The extinct South American mammals that evolved to look exactly like horses and camels

Nature · Published 3 October 2026 Swipe more like this →

In short: For millions of years, South America was an evolutionary laboratory. Among its most successful residents were the Litopterna, a group of hoofed mammals that independently mirrored the anatomy of northern horses and camels. Long considered a biological mystery, these creatures are finally revealing their true place on the tree of life.

The Litopterna were a diverse order of endemic South American ungulates that thrived from the Paleocene until the late Pleistocene. While they were once thought to be descendants of North American immigrants, recent research suggests a more complex origin. Modern molecular data, including collagen and DNA analysis, now place them within the Pan-Perissodactyla clade, making them distant relatives of modern horses, rhinoceroses, and tapirs.

Their evolution is a masterclass in convergent development. The proterotheriid family, for instance, evolved limb structures and hooves remarkably similar to modern equines, with some species like Thoatherium achieving even greater toe reduction than horses. Meanwhile, the macraucheniids developed long necks and limbs, with high-set nasal openings that led early researchers to speculate they possessed trunks, though modern theories suggest they may have had prehensile lips or specialized filtering noses.

The group's history was long clouded by the 'Ancient Immigrants' hypothesis, which proposed they arrived from North America during the Paleocene. However, recent morphological studies of dental traits—such as specific molar cusp patterns—show no clear link to North American Mioclaenidae. Instead, evidence points to South American 'condylarths' like the Didolodontidae as their closest kin, suggesting these animals were distinct, southern-evolved lineages that thrived in isolation for tens of millions of years.

The decline of the Litopterna coincided with the Great American Biotic Interchange, when the Isthmus of Panama connected the continents, introducing competition from northern mammals. While some macraucheniids survived until the end of the Pleistocene, most vanished around 12,000 years ago. Recent findings even suggest some, like Xenorhinotherium, may have persisted into the late Holocene, potentially crossing paths with the first humans to reach the continent.

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