Imagine a dinosaur; a sauropod such as a Brachiosaurus, or Diplodocus, one of the largest animals ever to roam the land. A plant-gobbling true giant of a beast, with a graceful long neck and tail. Now imagine that same dinosaur shrunk down to the size of a cow.

Strange as it may seem, such bizarre animals did exist. A number of astonishing fossil discoveries have confirmed that dinosaurs really did come in dwarf form. Their existence not only provides some fundamental insights into dinosaur evolution, it may even help explain why no large dinosaurs survive today.

Transylvanian dwarfs

The strange story of dwarf dinosaurs has a long and convoluted history, beginning in Transylvania in the closing years of the nineteenth century. Franz Nopcsa, a local aristocrat, became fascinated by the fossils turning up on his family estate. He showed them to a leading geologist of the day, who identified the fossils as dinosaurs. Nopcsa was thrilled.

There is also a report of an island dwarf sauropod from the Cretaceous of Saudi Arabia

He began studying the dinosaurs, and it wasn’t long before he realised there was something strange about them: they all seemed unusually small. As Nopcsa learned more about dinosaurs, he recognised that one of his new species was a long-necked sauropod, closely related to others turning up in North America and India at the time. Nopcsa’s sauropod looked like a tiny version of large Indian specimens that had been named Titanosaurus - but that name seems hopelessly inappropriate for Nopcsa’s small beast. Eventually it was named Magyarosaurus.

By 1912, Nopcsa knew enough about the dinosaurs on his estate, and the prehistoric world they had once occupied, to make a bold claim. He said his studies had revealed no less than a prehistoric dinosaur island - one that was populated by tiny versions of the giant beasts that walked the world elsewhere at the time.

The idea wasn’t as crazy as it might sound: scientists in pre-war Europe already knew that strange things can happen to animals on isolated islands. For instance, they had found good evidence that some Mediterranean islands had been populated by dwarf elephants and hippopotamuses during the Stone Age, a few thousand years ago. Even so, Nopcsa’s idea wasn’t an instant hit with his colleagues. How could he be sure his dwarf dinosaurs were not simply the babies of larger species?

Unfortunately for Nopcsa, world events intervened before he could strengthen his case. The First World War broke out 20 months after he came up with his dwarf dinosaur theory.

Europasaurus in Germany was just one dwarf dinosaur. But in Romania there were lots of dwarf species

In the fallout from the war, Europe’s political boundaries were redrawn and Transylvania became part of Romania. Nopcsa lost his estate - and access to undiscovered fossils there - during the transition. He continued to write about the dinosaurs, but financial and personal problems drove him to suicide in the 1930s.

Astonishing little dinosaurs

The story then jumps forward into the 21st Century, when out of the rocks of northern Germany tumbled another diminutive dinosaur. Martin Sander at the University of Bonn led the excavations. His team found fossilised bones belonging to at least 11 skeletons, the largest 6 metres long and the smallest just 1.7 metres. All belonged to the same species, which in 2006, was given the name Europasaurus.

Sander assumed all 11 were juveniles, but a close look at the internal structure of the fossil bones revealed that the largest had features only seen in mature adult dinosaurs. There could be no doubt: Europasaurus was a dwarf.

There were a few other dwarf plant eaters, and maybe even a meat eater - it’s a really weird fauna

These little dinosaurs astonished experts. But within just a few years, the story deepened. By 2010 Sander and his team had turned their attention to Nopcsa’s Romanian dinosaurs. They proved that Magyarosaurus, Nopcsa’s tiny sauropod, was indeed the dwarf that he’d suspected it to be.

Dwarf dinosaurs are turning up elsewhere too now: one dinosaur from the UK may have been a dwarf. “There is also a report of an island dwarf sauropod from the Cretaceous of Saudi Arabia,” says Sander.

It now seems that dwarf dinosaurs may be more common than Nopcsa realised a century ago. But his Romanian beasts still stand out as something special, says Stephen Brusatte at the University of Edinburgh, UK, who has spent time studying them. “Europasaurus in Germany was just one dwarf dinosaur - but in Romania there were lots of dwarf species,” he says. “Alongside Magyarosaurus there were a few other dwarf plant eaters, and maybe even a meat eater - it’s a really weird fauna.”

A comparison helps to emphasise Brusatte’s point. The Romanian dwarf dinosaurs lived during the Cretaceous, the final chapter of the dinosaur’s 170-million-year-long reign. In Cretaceous South and North America, 40-metre-long sauropods rubbed shoulders with 12-metre-long dinosaur predators. The Romanian sauropods and predators would barely have reached the knees of their American cousins. Why was there such a size difference?

Big versus small

Answering this question is as much about explaining why the American dinosaurs were massive as it is about accounting for the diminutive size of the Romanian animals. There may, in fact, be a simple reason why some dinosaurs grew so large, says Roger Benson at the University of Oxford, UK: they did so because they could.

