Earliest Complete Fossil of World's Longest Lasting Mammal Uncovered

Researchers have uncovered a 160 million-year-old fossil of the most successful order of mammals the world has ever seen, making it the earliest complete remains of the creature ever discovered, according to a study published in Science. 

Known as multituberculates, the animals are characterized by tiny bumps on their back teeth and occupied an environmental niche similar to today's rodents. In all, fossil records reveal they walked the Earth between 170 million and 35 million years ago, living alongside and ultimately outliving the dinosaurs. The latest fossil, according to researchers, dates back 160 million years.

However, despite their overwhelming success, fossils of the animal tend to be fragmented.

"Now we finally have a compelte skeletal fossil that allows us to paint a coherent picture of the evolutionary origin of these prolific and important ancient mammals," Zhe-Xi Luo, a professor of organismal biology and anatomy at the University of Chicago, said of the find.

Luo and his team discovered the fossil in the Liaoning Province of China. From the animal Rugosodon eurasiaticus, the remains reveal key insights into the adaptations that allowed multituberculates to go on to claim the title as most abundant mammal during the Mesozoic Era.

For example, the remains reveal surprisingly mobile and flexible ankle bones, suggesting it was fast on its feet and agile, probably spending most of its time on the ground. According to the scientists, this feature may also represent the anatomical basis for the diverse adaptations seen in later multituberculates, including tree climbing and tunnel digging.

Dental features, meanwhile, point toward a versatile omnivore that fed on leaves and seeds as well as worms and insects, indicating, interestingly enough, that the later rise of numerous herbivorous multituberculates came from an omnivorous ancestor.

"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 adaptations, 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."