Hollywood and the television are obsessed with dinosaurs, and the most charismatic of them all has to be Tyrannosaurus rex. With its colossal jaws and powerful build, it terrorised the residents of the upper Cretaceous, despite having remarkably short forelimbs. Joel Hutson from Northern Illinois University, USA, explains that they were barely longer than an adult human's arms although they appear to have been put to good use grabbing prey. Yet it was not clear how the vicious predators moved the joints in their stunted limbs to grip their victims. So, when Hutson's thesis advisor, J. Michael Parrish, received casts of the shoulder and forelimb bones from Sue – the Chicago Field Museum's T. rex skeleton – he set about recreating the limb's movements. However, Hutson admits that he was taken aback when he saw how Parrish manoeuvred the bones. ‘I thought, “This is unrepeatable”’, he recalls, adding, ‘I said, “If I do this I have to find a way to make this more scientific”’. Determined to set range of motion studies on a more solid footing, Hutson decided to test how reliable the technique is on two modern – and relatively close – relatives of the T. rex: the ostrich and the American alligator (p. 2030).

Obtaining three culled alligators from a reserve in Louisiana and three ostrich wings from local butchers, Hutson and his wife, Kelda, decided first to find out whether they could correctly reassemble the limb bones and measure the elbow's range of motion. Stripping the bones of flesh and arranging them in a horizontal plane, the duo measured the elbow's range of motion with a protractor as they flexed and extended the limb by moving the radius and ulna around the skeletonised elbow joint. Then, having convinced themselves that they could measure the elbow range of motion, the duo embarked on a week-long dissection marathon to find out how the presence of soft tissues affected their measurements.

Painstakingly manipulating the intact limbs in the vertical plane and measuring the full extent of the elbow rotations with an inclinometer while flexing them, the husband and wife team then carefully removed the limbs' skin before repeating the elbow rotations. Systematically dissecting away the muscles and tendons, joint capsules and ligaments, and finally the cartilage, the duo repeatedly measured the elbow's range of motion at each stage, taking it in turns to manipulate the limb to introduce the variation between experimenters that Hutson was sure would make range of motion studies almost unreproducible.

However, when Hutson analysed the elbow rotation angles he was shocked to see that their measurements were reproducible. ‘Sometimes our range of motion measurements are up to 70 deg apart, but when you take those hundreds of measurements and average them out, the statistical analysis said there was no difference between experimenters’, says Hutson. And when he analysed the impact of the animal's soft tissues on the elbow's mobility, Hutson could see that the bulk of the limb impeded the elbow's rotation, which increased as the duo stripped away the tissue. Hutson says, ‘Our summary says that what people do with fossil bones is probably a conservative underestimate of what the range of motion was in the elbow joint in real life.’

Having convinced himself that palaeontological studies of the range of motion of fossil joints are reproducible, Hutson says that he is impressed by the accuracy of previous studies. He adds, ‘Those scientists have vast accumulated knowledge in comparative anatomy; they intuitively know where the tendons and muscles were and they knew what they were doing in their range of motion studies.’

J. D.
K. N.
A test of the validity of range of motion studies of fossil archosaur elbow mobility using repeated-measures analysis and the extant phylogenetic bracket
J. Exp. Biol.