Morphological novelties from additional specimens of the Middle Miocene Proteropithecia neuquenensis (Primates, Platyrrhini), and new absolute age of its type locality in Argentine Patagonia

Long before humans walked the earth, South America was home to a diverse array of monkeys that lived in the southern reaches of the continent, an area now known as Patagonia. These ancient primates, known as platyrrhines, thrived during a warm period in the Middle Miocene epoch, roughly 20 to 15 million years ago. While many of these species are known only from scattered fragments of bone and teeth, one particular group, the pitheciines, stands out for its unique way of eating. Unlike most monkeys that rely on soft fruits or leaves, pitheciines evolved to crack open hard seeds and nuts, a diet that required specialized teeth and strong jaws. Today, this group includes living monkeys like the saki and the uakari, but their deep history in South America has remained somewhat mysterious, particularly regarding when and how they first developed these hard-shelled food adaptations in the far south.

For decades, scientists have known about a fossil monkey called Proteropithecia neuquenensis, found in the Collón Curá Formation in Argentina. This species is significant because it represents the youngest known primate fossil from Patagonia, marking the end of an era for these animals in that region. However, the picture of this creature was incomplete. The fossil record consisted mostly of isolated teeth, making it difficult to understand how its jaw worked or to confirm its exact place in the family tree of modern monkeys. Researchers suspected it was an early member of the pitheciine family, but without more complete specimens, the evidence was thin. Now, a team of researchers has uncovered seven new specimens from the same site, including the first-ever lower jawbone with teeth still in place, a complete canine tooth, and the first upper molars ever found for this genus. These new finds, combined with a fresh, precise measurement of the rock layers where they were found, have filled in the missing pieces of the puzzle.

The journey to these discoveries began at a canyon called Cañadón del Tordillo in the Neuquén Province of Argentina. The site is a rugged landscape of volcanic rock and ancient riverbeds. To understand the age of the fossils, the team first had to pin down the timeline of the rock layers. They collected samples from a thick layer of volcanic ash, known as the Pilcaniyeu Ignimbrite, which sits just below the layers containing the monkey fossils. By analyzing tiny crystals of zircon within this ash using advanced laser technology, they determined that the ash was deposited approximately 15 million years ago. This new date is slightly younger than previous estimates, providing a tighter and more accurate window for when these monkeys lived. It places them firmly in the Middle Miocene, just before the climate began to cool and dry out, which eventually led to the disappearance of primates from this southern latitude.

With the timeline established, the researchers turned their attention to the fossils themselves, which were found in the layers of sediment sitting directly on top of that volcanic ash. The most exciting find was a partial lower jawbone, or symphysis, which is the part of the jaw where the two halves meet at the front. This specimen was remarkable because it preserved the roots of the front teeth and the lower premolars in their original positions. By using high-resolution 3D scanning, the team was able to fit a complete lower canine tooth, found nearby, perfectly into the empty socket of the jaw. This allowed them to reconstruct the exact shape and orientation of the front of the monkey's mouth. They saw that the front teeth were tilted forward, a feature known as procumbency, and that the lower canine was large and projecting. These are classic traits of modern pitheciines, which use their front teeth to pry open hard shells.

The new jaw also revealed the first known lower premolar, a tooth that sits just behind the canine. This tooth was unusually large and projected outward, a specialized feature seen in modern seed-eating monkeys. The researchers also found the first upper molars for this species. These back teeth showed a subtle but important detail: the enamel on their surface was slightly wavy or crenulated. This texture helps in crushing hard seeds without breaking the tooth, a trait shared with living pitheciines. The combination of a forward-leaning jaw, a large canine, a specialized lower premolar, and wavy enamel on the back teeth strongly suggests that Proteropithecia was indeed a seed-eater, adapted to a diet of hard nuts and seeds just like its modern relatives.

The discovery also sheds light on the evolutionary history of these monkeys. For some time, scientists have debated whether certain ancient Patagonian monkeys were direct ancestors of today's pitheciines or if they were on a separate evolutionary branch. The new fossils show that Proteropithecia shares a very specific set of features with other ancient Patagonian monkeys, such as Soriacebus and Mazzonicebus. The researchers argue that these similarities are too complex to be a coincidence. Instead, the evidence supports the idea that these ancient monkeys were part of a long, continuous lineage that eventually gave rise to the living pitheciines we see today. The fact that Proteropithecia appears so late in the fossil record, with such advanced features, suggests that this group of monkeys had already established its unique way of life millions of years ago and persisted in Patagonia until the very end of the Miocene.

By bringing together a precise date for the rocks and a much more complete picture of the animal's anatomy, this study transforms our understanding of Patagonian primates. It confirms that the specialized adaptation for eating hard seeds was present in South America much earlier and for a longer duration than previously thought. The new fossils do not just add a few more teeth to a collection; they provide the first clear view of how the front of the jaw was built and how the teeth fit together. This allows scientists to see the animal not just as a set of isolated parts, but as a functional whole, capable of cracking the hardest seeds in a changing world. The story of Proteropithecia is now clearer: it was a resilient, specialized monkey that thrived in the southern forests of the Middle Miocene, carrying the evolutionary torch for a lineage that survives to this day.