Big 'groundhog' fossil from age of dinosaurs rewrites mammal history

There is no doubt scientists had a lot of luck on their side in discovering a critter from the age of dinosaurs that rewrites our understanding of the history of early mammals.

The researchers said on Wednesday they unearthed in Madagascar the fossil of a remarkable creature resembling a big groundhog that lived about 66 million years ago and, at about 9 kilograms (20 pounds), was enormous compared to most other mammals of the Mesozoic Era.

Judging from a wonderfully preserved skull with a bizarre set of features, it was an active plant eater with strong jaws, keen sense of smell, well-developed hearing and terrific eyesight under low light conditions, they said.

It is named Vintana sertichi. Vintana means luck in the Malagasy language, referring to the fortuitous circumstances behind how it was found.

During 2010 excavations in Madagascar, the researchers collected a 58-kilogram (150-pound) block of sandstone chock-full of fish fossils. They used a computerized tomography (CT) scan at Stony Brook University in New York state to peer inside. As luck would have it, they saw more than just fish.

"We were astounded to see a mammal skull staring back at us on the screen," said Stony Brook University paleontologist David Krause, who led the study published in the journal Nature.

• Summary of the paper in Nature

"It was dawning on me that I was experiencing the most incredible bit of luck I had ever been part of," added Joe Groenke, Krause's technician and the first to view the CT images.

Groenke spent half a year extracting the 12.5-centimetre (5-inch) long skull from the sandstone, one sand grain at a time.

Krause called Vintana the second-largest mammal known from the age of dinosaurs, when most mammals were shrew-sized, behind only the badger-like Repenomamus from earlier in the Cretaceous Period. Vintana lived immediately before the dinosaurs were wiped out by an asteroid that struck Earth, a disaster that paved the way for mammals to dominate the land.

Vintana is a member of a poorly understood group of primitive mammals called gondwanatherians that lived on the southern supercontinent of Gondwana and until now was known only from isolated teeth and jaw fragments.

Its well-preserved skull, though lacking the lower jaw, provided the first good evidence of this group's lifestyle and its relationships to other early mammals.

The scientists determined that gondwanatherians were closely related to multituberculates, a group of rodent-like Mesozoic mammals that thrived on the northern continents, as well as to another obscure group called the haramiyidans.

Shakes the family tree

"In essence, it really shakes up the early mammalian family tree and helps to reorganize it," Krause said.

Connecting the three groups also enables scientists to estimate the appearance of Earth's first true mammals at well before 200 million years ago, Krause said.

Its skull boasts a combination of primitive features and advanced ones like a cheek bone with a dagger-like feature for attachment of massive chewing muscles like in an Ice Age giant ground sloth. Its down-turned snout resembles a walrus. It has huge eye-sockets, a big nasal cavity and fairly small braincase.

"Throw together some anatomical features from ancient mammal-like reptiles, Pleistocene ground sloths, an extant rodent and maybe a few bits and pieces from the Muppets on 'Sesame Street' and you might get something that resembles the cranium of Vintana," Krause said.

It is only distantly related to today's mammals and was not a member of any of the three existing groups: placentals, marsupials and monotremes. "It is one of those evolutionary experiments in 'mammalness' that did not make it," Krause said.

The closest modern comparison to Vintana is a large, semi-aquatic South American rodent called a nutria, Krause said.

Zhe-Xi Luo, a University of Chicago expert on early mammalian evolution who reviewed the study, called Vintana the "discovery of the decade for understanding the deep history of mammals."

"This mammal helps to stretch our imagination of what is possible by evolution beyond our stereotypes from the extant mammals," Luo said. "Early mammal history is our own history - that is why a discovery of this kind is important, because it may prompt us to re-think our own evolutionary past."