This tiny dinosaur-era mammal may hold the secret to surviving Earth’s worst extinction.
Mammals lived alongside dinosaurs for millions of years until a catastrophic event 66 million years ago wiped out about 75% of life on Earth. Even after this mass extinction, some species endured. Among the survivors were small, rodent-like mammals from the genus Cimolodon. These animals belonged to a group known as multituberculates, which first appeared during the Jurassic Period and survived for more than 100 million years before eventually disappearing. By studying them, scientists gain insight into how early mammals made it through the extinction and later evolved into the wide range of species seen today.
New Species Found in Baja California
Researchers led by the University of Washington have identified a previously unknown species within this genus using a fossil discovered at a site in Baja California. The fossil is estimated to be about 75 million years old. The new species, named Cimolodon desosai, was roughly the size of a golden hamster. Scientists believe it moved both on the ground and in trees and likely fed on fruits and insects.
The findings were published April 22 in the Journal of Vertebrate Paleontology.
A Common Mammal With Survival Advantages
“The genus Cimolodon was a pretty common mammal during the Late Cretaceous, the last epoch of the Age of Dinosaurs. Cimolodon fossils have been found throughout western North America, from western Canada down through Mexico,” said senior author Gregory Wilson Mantilla, a UW professor of biology and curator of vertebrate paleontology at the Burke Museum. “This new species, Cimolodon desosai, was ancestral to the species that survived the extinction event. It and its descendants were relatively small and omnivorous — two traits that were advantageous for surviving.”
Rare Fossil Provides More Complete Picture
The fossil was first discovered in 2009 by Wilson Mantilla and his team. Unlike many similar finds, which often consist only of teeth, this specimen included a broader set of remains. The researchers recovered teeth, a skull, jaws, and parts of the skeleton, including a femur and an ulna.
“It’s very hard to find fossils at this site compared to other areas,” Wilson Mantilla said. “At first, my field assistant found just a little tooth poking out. If he had just found that, I would have been over the moon. But then when we looked inside the crack of the rock, we could see there was more bone.”
Because the team uncovered more than just teeth, they were able to better estimate the animal’s size, body structure, and movement. These details also provide a clearer picture of the environment it lived in and improve understanding of the multituberculate group overall.
Advanced Imaging and Identification
To study the fossil in detail, the researchers used digital imaging and micro-computed tomography, or micro-CT, to produce high-resolution images. They then compared the teeth of C. desosai with those of related species in the Cimolodon genus to confirm it represented a new species.
“That far back in time everything is named based on their tooth characteristics,” Wilson Mantilla said. “If you find a skeleton that’s missing teeth, sometimes it’s hard to attach it to a name.”
Naming the Species and Acknowledgments
The new species was named after Michael de Sosa VI, the field assistant who first noticed the fossil. De Sosa died while the team was still analyzing the specimen.
“He was a great field assistant, and he was like a little brother to me,” Wilson Mantilla said. “It’s a great specimen to be associated with.”
Reference: “Cranial and postcranial remains of a new species of Cimolodon (Mammalia, Multituberculata, Cimolodontidae) from the Upper Cretaceous (Campanian) El Gallo Formation of Baja California, México” by Gregory P. Wilson Mantilla, Isiah R. Newbins, David E. Fastovsky, Yue Zhang, Marisol Montellano-Ballesteros, Dalia García Alcántara and Meng Chen, 22 April 2026, Journal of Vertebrate Paleontology.
DOI: 10.1080/02724634.2026.2641109
Additional co-authors are Isiah Newbins, UW doctoral student in biology, David Fastovsky at the University of Rhode Island; Yue Zhang, who completed this research as a UW postdoctoral fellow in biology; Meng Chen, who completed this research as a UW doctoral student in biology; and Marisol Montellano-Ballesteros and Dalia García Alcántara at the Universidad Nacional Autónoma de México.
This research was funded by UC MEXUS-CONACYT, Dirección General de Asuntos del Personal Académico PAPIIT IN111209-2, the UW College of Arts and Sciences, the UW Department of Biology and the American Philosophical Society.
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