Dinosaurs were much taller than previously thought, according to a new study that found cartilage extended the limbs of these animals by up to a third of a metre in length.
The discovery has implications for the posture, flexibility, total body length, speed and feeding habits of dinosaurs, some of which are now believed to have grown to about the same height as a three-story building.
It could also mean museum curators may have to remount their dinosaur skeletons.
"Most estimates of dinosaur height simply stack the limb elements on top of one another without accounting for the soft tissues that were once present in the living animal," says lead author Dr Casey Holliday.
Holliday, an anatomy professor at the University of Missouri-Columbia , and colleagues Ryan Ridgely, Jayc Sedlmayr and Lawrence Witmer made the determination after analysing the cartilage present in ostriches and alligators, close modern-day relatives of dinosaurs. They also analysed the fossilized limbs of different dinosaurs, including Tyrannosaurus rex, Allosaurus, Brachiosaurus and Triceratops.
The scientists discovered that cartilage accounted for about 10% of the lengths of ostrich and alligator limbs. Using a "cartilage correction factor," they now believe carnivorous dinosaurs like Tyrannosaurus were only modestly taller when fleshed out. Herbivorous giants, such as Triceratops and Brachiosaurus, were likely a full foot more in height.
The findings are published in the latest issue of the journal PLoS ONE .
"We only see cartilages this big (today) maybe in some whale flippers, maybe some sea turtles," says Holliday said. "So regardless of Brachiosaurus being a foot taller overall, this animal had really, really thick cartilage caps - thicker than 3 to 4 inches (7 to 10 centimetres)."
Standing or crouching
He and other palaeontologists still theorise that sauropods stood upright with column-like legs. Meat-eating predators like T. rex, however, ran on two legs and had a crouched stance, which could explain why this species had less cartilage in its limbs than the plant-eaters.
"It take huge muscles to maintain that crouched posture, but even bigger muscles to move the animal at significant speeds," says Holliday. "So in a crouched animal, the longer the limbs, the bigger the muscles. Muscles are expensive tissues - heavy, high physiology, vascular - and they may result in having a slower animal."
Even without a sauropod level of cartilage, T. rex grew to at least 4 metres tall. Scientists aren't entirely sure why many dinosaurs were taller than today's animals, but more plants, different atmospheric concentrations of oxygen and carbon dioxide, and lighter body structures have all been proposed as possible explanations, according to Holliday.
Back to drawing board
Thomas Holtz, Jr, a senior lecturer on vertebrate palaeontology at the University of Maryland believes the conclusions in the new paper "are sound."
"The major important points are that, just like you see when carving a turkey or chicken leg, the bones themselves are not in direct contact, but there is sometimes a fair amount of cartilage between them," says Holtz. "So in order to correctly figure out the lengths and shapes of the different limbs of dinosaurs, we need to estimate the amount of cartilage."
John Hutchinson, a reader in evolutionary biomechanics at the University of London's Royal Veterinary College agrees cartilage needs to be accounted for.
"Some researchers tend to focus just on the skeletal remains of fossils and forget that dinosaurs had soft tissues that influenced almost every aspect of their biology," says Hutchinson.
Witmer adds, "We now have to go back to the drawing board and re-evaluate what we thought we knew about how dinosaurs worked, from walking and running to reaching and grasping. We need to start looking for new and different evidence."