For decades, its sole fossil was mistaken for a baby dinosaur’s arm (Picture: Getty images/ Catmando/Shutterstock)

It seems not all dinosaurs were gigantic beasts that would tower over humans. Some of them were barely the size of a mountain dog.

A new species of tiny dinosaur has been discovered, that would have weighed around 75 kilogrammes. That’s not much heavier than the average human.

For decades, its sole fossil was mistaken for a baby dinosaur’s arm. However, new analysis has revealed that not only was the bone structure that of a fully grown dinosaur, but that it probably belonged to an entirely unknown species.

The fossil suggests they were ancestors of a group of dinosaurs called sauropods and their close relatives, known as the sauropodomorphs.

While the heaviest sauropod dinosaurs could weigh up to 70 tonnes, this new species may have barely weighed 70 kilogrammes.

Still, scientists say they don’t have enough evidence to name the new species yet.

A new species of tiny dinosaur has been discovered, that would have weighed around 75 kilogrammes (Picture: Getty images/ Catmando/Shutterstock)

‘Until now, we didn’t know that early sauropodomorphs could get this small in the Jurassic, so the smallest skeletons were assumed to be babies,’ said Dr Kimberley Chapelle, the lead author of a new study on the bone.

‘We can now reassess these skeletons discovered in southern Africa and hopefully find a more complete individual from which we can name a new species.’

The bone had previously been misidentified as belonging to a Massospondylus. More recent research, however, has called this into question.

‘Massospondylus was first described in 1854 and was one of the first dinosaurs known from South Africa,’ said Kimberley.

‘At that time, there weren’t many early sauropodomorphs to compare it to, and a formal description of this species’ skeleton only came out in 2019.’

The heaviest sauropod dinosaurs could weigh up to 70 tonnes (Picture: Júlia d Oliveira)

This resulted in some deformed bones being redescribed as belonging to an entirely new species of sauropodomorph called Ngwevu intloko.

But while describing Ngwevu, the team got the hunch that many other specimens misidentified as Massospondylus over the years may be from another new species.

They also thought it was more likely that rather than a single species, a whole host of sauropodomorphs flourished across southern Africa.

Finally, it was a fossil of an upper arm bone that unravelled the mystery.

Comparing the mystery arm bone to other dinosaurs of a similar era revealed that it was much smaller than any of them.

In fact, the new fossil was the smallest sauropodomorph of its era and may have been under a metre long as an adult.

While it’s hard to tell much else from the arm bone alone, its shape suggests that the unknown dinosaur probably stood on two legs like its larger relatives.

Sadly, the small dinosaur did not survive evolution while its distant relatives went on to become the largest animals to walk the Earth for the next 100 million years.

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For decades, its sole fossil was mistaken for a baby dinosaur’s arm.