Hamster-Sized Mammal Survived Dinosaur Age on Pacific Coast

A newly discovered fossil in Baja California, Mexico, reveals a new species of hamster-sized mammal that lived alongside dinosaurs approximately 75 million years ago. Named Cimolodon desosai, the creature is providing insights into how mammals survived the Cretaceous-Paleogene (K-Pg) extinction event that wiped out the non-avian dinosaurs.

The discovery, published April 22 in the Journal of Vertebrate Paleontology, highlights the adaptability of early mammals. Researchers from the University of Washington, who led the study, found a remarkably complete fossil including skull fragments, jaws, and limb bones – a femur and an ulna – at a research site in Baja California. Initial findings in 2009 revealed only a single tooth, but further excavation uncovered the more substantial remains.

A Common Genus During the Age of Dinosaurs

Cimolodon was a widespread genus during the Late Cretaceous period, with fossils previously found throughout western North America, ranging from western Canada to Mexico. “The genus Cimolodon was a pretty common mammal during the Late Cretaceous, the last epoch of the Age of Dinosaurs,” explained Gregory Wilson Mantilla, a UW professor of biology and curator of vertebrate paleontology at the Burke Museum. “Cimolodon fossils have been found throughout western North America, from western Canada down through Mexico.”

This new species, Cimolodon desosai, is considered ancestral to the species that ultimately survived the K-Pg extinction. Researchers believe its small size and omnivorous diet were key factors in its survival. “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,” Wilson Mantilla stated.

Survival Traits: Small Size and an Omnivorous Diet

The K-Pg extinction event, triggered by a catastrophic asteroid impact 66 million years ago, resulted in the demise of approximately 75% of life on Earth. However, some animals, including the multituberculates – the group to which Cimolodon belongs – managed to endure. Multituberculates originated during the Jurassic Period and persisted for over 100 million years before eventually going extinct.

Studying these ancient mammals provides valuable insights into the evolutionary strategies that allowed mammals to not only survive the extinction event but also diversify into the wide range of species seen today. The relatively small stature and varied diet of Cimolodon desosai likely allowed it to exploit resources unavailable to larger, more specialized dinosaurs.

The fossil site in Baja California is known for being challenging to excavate. Wilson Mantilla noted, “It’s very hard to find fossils at this site compared to other areas. 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 discovery of Cimolodon desosai adds another piece to the puzzle of understanding mammalian evolution and resilience in the face of global catastrophe. Further research on this and other multituberculate fossils will continue to illuminate the pathways that led to the rise of mammals in the post-dinosaur world.