An illustration of Cimolodon desosai on the tree with a fruit in its mouth. It was about the size of a golden hamster. It likely scampered on the ground and in the trees and ate fruits and insects.
Photo: Andrey Atuchin.
Although the catastrophe at the end of the Cretaceous exterminated about 75% of life on Earth, including all the non-avian dinosaurs, some small mammals survived. Among them were rodent-like multituberculates: not rodents, but an extinct and highly successful group of mammals that had already lived alongside dinosaurs for more than 100 million years. One newly identified member of that group was a small mammal, about the size of a golden hamster, described in a recent paper published in the Journal of Vertebrate Paleontology.
The people who made up the creation myths in the Bible could have known nothing of this, of course. As their stories show, theirs was a narrow, parochial view of the world, centred on a small part of the Middle East and on the folk history of one people. Their modern legacy is a movement of equally incurious believers who treat those ancient stories as real history and their creation fantasy as real science. Consequently, the evidence revealed by palaeontologists is almost invariably too awkward for them to acknowledge honestly.
Sadly, creationists continue trying to boost their dwindling numbers, and soothe their fragile egos, by demanding the right to indoctrinate children at public expense. Behind that demand lies the curious belief that falsehoods become less false if enough people can be persuaded to believe them. The facts themselves are of little consequence. What matters is recruitment: encouraging ignorance, anti-science conspiracism and belief in magic, while leaving children ill-equipped to evaluate evidence, understand complex data, or perhaps become the scientists society will need in the future.
Nevertheless, the facts remain what they are, regardless of who accepts them. In this case, a research team led by the University of Washington has identified a new 75-million-year-old species of Cimolodon from a fossil discovered at a research site in Baja California, Mexico. The species, named Cimolodon desosai, was about the size of a golden hamster and probably scampered both on the ground and in trees, feeding on fruit and insects.
Background^ Multituberculates. Multituberculates were an extinct group of rodent-like mammals that lived alongside dinosaurs and survived the end-Cretaceous mass extinction, only to disappear later, leaving no living descendants. They were not true rodents — rodents evolved much later — but many multituberculates occupied similar ecological roles, scampering through trees, burrowing, feeding on plant material, fruit, seeds and insects, and exploiting the sorts of small-animal niches that helped some mammals survive in a world dominated by dinosaurs. The University of California Museum of Paleontology describes them as the only major branch of mammals to have become completely extinct, with no living descendants. [1]How the team made the discovery, and why it matters for understanding how some early mammals survived the end-Cretaceous mass extinction before later diversifying, is explained in a University of Washington news item by Sarah McQuate.
Their name means “many tubercles”, referring to the rows of small cusps on their cheek teeth. These teeth are often the main fossils by which they are identified, because small mammal skeletons are rarely preserved complete. The distinctive teeth made them efficient processors of plant material, and their fossil record shows a wide range of body sizes and lifestyles, from tiny mouse-sized animals to beaver-sized forms, including burrowers and squirrel-like tree climbers. [1]
Multituberculates were among the most successful mammals of the Mesozoic and early Cenozoic. They ranged from the Jurassic into the Late Eocene, were geographically widespread, and are known from most major landmasses, though the southern-hemisphere record is more uncertain. Their long history makes them a useful reminder that mammal evolution was not a straight line leading inevitably to modern placentals, marsupials and monotremes, but a branching bush containing whole groups that flourished for tens of millions of years and then vanished. [2]
The newly described Cimolodon desosai from Baja California belongs to this lost mammalian radiation. According to the University of Washington team, it was about the size of a golden hamster, probably moved both on the ground and in trees, and ate fruit and insects. Its small size and omnivorous diet are precisely the kinds of traits that may have helped related mammals survive the ecological chaos after the asteroid impact 66 million years ago. [3]
Multituberculates were not ancestors waiting politely in the wings for dinosaurs to disappear; they were already successful mammals, living, adapting and diversifying in the dinosaur world itself.
Researchers discover the fossil of a new hamster-sized mammal that lived alongside dinosaurs on the Pacific Coast
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 [2026] 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. 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.
Professor P. Gregory Wilson Mantilla, senior author
Department of Biology and Burke Museum of Natural History and Culture
University of Washington
Seattle, Washington WA, U.S.A.
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. 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.
Professor P. Gregory Wilson Mantilla.
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. If you find a skeleton that’s missing teeth, sometimes it’s hard to attach it to a name.
Professor P. Gregory Wilson Mantilla.
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. It’s a great specimen to be associated with
Professor P. Gregory Wilson Mantilla.
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.
Publication:
The significance of this discovery is not merely that another fossil mammal has been added to the catalogue of life from the age of dinosaurs. It is that Cimolodon desosai fits into a much larger and already well-supported picture: mammals were not sudden arrivals, conjured into existence in their modern forms, but members of ancient, branching lineages that had been evolving, diversifying and adapting long before the asteroid impact ended the reign of the non-avian dinosaurs.
Multituberculates, in particular, make nonsense of the simplistic creationist caricature of evolution. They were not “half-formed” mammals, nor failed experiments waiting for their inevitable replacement. They were successful, specialised animals that occupied real ecological niches for more than 100 million years. Some lived in trees, some probably burrowed, some ate insects, fruit or seeds, and some survived one of the greatest mass extinctions in Earth’s history before eventually disappearing without leaving living descendants. That is exactly what evolution predicts: not a tidy ladder of progress, but a branching, contingent history shaped by adaptation, extinction and chance.
For creationists, however, the problem remains the same as always: the evidence is not arranged to suit their mythology. A 75-million-year-old mammal from Baja California, belonging to an extinct lineage that lived alongside dinosaurs and helps explain the survival and later radiation of mammals, has no sensible place in a Bronze Age tale of recent special creation. It can only be forced into that story by ignoring the dates, ignoring the relationships, ignoring the geology and ignoring the wider pattern of the fossil record.
Science, by contrast, does not need to force the evidence to fit a sacred conclusion. It follows the evidence wherever it leads — in this case, to a small, hamster-sized mammal in Cretaceous Baja California, and from there to a deeper understanding of how mammals endured catastrophe and later came to dominate many of the ecological spaces left vacant by the dinosaurs. Once again, the fossils tell a story far richer, older and more interesting than creationism’s childish little myth.
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