In the vast arenas of evolutionary biology and paleontology, few stories capture the imagination quite like the rise of the swiftest land animals. North America’s fastest terrestrial mammal, the American pronghorn (Antilocapra americana), has long been a subject of fascination for scientists attempting to unravel the origins of its remarkable speed. Historically, the prevailing hypothesis credited the pronghorn’s exceptional velocity to evolutionary pressure exerted by the now-extinct American cheetah (Miracinonyx spp.), suggesting a predator-prey arms race that honed its capacity for rapid escape. However, emerging fossil evidence from the Miocene epoch now challenges this long-held narrative, offering profound insights into the evolutionary dynamics that sculpted the pronghorn’s agility millions of years prior to the American cheetah’s arrival.
Recent research, spearheaded by a team at the University of Michigan and published in the Journal of Mammalogy, delves deeply into the morphology of fossilized ankle bones—specifically the astragalus—of ancient relatives of the modern pronghorn. This bone, integral to the ankle joint, serves as a critical biomechanical indicator of locomotor adaptations. The study reveals that these early pronghorn ancestors possessed anatomical characteristics optimized for swift movement not only millions of years before the arrival of the American cheetah but also before major shifts in North America’s environmental landscape. This discovery upends the classical model of predator-driven evolutionary acceleration, pointing instead to intrinsic evolutionary trends independent of external predation threats.
By meticulously examining the astragali from Miocene fossils unearthed in the Mojave Desert’s Dove Spring Formation, dated between 8 and 12.5 million years ago, the research team identified an unexpected pattern of functional trait consistency. Contrary to expectations shaped by environmental changes—from dense, unbroken forests giving way to patchy woodlands and arid grasslands—the ankle morphology of these ancient artiodactyls exhibited remarkable stability. This suggests a locomotor strategy optimized for navigating a heterogeneous mosaic of habitats rather than purely open terrain, highlighting the complexity of adaptive responses in evolutionary history.
Paleontologist Anne Kort, a principal investigator in this project, emphasizes that the American cheetah’s influence may have been overestimated. The cheetah analog, often portrayed as a hypercarnivorous pursuit predator exerting strong selective pressure, appears less “cheetah-like” in biomechanical terms than originally assumed. The pronghorn’s ancestors, it seems, had already evolved an efficient running capability—reflected in the astragalus morphology—well before the predator emerged. This raises intriguing questions about the underlying drivers of cursoriality and the evolutionary pathways animals travel to adapt to changing environments.
The implications of this study resonate far beyond paleontology. Given the pronghorn’s extraordinary locomotor abilities enable it to traverse large distances across fragmented landscapes, insights into its evolutionary history inform contemporary conservation and wildlife management strategies. Fabian Hardy, lead author and assistant professor at Slippery Rock University, underscores how understanding the historical stability in pronghorn locomotion sheds light on their resilience in the face of modern challenges like urbanization and habitat fragmentation. Their ingrained capacity for movement may well buffer them against some of the ecological pressures posed by human activity and climate change.
Contextualizing these findings within broader mammalian evolution, the researchers note a trend of increasing cursoriality in artiodactyl mammals over the last 30 million years. However, the local environmental fluctuations in the Mojave region during the Miocene did not produce a corresponding microevolutionary shift in ankle morphology, indicating that functional traits could remain stable amidst significant habitat transitions. This decoupling of morphological adaptation from environmental change challenges simplistic models of linear evolutionary responses to habitat shifts and underscores the importance of mobility and behavioral adaptability.
The fossil assemblage analyzed provides a temporal window into a dynamic ecosystem undergoing dramatic shifts: the once continuous forests fragmented into an arid grassland-woodland mosaic. Such biome fragmentation typically exerts selective pressure favoring either specialized running adaptations for open habitats or forest-based maneuverability. Yet, the pronghorn relatives maintained an intermediate ankle structure, suggesting a versatile locomotor repertoire allowing efficient movement between isolated habitat patches, rather than specialization for exclusively open or closed environments.
Contrary to the expectation of progressive ankle shortening to enhance cursorial efficiency, the astragalus remained morphologically constant throughout the temporal range of the specimens, indicating that the ancient pronghorns already possessed an optimal locomotive form for their ecological niche. This finding invites a reevaluation of how ancient mammals responded to climatic and environmental pressures, where mobility and strategic habitat utilization may outweigh morphological specialization.
Moreover, the similarity of astragalus morphology between ancient pronghorn relatives and the extant American pronghorn underscores a remarkable evolutionary conservation of functional traits. This constancy extends back over 12 million years, implying that the pronghorn’s evolutionary design for speed is a deeply entrenched adaptation. It highlights an evolutionary trajectory where functional effectiveness is maintained over extended periods despite fluctuating external conditions.
The eventual disappearance of these small Miocene pronghorn ancestors toward the epoch’s close may reflect an ecosystem tipping point—an irreversible shift in community composition and resource availability. Such extinction events provide poignant reminders of the fragility of evolutionary equilibria and the potential for rapid environmental changes to outpace adaptive responses.
This study also contributes a historical dimension to contemporary conservation biology. By illuminating the stability and resilience embedded in anatomical traits over millions of years, the research offers a valuable lens for understanding how modern species may cope with ongoing environmental transformations. It encourages conservationists to consider long-term evolutionary contexts when devising management plans.
In sum, the University of Michigan-led research redefines our understanding of North America’s fastest land mammal’s evolutionary history. By decoupling pronghorn speed from the previously attributed predator-prey arms race with the American cheetah, it shifts the focus onto intrinsic locomotive tendencies established well before the arrival of complex predation pressures. Such insights enrich the scientific narrative of mammalian evolution and furnish critical knowledge for safeguarding biodiversity amidst the challenges of the Anthropocene.
Subject of Research: Evolutionary biomechanics and environmental adaptation of Miocene artiodactyls, focusing on the functional morphology of pronghorn relatives.
Article Title: Family-level ecometrics reveal functional trait stability in Miocene artiodactyls despite environmental change in the Mojave region
News Publication Date: 20-Jan-2026
Web References: 10.1093/jmammal/gyaf089
Keywords: Physical sciences, Earth sciences, Paleontology
