This is a 250-million-year-old limb bone that once belonged to a previously unknown species of mammal-like reptile known as a dicynodont.
The bone was unearthed near the village of Lisowice, in Poland, by scientists led by Grzegorz Niedzwiedzk from Uppsala University, in Sweden.
Dicynodonts were members of a very large group of reptiles that shared many behaviours in common with mammals. They were the dominant terrestrial herbivore until the rise of the dinosaur.
The new species – dubbed Lisowicia bojani – would have been 4.5 metres long, 2.5 metres high and weighed nine tonnes, making it the biggest dicynodont ever found, by a comfortable margin.
The find is described in the journal Science.