PARIS, Aug 11 (AFP) - The Tyrannosaurus rex's prehistoric equivalent of the Atkins diet had a formidable weight gain effect, adding up to 2.1 kilogrammes (4.6 pounds) to its fearsome mass every day, a group of Canadian and US scientists have concluded.
The all-meat intake of the so-called "king" of the dinosaurs bulked it up so much that by adulthood, an average T. rex weighed five tonnes, or as much as an African bull elephant, the researchers wrote in the latest issue of Nature magazine.
Although nothing remains of the dinosaurs except bones dating back between 65 and 100 million years, those fossils were enough for the team, led by Peter Makovicky of Chicago's Field Museum, to calculate body mass.
A study of growth rings in the bones from the remains of 20 dinosaurs of the Tyrannosaurus rex's theropod suborder (carnivores that walked on two feet and had small forelimbs) suggested the growth rate of the giant creatures.
But the team had to crunch that data using an innovative technique to get around the fact that many of the inner rings had become difficult to detect because of the expanding marrow core in a maturing animal.
The technique involved measuring the circumference of the main leg bone, the femur, to get an idea of the height and then correlating that with the growth rings from the smaller bones, such as vertebrae, ribs (including gastralia, or belly ribs found in dinosaurs), and the fibula to work out the overall mass.
The results showed that a Tyrannosaurus reached adulthood after 14 to 18 years, after which a few continued to grow at a much slower rate for a dozen years or so.
When mature, a T. rex stood as high as a giraffe, some five to six metres (16 to 20 feet), and from its head to its tail measured 14 metres (46 feet).
Although originally thought to have been the biggest of the predators ever to walk the planet, hence its name, the later discovery of fossils of other dinosaurs (Spinosaurus, Giganotosaurus and Carcharodontosaurus) showed there were even bigger carnivores that once trod the earth.
Some four-legged vegetarian dinosaurs eclipsed all of them, though, for instance the Brontosaurus which weighed 30 tonnes and was 20 metres long.