Dispersal of Pleistocene Equus (Family Equidae) into South America and calibration of GABI 3 based on evidence from Tarija, Bolivia.

Summary

# Editorial Summary: Equus Dispersal into South America and GABI 3 Calibration Fossil evidence from the Tolomosa Formation at Tarija, Bolivia fundamentally revises our understanding of when horses first arrived in South America during the Great American Biotic Interchange. MacFadden's analysis of 15 superposed faunal horizons containing Equus remains provides the first well-calibrated chronostratigraphic evidence placing this dispersal event significantly earlier than previously accepted—between approximately 0.99 and 0.76 million years ago during the middle Pleistocene Ensenadan South American Land Mammal Age, rather than the late Pleistocene as conventionally dated. This finding contradicts the long-held assertion that Equus arrived during GABI 4 (the fourth phase of the biotic interchange at ~0.125 Ma), instead positioning the genus within GABI 3, thereby redefining the biostratigraphic markers used to calibrate continental-scale mammalian chronology. For equine professionals, this work underscores the importance of recognising that South American horse populations have an evolutionary and biogeographic history extending hundreds of thousands of years deeper into the Pleistocene than previously documented, with implications for understanding adaptive radiation and extinction patterns in equids across the continent.

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Key Findings

• •Equus dispersed into South America during GABI 3 (approximately 0.99 to <0.76 Ma in the middle Pleistocene) rather than GABI 4 as previously hypothesized
• •Tarija, Bolivia fossil record documents Equus occurrence across 15 superposed faunal horizons in the Tolomosa Formation with independent chronostratigraphic calibration
• •The earlier dispersal timing of Equus into South America contradicts the previous hypothesis of occurrence at approximately 0.125 Ma defining the base of the Lujanian SALMA