Prehistoric Fossil Discovery Reveals Details About Earth's Most Successful Mammal Lineage
August 16, 2013

Prehistoric Fossil Discovery Reveals Details About Earth’s Most Successful Mammal Lineage

Brett Smith for - Your Universe Online

The 160 million-year-old fossil of a newly described species has revealed new details about the most successful mammalian lineage in Earth’s history.

Multituberculates were a group of extremely diverse rodent-like mammals, ranging from tree dwellers to fastidious burrowers. They existed for about 120 million years before being out-competed into extinction by more modern mammals in the Oligocene epoch around 35 million years ago.

According to a report published in the journal Science, a fossil of the species being called Rugosodon eurasiaticus shows evidence of traits that allowed multituberculates to flourish, such as teeth that allowed for eating both plants and animals and ankle joints that made for easy rotation.

"The later multituberculates of the Cretaceous [era] and the Paleocene [epoch] are extremely functionally diverse: Some could jump, some could burrow, others could climb trees and many more lived on the ground," explained study co-author Zhe-Xi Luo, a biologist at the University of Chicago. "The tree-climbing multituberculates and the jumping multituberculates had the most interesting ankle bones, capable of 'hyper-back-rotation' of the hind feet."

Luo noted that these highly mobile ankle joints are normally associated with animals that are exclusively tree-dwellers.

"What is surprising about this discovery is that these ankle features were already present in Rugosodon—a land-dwelling mammal," he said.

Study researchers said R. eurasiaticus, which resembles a small rat or a chipmunk, could eat a variety of food. The animal’s fossilized teeth confirmed a 2012 study of tooth types that found multituberculates subsisted on a carnivorous diet for much of their existence and later converted to an herbivorous one. The evolution of both multituberculates’ diet and their ankle structure most likely led to their proliferation on Earth for around 100 million years, study researchers said.

The R. eurasiaticus fossil discovered by Yuan and his team was found in ancient lake sediments in eastern China, suggesting that the animal may have lived on the lake’s shoreline. Based on an analysis of the evolved ankle joints and teeth of this early multituberculate, the researchers said that such adaptations probably arose very early in the evolution of the order. These early evolutions set the stage for the major diversification of multituberculates that ensued.

The discovery of the fossil also expands the distribution of some multituberculates from Europe to Asia during the Late Jurassic period, the researchers said.

"This new fossil from eastern China is very similar to the Late Jurassic fossil teeth of multituberculates from Portugal in western Europe," Luo said. "This suggests that Rugosodon and its closely related multituberculates had a broad paleogreographic distribution and dispersals back-and-forth across the entire Eurasian continent [sic]."

While multituberculates became extinct during the Oligocene epoch around 23 million years ago, other land vertebrates grew into larger sizes than they had in previous epochs, many taking on the appearances we are familiar with today. Animals such as horses, rhinoceroses and camels learned how to run during the Oligocene and quickly began to dominate plains around the Earth. Marine life also began to take on a modern appearance, with the emergence of bivalves and whales.