Beyond the desert: new paleoenvironmental evidence for Nothrotheriops (Xenarthra, Folivora) from the Late Pleistocene of Argentina

For millions of years, the Americas were home to a remarkable group of mammals known as ground sloths. Unlike the small, tree-dwelling sloths seen in Central and South American rainforests today, these ancient relatives were massive, slow-moving creatures that walked on the ground. They belonged to a larger family of animals called xenarthrans, which also includes armadillos and anteaters. While many of these giants vanished as the last ice age ended, their fossilized bones tell a story of how they lived, where they traveled, and what environments they could survive in. For a long time, scientists believed that one particular type of ground sloth, known as Nothrotheriops, was a creature of the dry, dusty deserts of North America. Its remains were most often found in places like the arid landscapes of the American Southwest and Mexico, leading researchers to picture it as an animal perfectly adapted to heat and a lack of water.

This picture, however, is being redrawn by a new discovery in Argentina. A team of researchers recently examined a fragment of a jawbone found in the Santa Fe Province of Argentina. This piece of bone, which belongs to a Nothrotheriops ground sloth, was unearthed from layers of earth that tell a very different story about the animal's home. The sediment surrounding the fossil suggests that when this sloth lived there, the landscape was not a desert at all. Instead, it was a wet, complex environment filled with rivers, marshes, and lush vegetation. This finding challenges the long-held idea that these animals were restricted to dry habitats and suggests they were far more adaptable than previously thought, capable of thriving in diverse and humid conditions during the last ice age.

The journey to this discovery began with a small, unassuming piece of rock containing a fragment of a right lower jaw. The fossil was found near the town of Puerto San Martín, in a region of Argentina known as the Northern Pampa. The researchers, Diego Brandoni and Raúl I. Vezzosi, carefully cleaned and studied the bone, which had been preserved in layers of earth known as the Puerto San Martín Formation. These layers are made of silty soil that was deposited by slow-moving rivers and shallow lakes, a stark contrast to the wind-blown sands of the deserts where Nothrotheriops is usually found. By measuring the jaw and comparing its shape and size to dozens of other ground sloth fossils from museums around the world, the team confirmed that this specimen belonged to the genus Nothrotheriops.

Identifying the specific species of the sloth proved to be a more difficult task. The genus Nothrotheriops contains two well-known species: one that was generally larger, and another that was slightly smaller. The researchers looked for specific features on the jaw, such as the length of the tooth row relative to the front part of the jaw, to make a precise identification. However, the front part of the jaw was missing from their specimen, making it impossible to measure the necessary proportions. Furthermore, the team noted that another ancient ground sloth from the same region, previously thought to be a different genus called Nothropus, might actually belong to the same group as Nothrotheriops. Because of these uncertainties and the missing parts of the jaw, the scientists decided it was most accurate to classify the specimen simply as Nothrotheriops sp., indicating it is a member of the group but not yet assigned to a specific species.

What makes this discovery truly significant is not just the identity of the animal, but the world it lived in. The geological evidence from the site where the jaw was found paints a vivid picture of the environment during the time the sloth was alive, roughly 90,000 to 118,000 years ago. This period corresponds to a warm, wet phase known as the Last Interglacial, when sea levels were higher and the climate in this part of South America was much more humid than it is today. The layers of rock containing the fossil show signs of low-energy river systems and marshy areas, with evidence of trees and dense vegetation growing nearby. This stands in sharp contrast to the dry, desert-like environments where Nothrotheriops fossils are typically found in the United States and Mexico.

The presence of a ground sloth in such a wet, vegetated landscape suggests that these animals were not limited to arid conditions. While they were certainly comfortable in dry places, this new evidence indicates they possessed a flexibility that allowed them to inhabit a wide range of environments, from the dry deserts of the north to the wetlands of the south. The researchers suggest that during these warm, wet periods, the expansion of rivers and marshes may have created a corridor that allowed these animals to move further south than they had before. The fossil record of this region, which includes other large mammals and evidence of diverse plant life, supports the idea of a dynamic landscape where different habitats existed side by side.

This finding adds a new chapter to the story of how life moved across the Americas. It suggests that the ground sloths were not just specialists of the desert, but generalists capable of adapting to changing climates and diverse ecosystems. As the climate shifted between wet and dry cycles over thousands of years, these animals likely followed the available resources, moving through forests, grasslands, and wetlands. The jawbone from Argentina serves as a tangible reminder that the past was more complex than a simple map of dry and wet zones. It shows that even creatures we think we know well can surprise us, revealing hidden capabilities and a broader reach than our previous maps allowed. The story of Nothrotheriops is no longer just a tale of desert dwellers, but a narrative of a resilient animal that thrived across a changing continent.