Ancient Howler Monkey Relative Was South America's First Known Leaf-Eating Primate

Fossils dating back 13 million years indicate that an ancient relative of the modern howler monkey was the first known leaf-eating primate in South and Central America.

This dietary shift led to an increase in body size and diversification from other primate species, according to a research team led by Johns Hopkins Medicine scientists.

Siobhán Cooke, an associate professor at Johns Hopkins University School of Medicine and lead researcher, stated that prior to this discovery, there was no evidence of leaf consumption among South American primates.

She noted that this finding could contribute to understanding ecological evolution within the Amazonian rainforest.

Unearthing Stirtonia victoriae

The findings, published on March 2 in PaleoAnthropology, detail the analysis of two mandible fossils from the Stirtonia victoriae species, an ancient howler monkey relative. Researchers indicate that the discovery establishes a timeline for this monkey's transition to a leaf-based diet, enabling larger body sizes and reducing competition for fruit among primates.

The fossils were initially discovered in 2016 at La Venta, a site in Colombia's Tatacoa desert, by Andrés and Rubén Vanegas. This location was previously a swampy forest and riverbank environment inhabited by diverse ancient fauna, including giant sloths and armored armadillos.

Limited fossil evidence for Stirtonia victoriae existed previously, consisting only of fragments from the face and head.

Clues to the Howler's Call and Evolution

The findings may also offer insights into the development of the howler monkey's distinctive 'howl.' The jaw structure of Stirtonia victoriae, characterized by a wide, deep mandibular body, suggests the potential for a hyoid neck bone similar to modern howler monkeys, which facilitates their calls. However, researchers state that it cannot be definitively confirmed if this behavior was present.

The research team utilized 3D modeling of the jaw fossils to analyze their structure, particularly the lower molars. This analysis informed their understanding of Stirtonia victoriae's diet, size, and its differentiation from other primate species during the Amazonian basin's formation.

Comparisons were made between Stirtonia victoriae fossils and other South American primate jaws, including those of modern howler, spider, and woolly monkeys, using a dataset compiled by Ryan Knigge. Similar to modern leaf-eating primates, Stirtonia victoriae exhibited large molars with shearing protrusions, adapted for breaking down carbohydrates from leaves.

Researchers estimated the body mass of Stirtonia victoriae to be between 17 and 22 pounds.

Broader Ecological Impact

This discovery suggests that the availability of leaves as a food source allowed Stirtonia victoriae to evolve a larger body mass and occupy a new ecological niche, distinguishing them from previously smaller South American monkeys in the fossil record.

Stirtonia victoriae is considered an early indicator of the diversification of primate lineages in South America.