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.

在中新世晚期到上新世早期，美洲草原栖息地的同时扩张和移民类群通过美洲生物大交换 (GABI) 首次出现在墨西哥，这表明迁移时间与环境条件变化之间可能存在联系。然而，GABI 迁移的原因和环境背景尚不清楚。本研究检查了墨西哥奇瓦瓦西部 Yepómera 动物群牙釉质样品的稳定同位素组成，以探索该地区在 4.99 至 5.23 Ma 之间的环境条件，就在 C ₄植物到达北美和南美之后和之前GABI 的第一次重大迁移。在 Yepómera，有明显的生态位划分为 C ₃饮食、混合饮食和 C₄饮食。尽管有预期，但马科动物（墨西哥 Dinohippus mexicanus、Nannippus aztecus、Astrohippus stockii和Neohipparion eurystyle）之间的生态位划分无法从碳同位素比率确定。釉质碳和氧同位素数据表明，这里是一个相对干燥、开放的栖息地，以稀树草原或草原为主，具有大量的 C ₄植被成分，气候比今天更温暖、更湿润。这些重建与5.23 Ma 之前C ₄生物量的增加一致，并表明 C ₄生长所需的条件墨西哥这个地区盛行植被。沿 GABI 迁徙路线的未来工作将导致对气候变化和动物群互换事件的生态响应有更全面的了解。