Big Discovery in a Tiny Mammal-Like Skull Found Under a Dinosaur’s Foot

Thursday, May 24, 2018

An artist’s rendering of Cifelliodon wahkarmoosuch. Its fossilized skull was found under a dinosaur’s foot by paleontologists in Utah.CreditJorge A. Gonzalez

In 2006 a team of paleontologists in Utah were examining the fossils of a large dinosaur when they discovered beneath its foot a tiny skull unlike anything they had seen in the area.

Now, scientists have found that the fossilized cranium belonged to an ancient relative of modern mammals that once scurried around North America some 130 million years ago. The new species, called Cifelliodon wahkarmoosuch, is a member of an extinct group of animals known as the haramiyids, which some researchers think bridged the transition between reptiles and mammals.

A reconstruction of a Cifelliodon skull made using computed tomography. Huttenlocker et al.

The finding, published Wednesday in the journal Nature, shows that haramiyids spread much farther across the globe and in a later time period than previously thought.

“When I first saw the specimen I was stunned,” said Simone Hoffmann a paleontologist from New York Institute of Technology who was not involved in the paper. Dr. Hoffmann, who wrote a perspective that accompanied the study, said she was surprised to see a haramiyid fossil among the thousands of dinosaur specimens found in North America from the Cretaceous period.

In the 1900s scientists had uncovered teeth and jawbones from haramiyids in parts of Eurasia that dated back to the Jurassic and Triassic periods, more than 145 million years ago. Then around 2014 and 2015, researchers found skeletons and soft tissue of haramiyids in China, sparking a debate about where the group belongs on the evolutionary tree. Some argue that its place is within the mammal family, while others have said that it exists just outside that classification.

“It’s a mystery group,” said Zhe-Xi Luo, a paleontologist at the University of Chicago and an author on the paper.

If haramiyids are mammals, then the group pushes back the birth of mammals to about 220 million years ago. But if they are not, mammals date to only about 185 million years ago. Dr. Luo said he and his colleagues place the haramiyids at the doorstep of mammals — close but just outside.

Scientists think haramiyids were diverse and possibly occupied similar ecological niches, eating insects, plants or meat. There is evidence that some of the creatures could glide like flying squirrels and others could swim. 

Adam Huttenlocker, a paleobiologist at the University of Southern California and lead author on the paper made the link that the fossil found in Utah belonged to a haramiyid. The team performed CT scanning of the skull, which showed that its brain was small, and it likely had a good sense of smell.

Though most furry creatures during the Mesozoic Era — which spanned the Triassic, Jurassic and Cretaceous periods — were about the size of a shrew or mouse, Cifelliodon was about as big as a rabbit or hare, and weighed about 2.5 pounds.

“For a Mesozoic proto-mammal it was pretty huge,” said James Kirkland, a paleontologist from the Utah Geological Survey who first found the skull. “It was like a godzilla proto-mammal.” 

The skull the team found only had one tooth, which they said was similar to a fruit-eating bat’s teeth. But that lonely tooth was important, said David Krause, a paleontologist from the Denver Museum of Nature and Science who co-wrote the accompanying perspective.

“That tooth is now largely responsible for extending the geographic range of the group to an entire new continent — North America — and to a significantly later time than is typical for haramiyidans,” said Dr. Krause.

Dr. Hoffmann added that despite the new finding, she is undecided on whether haramiyids belong inside or outside of the mammal group.

“We still need more fossils to tell that,” she said.