Bringing the elephant to the weigh station. Photo © Project Exploration

A good place to weigh really heavy things is the scale at a weighing station for trucks. In fact, the scale is designed to weight front and hind axles separately. Angus only needed to stand on the scale and Jeff Ogradnik and Dr. Sereno would have an answer to their question: Where is Angus' center of mass?

Dr. Sereno discovered that Angus carries almost 60% of his weight on his front feet and only 40% on his hind feet. The forefeet carry about 7,600 pounds, or almost 4,000 pounds (2 tons) each!

Based on the weight report, Dr. Sereno knew that Angus' center of mass was forward in his body, closer to his front feet than his back feet. Angus has a large head and a puny tail, so it makes sense that his hind legs carry less weight than his front legs. To confirm this finding, Sereno and Ogradnik measured elephant limb bones and found that the circumference of the front leg bone (the humerus) was one and a half times bigger than the circumference of the back leg bone (femur).

The weight report from the weigh station. Photo © Project Exploration

Dr. Sereno compared this with the circumference of Jobaria's leg bones. He concluded Jobaria has the opposite weight distribution than Angus the elephant, carrying more weight on the hind legs and less on the front legs. Jobaria's leg bones measured as the reverse of Angus' bones: Jobaria's back leg bone (femur) has a circumference of about 1.5 times that of Jobaria's front leg bone (humerus).

Jobaria's center of mass was therefore closer to its rear feet than its front feet. And the closer the center of mass is to the rear feet, the shorter the center of mass has to travel to be above the rear feet to allow the animal to stand on its back legs. So if an elephant (which has its center of mass near the front) could move its center of balance far enough back to rear on its hind legs, it logically follows that a sauropod like Jobaria—who's center of mass is already near its hind legs - would have an even easier time rearing on its hind legs than the elephant.