Mammals and dinosaurs coexisted on Earth until a catastrophic event 66 million years ago killed 75% of life on the planet . Despite the devastation, some animals survived, including rodent-like mammals in the Cimolodon genus. These creatures are part of the multituberculates , a group that arose during the Jurassic Period and survived over 100 million years before going extinct. Studying these animals helps researchers better understand how mammals survived the mass extinction event and then diversified into the variety of mammals around today.
A research team led by the University of Washington has identified a new species in the Cimolodon genus from a fossil the team discovered at a research site in Baja California. The researchers estimate that this fossil is about 75 million years old. The new species, named Cimolodon desosai , was about the size of a golden hamster, the researchers said. It likely scampered on the ground and in trees and ate fruits and insects.
The researchers published these findings April 22 in the Journal of Vertebrate Paleontology .
"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."
When Wilson Mantilla and his team discovered the fossil in 2009, they found 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."
The fact that the researchers uncovered more than just teeth for C. desosai means that they can better understand its size and shape and how it likely moved. It also helps fill out the picture of this genus and the habitat in which it lived, and contributes to a better understanding of the multituberculate group in general.
The researchers used digital imaging and a tool called micro-computed tomography, or micro-CT, to get high resolution images of the fossil. Then the team compared the teeth of C. desosai to those of its cousins in the Cimolodon genus to establish it as 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."
The team named this species after Michael de Sosa VI, the field assistant who first found it, because de Sosa died while they were still analyzing the fossil.
"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."
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.
For more information, contact Wilson Mantilla at gpwilson@uw.edu .
Journal of Vertebrate Paleontology
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
22-Apr-2026