While mammals dominate the planet nowadays, for millions of years we played second fiddle to reptiles – most notably the dinosaurs. While reptiles became the biggest land animals to ever walk the Earth, mammals were mostly rat-sized critters running around underfoot. But now palaeontologists have described a mammal ancestor from the Triassic Period that bucked the trend and grew to the size of an elephant, giving those early dinosaurs a run for their money.

The newly-described species, named Lisowicia bojani, belonged to a group of animals called dicynodonts. Living about 250 to 220 million years ago during the Triassic, this herbivorous group has been described as "mammal-like reptiles," and would one day give rise to modern-day mammals.

But Lisowicia is no ordinary dicynodont. While group members ran the size gamut from rat to ox, Lisowicia bojani trumped them all and stood as tall as an elephant, long before that was the norm. It measured 4.5 m (14.8 ft) long, 2.6 m (8.5 ft) high and weighed about nine tons, which the researchers say was around 40 percent bigger than the next-largest known dicynodont. That's also not far short of the biggest known Triassic dinosaurs.

But it wasn't just its size that made Lisowicia special – it lived about 210 to 205 million years ago, which is around 10 million years after most dicynodonts were thought to have died out.

"Dicynodonts were amazingly successful animals in the Middle and Late Triassic," Grzegorz Niedzwiedzki, co-author of the study. "Lisowicia is the youngest dicynodont and the largest non-dinosaurian terrestrial tetrapod from the Triassic. It's natural to want to know how dicynodonts became so large. Lisowicia is hugely exciting because it blows holes in many of our classic ideas of Triassic 'mammal-like reptiles'."

The research was published in the journal Science.

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