Scientists have found the remains of a giant herbivore that lived more than 200 million years ago.
In a paper published by the Science journal, Polish researchers claim their discovery overturns the notion that the only giant plant-eaters at the time were dinosaurs.
The creature, the size of an elephant and known as Lisowicia bojani, belonged to the same evolutionary branch as mammals.
It has been named after the village in southern Poland where its remains were found.
Similar fossils from so-called dicynodonts have been discovered elsewhere, but they were dated from an earlier period – before a series of natural disasters wiped out most species on the planet.
Grzegorz Niedzwiedzki, a palaeontologist at Uppsala University in Sweden who co-authored the paper, said: "We used to think that after the end-Permian extinction, mammals and their relatives retreated to the shadows while dinosaurs rose up and grew to huge sizes."
Researchers said the discovery of giant dicynodonts living at the same time as sauropods – a branch of the dinosaur family that later produced the iconic long-necked diplodocus – suggests environmental factors in the late Triassic period may have driven the evolution of gigantism.
Christian Kammerer, a dicynodont specialist at the North Carolina Museum of Natural Sciences, said the size of Lisowicia was "startling".
He added: "Large dicynodonts have been known before in both the Permian and the Triassic, but never at this scale."
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Mr Kammerer said that while dicynodonts and dinosaurs existed at the same time, there is no evidence yet that they lived in the same habitats.
He also questioned the study's conclusions about the Lisowicia's posture, but added: "Overall I think this is a very intriguing and important paper, and shows us that there is a still a lot left to learn about early mammal relatives in the Triassic."