A strange groundhog-like creature that lived 66 million years ago has been discovered, changing the way scientists think about the evolution of mammals on Earth.

The 20-pound critter, Vintana sertichi, is the largest known mammal in the Southern Hemisphere during the Mesozoic era, according to Agence France-Presse.

It is must larger than other mammals of that era, which were about the size of a mouse. The discovery means that mammals may have evolved much earlier than thought.

The 5-inch skull of the animal was found in a sandstone block that had been taken from Madagascar to a lab in New York. Paleontologists were not expecting to make this find.

The skull is about twice the size of that of a modern-day groundhog. It had rodent-like incisors and molars, indicating that its diet consisted of roots, fruit, and seeds. It also had large eyes that would have made it an ideal night forager, and its inner ear suggests that it could hear high frequencies. Its large nasal cavity also suggests that it would have had a very good sense of smell.

Vintana is a new genus and species from the early mammal subset called Gondwanatherians from the supercontinent Gondwana. This group of mammals were totally unknown to scientists just three decades ago.

The discovery pushes back the origin of this mammal’s family tree back 25 million years.

Vintana would have had to share its habitat with meat-eaters such as dinosaurs, crocodiles, snakes, giant frogs, lizards, fish, and birds.

The sandstone slab was recovered from Madagascar in 2010 during a search for fish fossils. Researchers were shocked to see a mammal skull after conducting a CT scan on the block at Stony Brook University in the Department of Radiology.

Researchers painstakingly spent six months gently removing the skull from the stone so that it could be compared to other fossils and living mammals.