Editor, Earth News
A burrow photographed from above, showing a cross section, with the entrance on the right side and chamber on the left
The world's oldest dinosaur burrows have been discovered in Australia.
Three separate burrows have been found in all, the biggest 2m long, each built to a similar design and just big enough to hold the body of a small dinosaur.
The 106-million-year-old burrows, the first to be found outside of North America, would have been much closer to the South Pole when they were created.
That supports the idea that dinosaurs living in cold, harsh climates burrowed underground to survive.
The only other known dinosaur burrow was discovered in 2005 in Montana, US.
Described two years later, this burrow dated from 95 million years ago and contained the bones of an adult and two juveniles of a small new species of dinosaur called Oryctodromeus cubicularis.
Now the older burrows have been found by one of the researchers who made the original Montana discovery.
"Like many discoveries in palaeontology, it happened by a combination of serendipity and previous knowledge," says Anthony Martin of Emory University in Atlanta, Georgia, US.
"In May 2006, I hiked into the field site with a group of graduate students with the intention of looking for dinosaur tracks. We did indeed find a few dinosaur tracks that day, but while there I also noted a few intriguing structures."
Martin returned to the site, a place dubbed Knowledge Creek that lies 240km from Melbourne, Victoria, to study these structures, once in July 2007 and again in May this year.
His first reaction was one of astonishment.
"I was scanning the outcrop for trace fossils, and was very surprised to see the same type of structure I had seen in Cretaceous rocks of Montana the previous year," says Martin.
That original structure turned out to be the burrow of O. cubicularis, which Martin described with colleagues David Varricchio from Montana State University, Bozeman, US, and Yoshi Katsura of Gifu Prefectural Museum in Seki City, Japan.
"So to walk up to the outcrop and see such a strikingly similar structure, in rocks only slightly older, but in another hemisphere, was rather eerie."
Within the rock, which forms part of the so-called Otway group of rocks that have yielded a rich diversity of vertebrate fossils, Martin found three separate burrows less than 3m apart, which he describes in the journal Cretaceous Research.
Two of the burrows formed a semi-helix, twisting down into the rock that was once soil.
The largest and best preserved, dubbed tunnel A, turns twice before ending in a larger chamber. In total, it is more than 2.1m long.
Martin calculates that an animal around 10kg in size would have made each burrow.
Modern animals which create such burrows include aardwolves, alligators, coyotes, gopher tortoises and striped hyenas. Twisting burrows can help stop predators getting in and keep the temperature and humidity constant.
Martin can't be sure which species of dinosaur made the burrows, but he is struck by how similar their designs are to the burrow made by O. cubicularis.
A variety of small ornithopod dinosaurs were also known to have lived in the area during the same time in the Cretaceous. These ornithopods stood upright on their hind legs and were about the size of a large, modern-day iguana.
Surviving the cold
Martin has ruled out a variety of other factors that could have created the burrows.
The fact that they were made by dinosaurs makes sense, he says.
Twenty years ago, researchers in Australia, including Patricia Vickers-Rich of Monash University in Clayton and Thomas Rich of the Museum of Victoria, first proposed that some dinosaurs may have climbed into burrows to survive harsh climates they couldn't escape from by migrating.
"It gives us yet another example of how dinosaurs evolved certain adaptive behaviours in accordance with their ecosystems," Martin says.
"Polar dinosaurs in particular must have possessed special adaptations to deal with polar winters, and one of their behavioural options was burrowing. It provides an alternative explanation for how small dinosaurs might have overwintered in polar environments."
Martin now hopes that palaeontologists will be on the look out for a range of different types of dinosaur burrow, and for dinosaurs that are physically adapted to burrowing into soil.