An Eocene time capsule
Fifty-two million years ago, during the Early Eocene epoch, South America was an island continent. Tectonic forces had detached it from Antarctica, but the Isthmus of Panama had not yet formed to connect it with North America. This long period of geographic separation, often called "splendid isolation," turned the continent into a massive natural laboratory for evolution. The fossil beds at Puesto La Leona in Argentina’s Chubut Province record this lost world.
The fossils are preserved within the Sarmiento Formation, a thick sequence of volcanic and pyroclastic deposits that accumulated across Patagonia. This formation spans a vast stretch of time from the middle Eocene to the early Miocene, documenting millions of years of mammalian evolution. The specific layers at Puesto La Leona correspond to the Casamayoran South American Land Mammal Age (SALMA), a biochronological unit defined by its distinctive fossil assemblages. The environment at the time was a humid, subtropical forest, different from the arid Patagonian steppe of today.
A menagerie of the unique
The fauna unearthed at Puesto La Leona is a collection of strange and wonderful creatures. Among the most significant finds are the oldest known bats in South America. Represented by just a few fossilized molars, their discovery confirms that these flying mammals had reached the isolated continent by the Early Eocene. This suggests a remarkable trans-oceanic dispersal event early in their evolutionary history.
The site also contains the remains of endemic South American hoofed mammals, or "ungulates." These animals belonged to orders like the Notoungulata and Litopterna, which evolved entirely in isolation and have no close living relatives. Notoungulates were incredibly diverse, with forms resembling anything from rabbits to rhinoceroses. Litopterns developed cursorial limbs, adapted for running. Recent DNA analysis of collagen from younger fossils has surprisingly shown that these strange beasts were actually true ungulates, most closely related to perissodactyls—the group that includes modern horses, rhinos, and tapirs. The Puesto La Leona deposits also contain fossils of xenarthrans (the ancestors of armadillos and sloths) and numerous marsupials, which filled many ecological niches occupied by placental mammals on other continents.