It is believed that the massive herbivore existed in the Triassic Period about 200 million years ago. What is strange about the discovery is that large mammals should not have existed at this time. Scientists believed that large mammals had died at the end of the Permian Period, 251.9 million years ago.
After that, only the smallest mammals existed when dinosaurs became the dominant species on Earth, with mammals recovering the crown about 50 million years ago.
However, the discovery of the huge remnant herbivore found in southern Poland says large mammals and large dinosaurs must have coexisted.
The creature was named Lisowicia bojani and belonged to the same evolutionary branch of mammals.
Grzegorz Niedzwiedzki, a paleontologist at the University of Uppsala in Sweden and co-author of the paper published in the journal Science, said: "We used to think that after the extinction of the Permian's end, mammals and their relatives retreated into the shadows while dinosaurs grew to enormous sizes. "
The newly discovered beasts are part of the family of dicinodonts and lived at the same time as other sauropods, a group of dinosaurs that eventually led to the long-necked diplodocus – one of the largest creatures ever to walk the Earth.
Experts theorize that environmental factors during the Triassic Period may have led to gigantism around the world.
Christian Kammerer, a specialist in dicnodonts at the North Carolina Museum of Natural Science, said Lisowicia's size was "staggering."
"Great dicnodonts were known before in both Permian and Triassic, but never on this scale.
"Overall, I think this is a very intriguing and important article, and shows us that there is still a lot to learn about the first mammalian relatives in the Triassic."