South America's proterotheriids evolved single-toed hooves 20 million years ago, roughly 10 million years before true horses did in North America, and they did it in woodlands, not grasslands. The answer lies in Patagonia's ancient mosaic landscapes where patches of forest were interspersed with grasslands. These animals were adapted to sprint across open ground to reach the next patch of forest, like land-dwelling whales swimming between islands.
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An Experiment in Locomotion
South America's long isolation as an island continent produced a unique evolutionary theater. Among its most curious actors were the proterotheriids, a family of extinct hoofed mammals belonging to the order Litopterna. These animals are a case of convergent evolution, developing horse-like bodies and single-toed hooves millions of years before true horses (family Equidae) did the same in North America. Fossils of these creatures are found primarily in the Santa Cruz Formation of Patagonia, a layer of volcanic and river-deposited sediment laid down during the Early Miocene, roughly 18 to 16 million years ago.
Unlike modern horses, which are part of the order Perissodactyla (odd-toed ungulates), proterotheriids were on a completely separate branch of the mammalian tree. They were generally small to medium-sized animals. One of the most well-known genera, Thoatherium, was only about 70 centimeters (2.3 ft) long, roughly the size of a gazelle. Their teeth were brachydont, or low-crowned, indicating they were browsers that fed on soft leaves and buds rather than the tough, abrasive grasses consumed by grazing horses.
The most striking feature of advanced proterotheriids like Thoatherium was their feet. They achieved a state of monodactyly—standing on a single hoof—that was even more extreme than that of modern horses. While today's horses still retain vestigial "splint bones" from their second and fourth toes, Thoatherium had lost these remnants completely. This anatomical parallel with North American horses, developed from a different ancestor in a different hemisphere, shows two distinct lineages arriving at a similar solution to environmental pressures.
A Forest Floor Sprint
The evolution of the single hoof in true horses is classically linked to the expansion of vast, open grasslands in North America. A single, sturdy hoof is an excellent adaptation for running at speed over hard, flat ground to escape predators. The puzzle with the proterotheriids is that they perfected this adaptation in a very different setting. Fossil plant evidence from the Santa Cruz Formation, including fossilized wood and phytoliths (plant silica particles), reveals that Early Miocene Patagonia was not a continuous prairie. Instead, it was a temperate mosaic of semi-arid forests, humid woodlands, and open grassy patches.
This environment suggests a different evolutionary driver for their single-toed feet. The prevailing hypothesis is that proterotheriids were not long-distance endurance runners but specialized sprinters. Their speed was an adaptation for dashing across dangerous open areas that separated patches of protective forest. In this view, the landscape was a collection of wooded "islands" in a sea of open ground, and the single hoof provided the burst of speed needed to minimize exposure to predators while moving between them. Their hindlimb morphology also suggests a rotary or half-bound gait, different from the transverse gallop used by modern horses. This entire group of uniquely South American animals thrived for millions of years before eventually dwindling. One of the last members, Neolicaphrium recens, survived until the Late Pleistocene.
💡Fun Facts
The genus *Thoatherium* possessed a single hoof even more specialized than that of modern horses, having completely lost the external splint bones that horses still retain.
Proterotheriids belonged to the Litopterna, an order of mammals so unique to South America they are sometimes called "meridiungulates," or "southern ungulates."
The decline of these native ungulates coincided with the Great American Biotic Interchange, which began around 2.7 million years ago when the Isthmus of Panama formed and allowed North American mammals, including true horses and predators, to enter the continent.
The first fossils from the Santa Cruz Formation, which holds the important specimens of these animals, were collected in the 1840s and sent to Charles Darwin for study.
This is a paleontological region, not a single site. Key fossils are housed in museums like the Museo de La Plata and the Museo Argentino de Ciencias Naturales in Buenos Aires, with year-round opening hours.
Admission
Varies by museum.
Accessibility
The Patagonian landscape is vast and rugged. Museum accessibility varies by institution.
