Discoveries of the Fossilized Jaw of an Ancient Monkey Species Stiltonia victoriae unveil insights from Colombia’s La Victoria Formation, indicating that early primates in South America adapted to leaf consumption, which enabled them to grow larger and explore new ecological niches. This remarkable find may also provide clues about the timeline of when this lineage developed the anatomical traits responsible for the powerful howls seen in today’s howler monkeys.

The ancient primate Stiltonia victoriae thrived in what is now Colombia during the Miocene epoch, approximately 13 million years ago.

Dr. Siobhan Cook, a researcher from Johns Hopkins University, stated, “Prior to this discovery, there was no evidence indicating that South American primates consumed leaves.”

This research helps address crucial questions about ecological evolution in one of the Earth’s most biodiverse regions.

“What evolutionary changes occurred in the Amazon rainforest during the existence of these monkeys?”

In their recent study, Cook and colleagues investigated two fossilized mandibles of Stiltonia victoriae from Colombia’s La Victoria Formation in the Tatacoa Desert.

The findings indicate when this ancient monkey developed the ability to eat leaves, expanding its diet beyond fruit. This adaptation enabled it to grow larger and lessen food competition among howler monkeys and other primate species in ancient ecosystems.

“Millions of years ago, ancient monkeys traversed trees in what is now the Tatacoa Desert, once inhabited by wetland grasses, forests, and riverbanks,” said Dr. Cook.

These monkeys coexisted with long-extinct fauna in the Amazon basin, including giant sloths and armored armadillos.

“Before this, fossil findings were scarce. With Stiltonia victoriae, we could only glean knowledge from a few facial and cranial bone fragments,” Cook remarked.

“The latest discoveries not only shed light on their biodiversity and dietary habits but may also provide insight into when howler monkeys developed their distinctive ‘howl’, the loudest vocalization among land mammals.”

The structure of the jaws indicated a broad and deep mandibular body, which may have allowed the hyoid bone to protrude, similar to modern howler monkeys, potentially enabling their iconic calls.

“However, we are still uncertain about their exact behavior,” Dr. Cook added.

Paleontologists employed scans of the jaw fossils to create a 3D model for detailed analysis.

From the structure of the mandibular molars, researchers determined the dietary patterns, size, and distinguishing features of Stiltonia victoriae, comparing it against 3D models of other South American primate fossils, including Stiltonia tatakoensis, a known ancestor of howler monkeys.

They also closely examined the jaws of modern howler monkey ancestors and their relatives, such as spider monkeys and woolly gibbons residing in rainforests.

“Like modern howler monkeys, Stiltonia victoriae possessed relatively large molars with protrusions to act as ‘scissors’ for efficiently grinding carbohydrates, an adaptation common in leaf-eating primates,” said Dr. Cook.

Through their research, the body weight of Stiltonia victoriae was reconstructed, revealing these monkeys weighed between 17 and 22 pounds (8 to 10 kg).

Dr. Cook highlighted, “Previous South American monkeys in the fossil records were significantly smaller. This suggests that for the first time, these monkeys had access to abundant food sources, primarily leaves, enabling them to evolve into a heavier ecological niche.”

This discovery marks the emergence of a large and diverse group of primates in South America.

“We can now accurately trace the origins of various modern lineages.”

These findings will be published in the journal Paleoanthropology.

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Siobhan B. Cook et al. 2026. Mandibular specimen of Stiltonia victoriae from La Victoria Formation, La Venta, Colombia. Paleoanthropology 1: 148-170; doi: 10.48738/2026.iss1.3992