A "bizarre" groundhog-like creature lived in the kingdom of the dinosaurs some 66 million years ago, according to a study Wednesday that rewrites the story of mammals.
Weighing some nine kilogrammes (20 pounds), Vintana sertichi is now the largest southern hemisphere mammal known to have existed in the Mesozoic era.
A "super heavyweight" compared to contemporary mammals that were no bigger than a mouse, the creature has a historic place in the book of life, the researchers said.
Its existence means that mammals -- which became the rulers of the planet after the dinosaurs were wiped out -- must have evolved millions of years earlier than thought, they said.
The 13-centimetre (five-inch) fossilised skull of the strange creature was discovered by chance in a 70 kg sandstone block that had been hauled from Madagascar to a New York lab.
"No palaeontologist could have come close to predicting the odd mix of anatomical features that this cranium exhibits," said David Krause of New York's Stony Brook University, who led the study published in Nature.
About twice the size of a modern-day groundhog, Vintana had rodent-like incisors and wear-proof molars, which were presumably used for tucking into roots, seeds and fruit.
Large eyes would have allowed it to see in low light, while the shape and size of its inner ear suggested the animal could hear higher frequencies than humans.
It was likely agile, and a large nasal cavity implied it would have had a keen sense of smell.
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A "remarkably bizarre" mix of primitive features and modern specialisations, Vintana represents a new genus and species of enigmatic early mammals called Gondwanatherians which lived on the supercontinent called Gondwana.
Only 30 years ago, Gondwanatherians were completely unknown.
The find allows the first insight into their habits and their place in the world during the twilight of the dinosaurs.
And it rearranges the family tree of mammals, pushing their origins back 25 million years.
The find is "the discovery of the decade for understanding the deep history of mammals," evolution expert Zhexi Luo from the University of Chicago said in a comment on the study.
Based on fossil finds, Vintana shared its environment with meat-eating and plant-eating dinosaurs, as well as crocodiles, snakes, giant frogs, lizards, fish and half a dozen bird species.
Its skull, minus lower jaw, was discovered when researchers examined a sandstone block collected in Madagascar in 2010 for its fish fossil content.
"We CT-scanned it here at Stony Brook University in the Department of Radiology and were astounded to see a mammal skull staring back at us on the screen," Krause told AFP.
"It was truly one of those electric moments of palaeontological discovery!"
It took six months to gently extract the skull from the rock so that the precious find could be compared against other fossils and living mammals.
As a tribute to the way in which it was found, Vintana was translated from the Malagasy word for "luck". Its second name comes from Joe Sertich, who collected the specimen.
Vintana was the product of some 20 million years of evolution after Madagascar split from India, to which the island had been connected for 30 million years during Gondwana's breakup.
The animal's lineage, however, eventually disappears.
"It is one of those evolutionary experiments in 'mammalness' that did not make it," said Krause.