A mouse-sized fossil mammal named Yeutherium Pressor was unearthed in Chilean Patagonia. The animal lived about 74 million years ago, sharing its habitat with the last non-avian dinosaurs until their extinction 66 million years ago.
Preliminary examination of its teeth showed pronounced ridges and cusps suited to cracking tough vegetation, leading researchers to infer a diet of hard plant material. Weighing an estimated 30–40 grams, Yeutherium stood as the smallest mammal yet recorded from South America during the Late Cretaceous, when the continent remained part of Gondwana.
The fossil emerged from the Río de las Chinas Valley in Chile’s Magallanes region, roughly 3,000 kilometers south of Santiago. A team from the University of Chile’s Paleontological Network, led by Hans Püschel and Alexander Vargas, removed a rock fragment that exposed a jaw section containing one complete molar and the roots of two others. The specimen displayed enough anatomical detail to place the species on an early mammalian branch that combined primitive and specialized traits.
“Finding a mammal from the era of the dinosaurs—and particularly in South America, where everything is quite unknown—makes it a very important discovery,” said Püschel, according to Semana. Mesozoic mammal material in the Southern Hemisphere generally consists of isolated teeth and bones, so an identifiable jaw offers rare insights.
The animal probably laid eggs like a modern platypus or carried young in a pouch, possibilities consistent with monotremes or early marsupials. Microscopic study revealed rounded, crenulated crests on the molars. “It has a dentition that resembles an orange juicer, with rounded and crenulated crests that would have served to better crush the food because they probably had a relatively hard diet in plant matter,” said Püschel.
Because the dental characters lie between generalized insect-eating teeth and the specialized grinders of later herbivores, Yeutherium could clarify the evolutionary shift toward plant-based diets. “The fossil allows us to understand what dental traits characterize this family and how that specialized morphology for crushing was reached,” said Püschel.
The Río de las Chinas site has yielded fish, reptiles, and dinosaur fragments, but mammals had remained elusive. Fieldwork scheduled for the next austral summer aims to uncover additional skeletal pieces that may confirm whether the creature laid eggs or reared pouched offspring.