Earliest complete fossil from rodent-like ancient mammal discovered

Kevin Jiang
Science Writer and Media Relations SpecialistUniversity of Chicago Medicine and Biological Sciences

Flexible ankles and versatile, ridged teeth were the key adaptations that allowed a group of rodent-like ancient creatures to become the most successful ancient mammals. UChicago scientists in the Aug. 16 issue of Science revealed the discovery of a 160 million-year-old fossil—the earliest known complete skeleton of a multituberculate—named Rugosodon eurasiaticus, a fast-running, agile omnivore similar to a modern-day African dormouse.

While the extinction of the dinosaurs 66 million years ago marked the beginning of the age of the mammals, early mammalian species were flourishing long before. Of these ancestral groups, the most prolific were the multituberculates, mammals characterized by numerous tiny bumps or cusps on their back teeth. Occupying similar environmental niches to modern rodents, multituberculates possessed a continuous fossil record from about 170 million years ago until their extinction about 35 million years ago. They lived and thrived alongside the dinosaurs for more than 100 million years, and outlived them by another 30 million years.

“Despite the success of the multituberculates, the earliest fossils are very fragmentary. Now we finally have a complete skeletal fossil that allows us to paint a coherent picture of the evolutionary origin of these prolific and important ancient mammals,” said Zhe-Xi Luo, professor of organismal biology and anatomy.

Discovered by Luo and his team in the Liaoning Province of China, the fossil of Rugosodon eurasiaticus reveals key adaptations that allowed multituberculates to be the most abundant mammal during the Mesozoic Era.

Of particular note are its surprisingly mobile and flexible ankle bones, which suggest that Rugosodon was a fast-running and agile mammal that mostly lived on the ground. This skeletal feature is thought to be the anatomical basis for the versatile and diverse locomotor adaptations, ranging from tree climbing to tunnel digging, which later multituberculates possessed.

Rugosodon draws its name from its rugose teeth, which were ornamented by numerous ridges and wrinkles. Its dental features are evidence that it was a versatile omnivore that fed on the leaves and seeds of gymnosperm plants and ferns, as well as worms and insects. Interestingly, this indicates that the later rise of numerous herbivorous multituberculates originated from an omnivorous ancestor.

Likely a nocturnal mammal, Rugosodon’s estimated body mass and feeding habits are analogous to a modern-day African dormouse, and it is thought to have lived on lakeshores in what is now western Liaoning Province. Scattered fossil teeth of multituberculates similar to Rugosodon have been discovered as far away as Western Europe. Thus, this new fossil provides fresh evidence that mammals in Europe and Asia had extensive interchanges during the Jurassic Era.

“Essentially, multituberculates were the first important mammal group to occupy an herbivorous niche. They were able to exploit a part of the ecosystem that was not accessible to many other vertebrates, including other Mesozoic mammals,” Luo said. “This superb feeding function, together with versatile locomotor adaptions, explains why multituberculates were so successful and diverse. Now we have a sense of what they started off with, thanks to the discovery of Rugosodon.”

The fossil of Rugosodon, now deposited in the collections of the Beijing Museum of Natural History, was studied by scientists from Chinese Academy of Geological Sciences, Beijing Museum of Natural History and the University of Chicago. The research was supported by the Ministry of Science and Technology of China, Ministry of Land Resources of China, Beijing Municipal Science & Technology Commission, the National Science Foundation, and the University of Chicago.