Tracking origin, home range, and mobility of Late pleistocene fossil horses from west-central Mexico

Abstract The use of radiogenic strontium (87Sr/86Sr) and stable oxygen isotopes (δ18O) in macrofossil remains has impacted paleomobility studies. We performed 87Sr/86Sr and δ18O analyses in dental enamel of three Rancholabrean horse species (E. cedralensis, E. conversidens and E. mexicanus) and in sediments and rocks from two fossil sites in west-central Mexico (La Cinta-Portalitos, and La Piedad-Santa Ana). The aim was to obtain information on geographical origin (on a regional and a local scale) and the mobility behavior of the horses, as well as to investigate the implications for paleoecological studies of large mammals. Enamel 87Sr/86Sr values show that the possible geographical origin of fossil horse populations from both sites is the west-central Trans-Mexican Volcanic Belt (TMVB). This was additionally corroborated by similar δ18O signatures for species and sites indicating that horses drank water with the same isotopic composition and lived in the same geographical region. To the north, 87Sr/86Sr from TMVB back-arc volcanic and carbonate rocks are more enriched in radiogenic strontium. To the south, Sierra Madre del Sur igneous rocks are generally more depleted in radiogenic 87Sr. Therefore, there is no evidence for horse migration or dispersion from these areas. However, in the case of the La Piedad-Santa Ana site, differences in 87Sr/86Sr between sediments and enamel could result from non-local residence within the TMVB. On the other hand, in La Cinta-Portalitos, similar strontium signatures of sediments and enamel indicate strictly local residence. We also detected that small differences in 87Sr/86Sr between the three species could have resulted from different residential mobility behavior. Finally, we suggest that populations of sympatric fossil horse species might have a similar geographic origin on a regional scale, but locally different, which might possibly be related to a mobility behavior, as exhibited by extant species of equids.