Tyrannosaurus-sized animals are just not there

Almost all of the large animals alive today are mammals – but none can reach the size of a giant dinosaur, says Benson, because of the way mammalian bodies generate internal heat. The bigger a mammal gets, the greater its body volume compared to its body surface area and the more difficult it becomes to get rid of unwanted heat. A very large mammal would run the risk of fatally overheating.

“Dinosaurs were almost certainly not warm blooded in the same way mammals are,” says Benson. “So maybe this thermal limit wasn’t a problem for them.”

Without that constraint on body size, herbivorous dinosaurs may have begun to evolve larger bodies for the protection it offered against predators. The predators may have responded by growing larger themselves, pushing the herbivores to grow larger still - and before too long, the world was populated by behemoths.

This is probably an oversimplification. Other factors may have helped dinosaurs grow so large, says Benson, including their strong but unusually lightweight bones. But this simple predator-prey theory for dinosaur size may actually help explain why Nopcsa’s dinosaurs were so small. By some quirk of fate, large predators apparently failed to reach his prehistoric island.

“Tyrannosaurus-sized animals are just not there,” says Brusatte. “If they were we would have found them by now.”

Perhaps it went like this: a prehistoric storm washed some large herbivorous dinosaurs off a large continent and out to sea, ultimately casting them ashore on Nopcsa’s isolated island. With no large predators to worry about, the dinosaurs no longer needed to maintain such big bodies - over the course of many generations, they gradually shrank.

More than 80% of living mammals weigh less than 1 kilogram. Almost zero dinosaurs other than birds did

We even know how the dinosaurs slimmed down. Earlier this year Sander’s team published their latest findings on Europasaurus, the German dwarf dinosaur. A careful look at its bones shows that it shrank through a form of arrested development. Europasaurus slowed down the pace of its growth so that as an adult it still had features - most obviously small size - that are otherwise seen only in juvenile dinosaurs. Europasaurus, and probably the dwarf dinosaurs of Romania too, evolved to hold onto their childhoods.

Why the size of a cow?

There is one remaining mystery, though. All of the dwarf dinosaurs found to date are still, fundamentally, pretty large animals. Europasaurus and Magyarosaurus managed to shrink, but each only reached the size of a cow. No dwarf dinosaur made it much smaller.

It wasn't just the dwarf dinosaurs that were relatively large. Earlier this year Benson and his colleagues looked at animal body size throughout the dinosaur age. In 170 million years, only one group of dinosaurs managed to evolve bodies less than 1 kilogram in weight: the group of dinosaurs that includes birds.

Other dinosaurs might not have achieved the level of smallness required to survive

“More than 80% of living mammals weigh less than 1 kilogram,” says Benson. “Almost zero dinosaurs other than birds did.”

Benson thinks birds might even have their small size to thank for their continued existence today. About 65 million years ago an asteroid slammed into Earth, triggering huge environmental disturbances that wiped out most animal species alive at the time. It might be no coincidence that the one group of dinosaurs to resist the turmoil was birds - the only group that included some truly tiny animals. The other dinosaurs “might not have achieved the level of smallness required to survive”, says Benson.

What’s interesting is that scientists now know how birds managed to become so tiny - and they used the same trick the dwarf dinosaurs did. Birds became small by lengthening their childhood, meaning they look a lot like baby dinosaurs. The difference is that birds took their shrinking act to a level that dwarf dinosaurs - for some reason - just couldn’t match.

“We need to know what birds had, and other dinosaurs lacked, that allowed them to be so small,” says Benson.

Mysteries remain

There’s no answer to this question yet. Maybe the mammals, birds and reptiles alive during the dinosaur’s era were so good at being tiny that they filled all of the ecological niches for small animals, leaving no room for small dinosaurs. Or maybe, says Brusatte, when the non-bird dinosaurs grew large they passed some sort of point of no return that made it impossible for them ever to become tiny again.

They certainly weren’t all big lumbering animals

Bhart-Anjan Bhullar at Harvard University, in Cambridge, Massachusetts, US, who helped establish that birds look like baby dinosaurs, thinks this might be the case. Dinosaurs like the giant sauropods may not have been able to shrink the “plumbing and wiring” of a working reproductive system to a size that would fit inside a really tiny animal, he says.

One way to explore the idea would be to carefully study the bones of sauropods and dwarf sauropods, looking for features that hint at when the animals became sexually mature. Looking at growth rates and sexual maturity in related groups like birds and crocodiles might help too.

While we await more scientific findings, the dwarf dinosaurs are also important for a simpler reason. They help remind us just how amazingly diverse dinosaurs could be. “They certainly weren’t all big lumbering animals,” says Brusatte.