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 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 20 pounds (9 kg), 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
150-pound (68-kg) 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.
 "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 5-inch (12.5-cm) 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.
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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."
(Reporting by Will Dunham; Editing by Marguerita Choy)
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