Small Australian dinosaurs endured around six months of darkness each year during the Early Cretaceous, with freezing temperatures and huge predators.
So the diminutive dinosaurs took refuge underground, according to a new study.
The findings, which will be presented at next month's Geological Society of America annual meeting in Portland, reveal how dinosaurs and certain other animals developed strategies for surviving harsh environments and predation.
Predator tracks near burrows in Australia indicate 1,700-pound carnivorous dinosaurs hunted diminutive 22-pounders, which had to think fast.
"What defense did they have other than running? How about staying out of sight in burrows?" Anthony Martin told Discovery News.
Martin, a senior lecturer in environmental studies at Emory University and an honorary research associate at Melbourne's Monash University, outlined his discoveries in a paper that will be published in the October issue of Cretaceous Research. They represent at least three years of work.
In 2006, he identified the 95-million-year-old skeletal remains of a small adult dinosaur and two juveniles in a fossilized burrow in southwestern Montana.
The species is now called Oryctodromeus cubicularis, meaning "digging runner of the lair." That same year he also found tracks for a carnivorous dinosaur in Victoria, Australia, where a theropod (two-footed dino) claw and a few related bones have been uncovered.
In 2007, while hiking in Victoria, Australia, Martin spotted three dinosaur burrows etched into a 105-million-year-old outcrop that's about six-feet long and one-foot in diameter. At least one of the burrows descends into a spiral and ends in an enlarged chamber.
"The burrows show a behavior that was probably related to surviving a polar winter, but also could have served as protection against large predators, such as the large theropod dinosaurs," he said, adding that Australia's big Allosaurus-like dinosaurs "must have had some physiological adaptations that helped them to survive cold, dark winters."
Average annual temperature in Early Cretaceous Victoria was likely less than 41 degrees Fahrenheit, "which means that wintertime temperatures were well below freezing, plus it was dark for five to six months of the year," he explained.
Patricia Vickers-Rich, chair of paleontology at Monash University, said of the Australian burrows, "We have wondered for some time what these structures were and when Tony found burrows with dinosaur bones in them in Montana, he became suspicious that these structures in southern Victoria were of a similar nature."
More recently, Martin has found insect and crayfish fossil burrows in Victoria that date back to the same period. The crayfish burrows are the oldest known for Australia. Mammal and turtle fossils suggest these animals might also have retreated into underground chambers.
Taken together, he said, the "dinosaur burrows and tracks, as well as crayfish and insect burrows, give us new insights on animals' adaptations to polar environments during the time of the dinosaurs."
In the long run, however, that adaptability wasn't enough to save the down under dinosaurs.
"Dinosaurs were a very successful group for more than 160 million years, but they still eventually went extinct for various reasons, so it's not surprising that even the best adapted ones, including burrowers, just wouldn't necessarily make it through every environmental change," he said.
Birds, such as puffins, swallows, kingfishers and some owls, do burrow, he added, but there's no evidence they are the direct descendants of the burrowing dinosaur lineages.