UCLA

Concurrent expansion of grassland habitat in the Americas and first appearances of immigrant taxa to Mexico via the Great American Biotic Interchange (GABI) during the Late Miocene to Early Pliocene suggest a possible link between the timing of migrations and changes in the environmental conditions. However, the causes and environmental context of GABI migrations are not well understood. This study examined the stable isotope compositions of tooth enamel samples of the Yepómera fauna from western Chihuahua, Mexico, to explore environmental conditions of this region between 4.99 and 5.23 Ma, just after the arrival of C4 plants in North and South America and before the first major migration of GABI. At Yepómera, there was distinct niche partitioning into C3 diets, mixed diets, and C4 diets. Despite expectations, no niche partitioning between equid species (Dinohippus mexicanus, Nannippus aztecus, Astrohippus stockii, and Neohipparion eurystyle) can be determined from carbon isotope ratios. The enamel carbon and oxygen isotope data suggest a relatively dry, open habitat dominated by either savanna or grassland, with a substantial C4 vegetation component and a warmer and somewhat wetter climate than today. These reconstructions are consistent with a rise in C4 biomass before 5.23 Ma and suggest that the conditions needed for growth of C4 vegetation were prevalent in this region of Mexico. Future work along the GABI migration route will lead to a more complete understanding of the ecologic responses to changing climate and faunal interchange events.