Climate seasonality and dietary inferences by means of stable isotope analyses on Hipparion teeth of the late Miocene from Batallones-10 (Torrejón de Velasco, Madrid Basin, Central Iberian Peninsula)

Abstract

The Cerro de los Batallones (Madrid Basin, Spain) paleontological complex is composed of 9 fossil sites that contain a massive amount of remains belonging to a diverse late Miocene mammalian fauna. Batallones-10 stands out for containing exceptionally well-preserved remains of herbivores, being the equid Hipparion the best-represented taxon. In this study, we serially sampled the enamel of seven lower third molars of the equid Hipparion from Batallones-10 and analyzed their δ18O and δ13C isotopic variations with the aim of evaluating the climate seasonality at this period of time as well as the diet and habitat use of this equid. Additionally, stable isotope analyses were also used to address taphonomical questions about the mode of formation of the Hipparion assemblage. The sinusoidal δ18O curves displayed by Batallones-10 individuals together with the wide δ18O ranges were indicative of a significant hydrological seasonality in the Madrid Basin around 9.1 Ma. δ13C curves were flatter and pointed to a C3 diet in a woodland to mesic C3 grassland habitat. The inter-individual comparisons allowed us to detect some statistically significant differences in the δ18O and δ13C mean and range values, which points to some differences in the drinking water sources, plants ingested and habitat use. These differences suggest that the accumulation of the Hipparion individuals at Batallones-10 occurred as a gradual process and not as a single death event. When isotopic data from other Spanish Vallesian sites were analyzed, we observed that, during the MN9 (early Vallesian) biochronological unit, there was a less seasonal hydrological regime than during the MN10 (late Vallesian) unit, which corroborates previous studies that suggested an increase in the seasonality of precipitation during the Vallesian. This study supports the efficacy of stable isotope analyses on serially-sampled teeth of fossil vertebrates as a highly informative proxy to infer paleoclimatic and paleoenvironmental seasonality, paleoecological features as well as the taphonomical history of fossil sites.