The examination of fossilized mandibles belonging to the extinct primate species Stirtonia victoriae, unearthed from Colombia’s La Victoria Formation, provides compelling evidence suggesting that an evolutionary shift towards folivory, or leaf consumption, facilitated the growth in size and diversification into novel ecological roles for early South American primates. Furthermore, these discoveries may offer insights into the temporal emergence of the anatomical characteristics that subsequently enabled the formidable vocalizations of present-day howler monkeys.
The mantled howler (Alouatta palliata) in Panama. Image credit: Ariel Rodriguez-Vargas / CC BY 4.0.
The primate known as Stirtonia victoriae inhabited the region now recognized as Colombia during the Miocene epoch, approximately 13 million years in the past.
“Prior to the identification of Stirtonia victoriae, tangible evidence of frugivorous diets among South American primates was notably absent,” stated Dr. Siobhán Cooke, a researcher affiliated with Johns Hopkins University.
“This finding holds significant potential for elucidating critical aspects of ecological evolution within one of Earth’s most biologically rich locales.”
“What novel developmental processes were unfolding in the nascent Amazonian rainforest ecosystem during the period these simians existed?”
In the course of their recent investigation, Dr. Cooke and her associates meticulously analyzed two fossilized mandibular specimens of Stirtonia victoriae recovered from the La Victoria Formation, situated in Colombia’s Tatacoa desert.
These findings precisely delineate a period during which this primate species acquired the capacity for leaf consumption. This dietary expansion, extending beyond frugivory, permitted an increase in their physical stature and alleviated competitive pressures for sustenance between these ancient howler monkey relatives and other primate taxa.
“Millennia ago, these ancestral primates navigated the arboreal landscape of what is presently the Tatacoa desert, an area then characterized by marshy grasslands, dense woodlands, and riverine environments,” Dr. Cooke remarked.
Their contemporary fauna included long-extinct members of the Amazonian basin’s biodiversity, such as colossal ground sloths and immense, armored armadillos.
“Previously, our knowledge of Stirtonia victoriae was constrained by a scarcity of fossil discoveries, primarily limited to a few cranial and facial bone fragments,” Dr. Cooke elaborated.
“Beyond contributing crucial data on the biodiversity, dietary habits, and body mass of these extinct primates, these recent discoveries may shed light on the evolutionary juncture at which the howler monkey attained its characteristic, sonorous vocalizations, renowned as the loudest among terrestrial mammals.”
“The mandibular structure featured a broad, deep corpus, a characteristic that might have supported the expansion of the hyoid bone, analogous to the inflationary mechanism observed in modern howler monkeys, thereby producing their iconic calls.”
“Nevertheless, definitive confirmation of such vocal behavior remains elusive.”
The paleontological team employed advanced scanning technologies to generate three-dimensional digital reconstructions of the mandibular fossils, facilitating detailed analysis.
By scrutinizing the morphology of the lower molars, researchers were able to ascertain the dietary preferences of Stirtonia victoriae, estimate their size, and understand how this species diverged from other primate lineages that coexisted during the formative stages of the Amazonian basin.
The fossilized mandibles of Stirtonia victoriae were juxtaposed with three-dimensional models of mandibles from other South American primates of the same era, including the smaller Stirtonia tatacoensis, another progenitor of howler monkeys.
Additionally, contemporary howler monkey mandibles and those of their closely related extant ancestors, such as the arboreal spider monkeys and woolly monkeys dwelling in rainforest environments, were subjected to rigorous examination.
“Similar to extant howler monkeys, Stirtonia victoriae possessed comparatively substantial molars equipped with cuspal projections functioning as shearing surfaces during mastication, thereby aiding in the breakdown of complex carbohydrates – an adaptation typically associated with folivorous primates,” Dr. Cooke observed.
The research team also undertook reconstructions of the estimated body mass for Stirtonia victoriae, concluding that these primates likely weighed between 17 and 22 pounds (approximately 8–10 kg).
“Prior to this finding, the fossil record of South American monkeys indicates significantly smaller body sizes,” Dr. Cooke pointed out.
“This suggests that these primates benefited from an abundant food supply, predominantly leaves, which enabled them, for the first time, to evolve and occupy an ecological niche conducive to larger body mass.”
“The discovery of Stirtonia victoriae represents the incipient stage of a large and diverse primate radiation within South America.”
“We can now commence the process of identifying the evolutionary emergence of various modern lineages.”
The findings of this investigation are slated for publication in the journal PaleoAnthropology.
_____
Siobhán B. Cooke et al. 2026. Mandibular Specimens of Stirtonia victoriae from the La Victoria Formation, La Venta, Colombia. PaleoAnthropology 1: 148-170; doi: 10.48738/2026.iss1.3992
