixed fossil accumulation that may have been resulted from the co-occurrence of attritional and not attritional mode of accumulations.