NEW YORK—During the dinosaur age most mammals were puny, generally weighing less than a pound. Now a bizarre fossil skull from Madagascar has revealed a comparative giant, one that clocked in at maybe 20 pounds (9 kilograms).

“It was a monster,” said David Krause of Stony Brook University in New York, who led the discovery team. “It looks like a big groundhog.”

It’s the second heaviest mammal known from the dinosaur era, which ran roughly from 250 million years ago to 65 million years ago, and the most massive of that time from Southern Hemisphere.

Krause said his best guess is that the creature might have measured 20 inches (51 centimetres) to 24 inches (61 centimetres) from nose to rump. It lived sometime between 66 million and 72 million years ago.

In a paper released Wednesday by the journal Nature, Krause and colleagues named the creature Vintana sertichi. The first name, which means “luck” in the Malagasy language of Madagascar, was chosen because the skull appeared unexpectedly. When scientists did a CT scan of a large sandstone block to look for fish fossils, “we saw this thing staring back at us,” Krause said. “We were just amazed.”

The second name honours Joseph Sertich, now a curator at the Denver Museum of Nature & Science, who collected the sandstone block in 2010.

The 5-inch-long skull gives scientists their first good window into a poorly understood group of ancient Southern Hemisphere mammals that had been known only from isolated teeth and bits of jaw. They went extinct long ago, without leaving any descendants today.

Now researchers can see a face, and it is bizarre, Krause said. The skull is very tall in comparison to its length. The eye sockets are huge. Weird flanges by the bottom jaw once anchored chewing muscles.

The skull also revealed that the brain was tilted at a strange angle not seen in other animals. And it displayed an odd mix of primitive characteristics with more advanced ones.

Analysis suggests Vintana was an agile plant-eater with good eyesight in low light and a good sense of smell. Such abilities probably came in handy to avoid the predatory dinosaurs and other beasts that shared its environment, Krause said.

“It would have been a very fine hors d’oeuvre” for a dinosaur, Krause said.

John Flynn, an expert in fossil mammals at the American Museum of Natural History in New York, who didn’t participate in the discovery, called the find “fantastic” because of its good preservation and relative completeness, compared to fossils of other Southern Hemisphere mammals of the time.