Teenage tyrannosaurs grew double-quick

Tyrannosaurus rex, the largest carnivore ever to walk the Earth, was even bigger and heavier than we thought, new research shows. And juveniles must have grown around twice as quickly as previously thought.

13 October 2011, by Tom Marshall

Tyrannosaurus rex, the largest carnivore ever to walk the Earth, was even bigger and heavier than we thought, new research shows. And juveniles must have grown around twice as quickly as previously thought.

Size comparison between adult, juvenile and human

Scientists at the Royal Veterinary College (RVC), the University of Liverpool and Chicago's Field Museum used precise laser-scanning techniques to analyse the skeletons of four big T. rex, as well as one that's thought to come from a young tyrannosaur.

Using computers to reconstruct the surrounding flesh, they concluded the dinosaurs probably grew from 10kg hatchlings to 7-9 tonne adults by the time they were around 16 years old. That implies they could have put on as much as 1790kg a year. Earlier studies estimated adult T. rex weighed a comparatively svelte 5-7 tonnes.

Compared to modern animals, the adult T. rex was a super-heavyweight, especially around its haunches and tail, although relatively speaking its legs were more lightly muscled than those of a youngster. 'The total limb musculature of even an adult T. rex probably was relatively larger than that of a living elephant, rhinoceros or giraffe, partly because of its giant tail and hip muscles,' says Professor John Hutchinson of RVC, a specialist in evolutionary biomechanics and one of the lead authors of the paper, which appears in PLoS ONE.

'Yet proportionately its lower leg muscles were not as large as those of living birds, and those muscles seem to limit the speed at which living animals can run. So T. rex still wasn't the fleetest of land animals, although juveniles would have been relatively athletic,' he adds.

Older dinosaurs would have become comparatively slower and less agile as their bodies got heavier and their legs relatively less muscular, and as their centre of mass shifted forward. They may still have been capable of running faster than young ones hitting speeds of between ten and 25 miles (17-40km) an hour – not far from modern human running speeds – but their turning circles and general manoeuvrability would have been far worse than in their salad days, though.

The new estimates of T. rex size are likely to be more accurate than earlier efforts, as they're based on precisely scanning a variety of T. rex skeletons and then building computer models that represent the muscle and flesh that would have been needed for the dinosaurs to stand up and walk, rather than on physical scale models and equations derived from modern animals.

'This study underscores the value of having complete specimens of different ages,' says fellow lead author Dr Peter Makovicky of the Field Museum, which houses 'Sue', one of the biggest and most complete T. rex skeletons ever discovered, and one of those analysed. 'T. rex represents a biological extreme in being one of the largest bipeds that ever lived. Putting numbers on that requires access to the dimensions of whole skeletons.'

T. rex is the only big carnivorous dinosaur of which we have enough skeletons, covering a wide enough age range, to make this kind of analysis possible. But Hutchinson says the laser-scanning technique is becoming an increasingly vital tool in understanding the bodies and lifestyles of long-extinct animals, and that it could be applied to many other prehistoric species.

The results mean we may have to rethink how T. rex fitted into its environment. Bulky adults may have had to focus on different prey and use different hunting tactics from those that worked for nimble youngsters. They probably had to move onto bigger, slower prey, and it's possible that they started to use sneakier tactics.

'Is it possible for an animal that big to be an ambush predator? We're not sure,' Hutchinson comments. 'Certainly there would have been features in the landscape that it might have been able to take advantage of. But even as an adult, T.rex would probably have been fast enough to catch a duck-billed dinosaur or Triceratops, which we think were its main prey.'

Hutchinson says he's now doing further related work within his speciality of giant land animals, addressing questions like why rhinoceroses, which can jump and gallop, are so much more athletic than elephants, which can't do much more vary their walking speed.

Another current project aims to reconstruct Ichthyostega, a very early four-legged animal that's thought to have been one of the first creatures to emerge from water to live on dry land. Previous impressions of its build have all been based on flattened two-dimensional fossils, whereas Hutchinson's team are using a variety of high-tech tools to three-dimensionally scan skeletons, even parts still entombed within rock. He thinks the project will greatly change our ideas about these animals' behaviour and appearance.

'We're ending up with an animal that looks very different from previous reconstructions,' he says. 'Indeed, that's what palaeontologists who apply the latest 3D imaging technologies are increasingly finding with fossil animals - exciting times!'