Key Facts
- 13.3-million–13.6 million-year-old fossils from an extinct relative of the howler monkey show signs of being the first leaf-eating primate in South and Central America.
- Fossils from jaws of Stirtonia victoriae may have space for a specialized bone that may have acted as a resonating chamber, indicating how the modern howler monkey got its eponymous "howl."
- Analysis of jaw fossils indicates how modern monkeys from biodiverse Amazonian basin began differentiating from one another millions of years ago.
Thirteen million years ago, a medium-sized monkey known for guarding its territory among the treetops with a fearsome "howl" started doing something new. This monkey, one of the oldest known ancestors of the modern howler monkey, started eating leaves, causing them to evolve a larger body size and differentiate themselves from other primates, says a team of researchers led by a scientist at Johns Hopkins Medicine.
"Before the discovery ofStirtonia, we didn't have any evidence of leaf consumption in South American primates," saysSiobhán Cooke, MPhil, Ph.D., associate professor in the Center for Functional Anatomy and Evolution at the Johns Hopkins University School of Medicine. "This can help us answer important questions about ecological evolution in one of the most biodiverse areas of the planet. What new things were occurring in the development of the proto-Amazonian rainforest when these monkeys were alive?"
A report of the findings, led by Cooke, was published March 2 inPaleoAnthropology. In the paper, scientists analyzed two mandible fossils from theStirtonia victoriae species, an ancient relative of the modern howler monkey.
The findings pinpoint a timeframe for the monkey's leaf-eating ability, expanding its diet beyond fruit, letting them grow larger in size and easing up competition for food among these ancient howler monkeys and other species of primates, say the researchers.
Cooke's collaborators, brothers Andrés and Rubén Vanegas, who lead a paleontological museum in Colombia, South America, first uncovered the specimens in 2016 from a fossil-rich site known as La Venta, in the Tatacoa desert of Colombia. In a prior study published in 2024, a team of researchers including Cooke analyzed the fossil of a terror bird found in the same desert.
Millions of years ago, the ancient monkeys climbed from tree to tree in what is now the Tatacoa desert, but which was once full of swampy grass, forests and riverbanks, Cooke says. They lived among the Amazon basin's long-extinct fauna, among them giant sloths and huge armored armadillos.
Previously, researchers had few other fossil finds from Stirtonia victoriae to learn from, relying only on a few fragments of bone from their face and head, Cooke says.
In addition to providing key information about biodiversity, diet and body size of these monkeys, the most recent discoveries may also reveal when the howler monkey acquired its eponymous "howl" as the loudest land mammal.
"The jaw had a wide, deep mandibular body, which may have allowed for the hyoid neck bone to balloon the same way as modern howler monkeys, resulting in their iconic call," Cooke says. "However, we cannot be certain that they engaged in this behavior."
Finding an ecological niche
Led by Cooke, the team of scientists scanned the jaw fossils and created a 3D model of the bones to analyze them. From the structure of the lower molars, the researchers can determine whatStirtonia victoriae ate, how large they were and how this species differentiated themselves from the other primate families living as the Amazonian basin was forming.
Cooke worked with collaborator Ryan Knigge, a researcher at University of Minnesota Medical School, who created a dataset of information on the jaws of platyrrhines (monkeys that live in South America, Central America and Mexico). With this information, scientists compared theStirtonia victoriae fossils to 3D model jaws of other South American primates that lived during that time, including the smallerStirtonia tatacoensis species, another howler monkey ancestor. The scientists also took a close look at the jaws of modern howler monkeys and their close-living ancestors, including rainforest-dwelling spider monkeys and woolly monkeys.
Like modern howler monkeys, Stirtonia victoriae had relatively large molars with protrusions that act as "shears" when they grind against one another to help break down carbohydrates, which are adaptations scientists typically see in leaf-eating primates, Cooke says.
The researchers also reconstructed the body mass of Stirtonia victoriae and determined that these monkeys weighed between 17 and 22 pounds, Cooke says.
"Prior to this, the South American monkeys we have in the fossil record are much smaller," Cooke says. "This tells us that these monkeys had a plentiful food source, mainly leaves, that allowed them for the first time to evolve and occupy an ecological niche that allows for larger body mass."
Stirtonia victoriae marks the very beginning of a large, diverse group of primates in South America, Cooke says.
"Now, we can begin to pinpoint when different modern lineages started to evolve," she says.
In addition to Cooke, Knigge and Andrés Vanegas, other researchers include Savannah Cobb from the Johns Hopkins University School of Medicine, Melissa Tallman from the NYCEP Morphometrics Group, Laura Stroik from Grand Valley State University, Brian Shearer from University of California, San Francisco, Stephanie Palmer from Lake Erie College of Osteopathic Medicine, Zana Sims from University of Minnesota, and Luis Ortiz-Pabón and Andrés Link From Universidad de los Andes.
This work is supported by the Leakey Foundation, the National Geographic Waitt Grants Program, the Grand Valley State University Center for Scholarly and Creative Excellence, the National Science Foundation and the University of Minnesota Graduate School.
