As if dinosaurs weren't already giant to begin with, new research indicates they were even taller than was thought.

Although researchers had a good idea how tall dinosaurs stood based on their skeletons, it turns out that parts of their bodies that didn't fossilize might have boosted their height by at least 10 percent.

The ends of many dinosaurs' long bones, including leg bones such as the femur or tibia, are rounded and rough and lack major bony joint structures. Instead, very thick layers of cartilage probably helped form the joints connecting these bones, "and would have added significant height to certain dinosaurs," explained researcher Casey Holliday, an evolutionary anatomist at the University of Missouri. In contrast, mammals have bony joint structures and much less of the soft-tissue cartilage.

The scientists reached these conclusions about dinosaurs by investigating ostriches and alligators, the closest, modern-day relatives of the extinct giants. They made casts of their bones with cartilage and then removed the cartilage and compared the bones with the casts. They found the lengths of alligators' and ostriches' limbs included between 6 percent and 10 percent cartilage.

They next studied the fossilized limbs of different dinosaurs, including famous carnivorous theropods such as T. rex and Allosaurus as well as giant herbivorous sauropods and ornithischians such as Brachiosaurus and Triceratops, respectively. From the evidence, the researchers concluded that certain dinosaurs possessed a significant amount of cartilage in their joints.

Although their analysis suggested many theropods were only modestly taller, ornthischians and sauropods might have been 10-percent taller or more. For example, Brachiosaurus, once thought to be 42 feet tall, may actually have been more than 1 foot (12.8 cm) taller.

"This may seem trivial — however, this is a large amount of cartilage," Holliday said.

The extra cartilage may have helped the giant herbivores absorb the increased amount of stress resulting from their enormous sizes, Holliday conjectured, "but we're still not really sure." He added that birds, the closest living relatives of the extinct theropods they studied, also have less cartilage in their joints, and that "less cartilage might be a sign of a more active lifestyle, higher metabolic rate and faster growth rate that we see in theropods and birds."

In the future, this research could help shed light on dinosaur movements and posture, Holliday said. "We could use what we know about cartilage to make 3-D models of their joints to try and test how they were able to move," he told LiveScience.

"Bones can't always speak for themselves," added researcher Lawrence Witmer, an anatomist at Ohio University. "To understand how dinosaurs moved, we need to analyze the bones as they were inside their bodies, including their cartilage."

Holliday, Witmer and their colleagues detailed their findings online Sept. 30 in the journal PLoS ONE.