DNA can't survive for more than a few million years at most, putting paid to any plans to take the grandkids to a real Jurassic Park one day.
Even at the ideal temperature of minus five degrees centigrade, says a Murdoch University team, all bonds in a DNA strand would be completely destroyed in bone after around 6.8 million years.
But while that's nowhere near the 65 million years you'd need to find any in a Tyrannosaurus rex, it's a lot more than the current record of 450,000 to 800,000-year-old DNA found in Greenland ice cores.
The team studied 158 fossilised leg bones belonging to three species of the moa, an extinct group of birds that once roamed New Zealand.
“It has been agonisingly difficult to estimate the rate of DNA decay before now because finding a large set of DNA-containing fossils with which to make meaningful comparisons are exceedingly rare,” says Dr MikeBunce.
“Environmental conditions like temperature, degree of microbial ‘attack’ and oxygenation, can affect the DNA decay process and make it hard to detect a basic rate of degradation."
But, he says, the moa bones have allowed the team to study the comparative DNA degradation, as they come from different ages but all experienced the same environmental conditions.
The bone specimens were carbon dated at between 600 and 8000 years old and, by looking at the varying degrees of DNA degradation in each specimen, the team was able to calculate a DNA half-life of 521 years.
They found that the estimated decay rate in the specimens was almost 400 times slower than predicted from simulation experiments carried out in the lab.
"If the decay rate is accurate then we predict that DNA fragments of sufficient length will preserve in frozen fossil bone of around one million years in age," says Bunce.
"Ultimately, the models might enable better estimates of which fossils might work, and prevent the destructive sampling of rare fossils which are thought unlikely to yield DNA."