Fossilised dinosaur guts unearthed in outback Queensland could back a long-standing theory on the diet of sauropods, the biggest reptiles to roam the planet.

For nearly 150 years, it’s been thought that sauropods — a group of four-legged, long-necked and relatively small-headed creatures like the Brachiosaurus — were herbivores.

This was based on several assumptions, including their flat teeth and overall large body that would make it hard for them to chase down moving prey.

But direct evidence of what these animals ate has been lacking.

Now, in today’s issue of the journal Current Biology, researchers report their first ever discovery of fossilised intestinal contents — referred to as “cololite” — from a sauropod.

The shapes of leaves could clearly be seen in samples of Judy the dinosaur’s gut fossils. (Supplied: Australian Age of Dinosaurs)

The cololite was found inside a relatively complete skeleton of Diamantinasaurus matildae in 2017 at Belmont Station, near Winton, with much of it sealed over by a layer of mineralised skin.

“What was really exciting about it [the gut fossil] is in places you could see the folds of the gut,” Belmont Station owner and palaeontologist David Elliott said.

An analysis of the fossil showed a young Diamantinasaurus specimen, nicknamed “Judy”, whose gut contained small voids — impressions left behind by decomposed plants.

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Palaeontologist and study lead author Stephen Poropat, from Curtin University, said Judy seemed to have eaten plants from a range of heights.

“There’s relatives of modern-day monkey puzzle trees, we have seed ferns, which are a totally extinct group,” he said.

“And we also have leaves from angiosperms, which are flowering plants. Back in the Cretaceous period when Judy was alive, they would have looked somewhat similar to modern-day magnolias.”

Judy the dinosaur, who was found by Bob Elliott, is the most complete Diamantinasaurus specimen to be discovered. (Supplied: Australian Age of Dinosaurs)

The study has shown not only what a sauropod like Diamantinasaurus ate, but how they ate too.

Uppsala University palaeontologist Martin Qvarnström, who was not involved in the study, said the size of the food fragments found in Judy’s gut supported the idea that sauropods fermented their food rather than chewed it.

“The tricky part here was proving the plant material wasn’t just random debris, and here the authors make a strong case, especially with evidence of the plants being sealed inside mineralised skin,” he said.

“The preservation of both gut contents and skin is unusual.”

A small piece of fossilised Diamantinasaurus skin compared to fossil of skin overlaying gut contents. (Supplied: Australian Age of Dinosaurs)

What was life like for Judy in ancient Winton?

Diamantinasaurus were first described in 2009 and lived in the mid Cretaceous, about 101 to 94 million years ago.

They were part of the sauropod group of dinosaurs, which lived from about 201 million years ago until the end of the age of dinosaurs about 66 million years ago.

When Diamantinasaurus lived, Australia was closer to the South Pole, and what is now outback would have been a wet flood plain covered in conifers, gingkos, seed ferns and other plants — plenty of food for a growing dinosaur.

The largest-known sauropod eggs are smaller than a soccer ball so when the baby reptiles hatched, they may have been no bigger than 50 centimetres from tip to tail.

Dr Poropat said they would then have to grow very fast to avoid falling victim to predators.

“I often think about sauropods in similar context of turtles, where turtle mothers will lay a large number of eggs … and then basically abandon them,” he said.

“And when the babies hatch, they’re ready to go.

“For sea turtles that’s the sea, but for sauropods they might have just needed to get into a forest … where they were surrounded by food and also had places to shelter.”

Changing tastes with age

Dr Poropat said Judy was a sub-adult and probably measured about 11 to 12 metres.

A fully grown Diamantinasaurus would have reached about 16m long.

Fully grown Diamantinasaurus would have grown to about 16m long with a shoulder height of about 2.5m. (Illustration: Travis Tischler)

He theorised Judy (who is referred to as “she” although her gender isn’t known) was in a transition phase where she was eating some of the plants she consumed when she first hatched.

“We actually speculate that Judy might have had a diet different to an adult Diamantinasaurus,” he said.

“She couldn’t reach up into the tops of conifers, but also she’s starting to eat the conifer foliage that is targeted maybe more so by adults.

“So it’s kind of cool to think that sauropods would have changed their diet as they grew up, just as many animals do.”

Palaeontologist Stephen Poropat at the excavation of Judy in 2017. (Supplied: Australian Age of Dinosaurs)

Miami Frost Museum of Science palaeontologist Cary Woodruff, who specialises in sauropods but was not involved in the study, said capturing this transitionary period and its implications for sauropod growth were the crowning jewel of the new study.

“Previous studies have shown that in some species, the young had pointed snouts, likely for selective feeding, and when they grew up, they had broad ‘muzzles’ for grazing,” he said.

“And in some, the young had teeth designed for both soft and coarse vegetation.

” Imagine a Swiss Army knife in your mouth; no matter the plant, you have the right tool for the job. ”

Dr Woodruff said Judy captured a “moment in time” where the Diamantinasaurus had this Swiss Army knife.

Australian Age of Dinosaurs executive chairman David Elliott. (ABC Western Queensland: Ellie Grounds)

Mr Elliott said Judy the dinosaur, who scientists named after his wife, was likely to be given her own exhibit at a new building at the Australian Age of Dinosaurs museum in Winton.

He said galleries were currently being designed and Judy was a prime candidate for a skeleton reconstruction.

“We want to tell the story of Australian evolution as how from the very first life forms how Australian life has changed over millions of years.”