Most of the mammals that existed when the dinosaurs roamed on earth were small but, in 2010, scientists searching for fish fossils in Madagascar accidentally discovered the near complete skull of a groundhog-like animal that was comparatively larger than its modern-day counterparts.

The animal, which lived with the dinosaurs about 72 to 66 million years ago, belonged to a group of primitive mammals known as gondwanatherians.

In a new study published in the journal Nature on Nov. 5, David Krause from the Stony Brook University in New York, and colleagues, who discovered the skull of the animal, described the creature as large-eyed and agile.

The animal, which the researchers have called "Vintana sertichi," was way bigger than most other mammals of the era.

While mammals at the time typically weighed less than one pound, it was estimated to be about 20 pounds. The Vinta is also believed to have primarily fed on plants and was equipped with keen senses of smell and hearing.

"Its craniofacial anatomy reveals that it was herbivorous, large-eyed and agile, with well-developed high-frequency hearing and a keen sense of smell," the researchers wrote. "The cranium exhibits a mosaic of primitive and derived features, the disparity of which is extreme and probably reflective of a long evolutionary history in geographic isolation."

The well-preserved skull, which measures about five inches long, has allowed researchers to gather more information about the ancient animal, but another important implication of the discovery is that it could fill in gaps in the understanding of mammalian evolution.

Scientists have only known about the gondwanatherians based on a few teeth and jaw fragments, and they did not have sufficient information about this group of extinct animals to place them on the evolutionary tree.

Analyses of the Vintana skull and comparison of the animal to other mammals have allowed the researchers to gather more information about gondwanatherians.

"The discovery of Vintana will likely stir up the pot," said Krause. "Including it in our analyses reshapes some major branches of the 'family tree' of early mammals, grouping gondwanatherians with other taxa that have been very difficult to place in the past."

Krause and colleagues determined that this animal group is closely related to another group of rodent-like mammals called multituberculates.

Along with a few other animal groups, the gondwanatherians and multituberculates make up the evolutionary tree branch known as Allotheria, which included some of the most diverse, successful and long-lasting mammal groups on the planet for over 100 million years.

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