Mammals once shared the planet with dinosaurs until a catastrophic event 66 million years ago wiped out about 75% of all life on Earth. Even so, some species managed to survive. Among them were small, rodent-like mammals belonging to the genus Cimolodon. These animals were part of the multituberculates, a long-lived group that first appeared during the Jurassic Period and persisted for more than 100 million years before eventually going extinct. By studying them, scientists can better understand how early mammals endured the mass extinction and later evolved into the diverse forms we see today.
A team led by researchers at the University of Washington has identified a new species within this genus based on a fossil uncovered at a site in Baja California. The fossil is estimated to be around 75 million years old. The newly named species, Cimolodon desosai, was roughly the size of a golden hamster. According to the researchers, it likely moved both on the ground and in trees and had a diet that included fruits and insects.
The findings were published 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.”
Fossil Discovery Reveals Rare Skeletal Details
The fossil was first uncovered in 2009 by Wilson Mantilla and his team. Unlike many similar finds, which often consist only of teeth, this specimen included a broader range of remains. The team 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.”
Having more than just teeth allowed researchers to estimate the animal’s size, body structure, and likely movement. These details also help scientists better reconstruct the environment it lived in and expand knowledge of multituberculates as a whole.
Advanced Imaging Helps Identify New Species
To analyze the fossil, the team used digital imaging along with micro-computed tomography, or micro-CT, which produces highly detailed images. They then compared the teeth of C. desosai with those of related species in the Cimolodon genus to confirm it was distinct.
“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.”
Honoring the Fossil’s Discoverer
The species was named in honor of Michael de Sosa VI, the field assistant who first spotted the fossil. De Sosa passed away while the research team was still studying 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.”
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.
