What drives divergence?: Horse evolution has long been held as a classic example of adaptive radiation. It has been thought that an increase in the height of cheek teeth opened up new grass resources, leading to divergence. Cantalapiedra et al. , however, found that although the Equinae have experienced high levels of divergence, these splits do not appear to have been related initially to specific phenotypic changes. Instead, it seems that external environmental drivers and patterns of migration and isolation initiated population divergence, with phenotypic changes emerging once lineages had begun to divide. Science , this issue p. 627

Background: Stable isotope ratios (¹³C/¹²C and¹⁸O/¹⁶O) in fossil teeth and bone provide key archives for understanding the ecology of extinct horses during the Plio-Pleistocene in South America; however, what happened in areas of sympatry betweenEquus (Amerhippus)andHippidionis less understood.Results: Here, we use stable carbon and oxygen isotopes preserved in 67 fossil tooth and bone samples for seven species of horses from 25 different localities to document the magnitude of the dietary shifts of horses and ancient floral change during the Plio-Pleistocene. Dietary reconstructions inferred from stable isotopes of both genera of horses present in South America document dietary separation and environmental changes in ancient ecosystems, including C₃/C₄transitions. Stable isotope data demonstrate changes in C₄grass consumption, inter-species dietary partitioning and variation in isotopic niche breadth of mixed feeders with latitudinal gradient.Conclusions: The data forHippidionindicate a preference varying from C₃plants to mixed C₃-C₄plants in their diet.Equus (Amerhippus)shows three different patterns of dietary partitioningEquus(A.)neogeusfrom the province of Buenos Aires indicate a preference for C₃plants in the diet.Equus(A.)andiumfrom Ecuador andEquus(A.)insulatusfrom Bolivia show a preference for to a diet of mixed C₃-C₄plants, whileEquus(A.)santaeelenaefrom La Carolina (sea level of Ecuador) and Brazil are mostly C₄feeders. These results confirm that ancient feeding ecology cannot always be inferred from dental morphology. While the carbon isotope composition of horses skeletal material decreased as latitude increased, we found evidence of boundary between a mixed C₃/C₄diet signal and a pure C₄signal around 32° S and a change from a mixed diet signal to an exclusively C₃signal around 35°S.We found that the horses living at high altitudes and at low to middle latitude still have a C₄component in their diet, except the specimens from 4000 m, which have a pure C₃diet. The change in altitudinal vegetation gradients during the Pleistocene is one of several possibilities to explain the C₄dietary component in horses living at high altitudes. Other alternative explanations imply that the horses fed partially at lower altitudes.