“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.

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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.