Zigzagging backbones helped turn dinosaurs into giants

Sauropods were colossal creatures, reaching up to 50 meters in length and weighing as much as 77 metric tons—14 times the weight of modern-day African elephants. And the necks of some of these dinosaurs made up much of their entire body lengths —up to 15 meters in the aptly named Supersaurus. Now, scientists have suggested a new reason why these dinos might have been able to grow so massive: special “toothed” bones in their backs that fit together neatly like the pieces of a jigsaw puzzle. The new work helps researchers understand the growth of these behemoths by revealing just how their backbones could withstand the enormous stresses imposed by their massive body weight.

Scientists—and curious schoolchildren—have long wondered why sauropods were so big and how they got that way. The larger the body, the harder it is to support, especially when much of its weight is hovering in the air. Over the years, scientists have noted many adaptations that helped sauropod bodies carry their massive weight without injury. For example, the dinosaurs had weight-bearing pillarlike legs, small heads, and bones peppered with air sacs. To further explore how sauropods wore their weight, vertebrate paleontologist John Fronimos—a Ph.D. graduate from the University of Michigan in Ann Arbor—started studying the fossil remains of Spinophorosaurus nigerensis, a sauropod with vertebrae as high as half a meter. When Fronimos was examining the massive fossils while visiting a museum in Spain, he came across something unusual: deep, zigzagging lines in the vertebrae where the top half of the backbone contacts the bottom half.

In mammals and dinosaurs, each vertebra is made of two bones that slowly fuse over time: the main, often cylindrical body of the vertebra called the centrum and the arc of bone that houses the spinal cord, called the neural arch. The two bones wrap around the spinal cord like a padlock, touching once they come together. Before that happens, bone growth takes place between the two pieces, where soft cartilage generates new cells. Exactly when fusion happens depends on the animal. Human vertebrae fuse into solid units by age 6 or 7. But in the giant sauropods, fusion didn’t finish until they were well into their 20s, when they were almost fully grown.

This presented a pretty big problem, Fronimos says. That’s because the spots that allowed the bones to grow were also weak, subject to both sudden injury and the long-term stress that came from supporting such a hefty load. Enter the zigzagging lines: Instead of two flat surfaces where the bones came together, as in humans, these sauropods had grooved surfaces that fit together almost like puzzle pieces. Mathew Wedel, a vertebrate paleontologist at the Western University of Health Sciences in Pomona, California, likens the bones to tennis shoes. “They basically went from having slick soles to having treads.”

Aside from providing better grip to the two bone parts, so one half was not pulled off the other, the treads “increase the surface area of contact between the bones,” Fronimos says. “This means that the same amount of force is distributed over a larger area, reducing the stress at any one point.” That helped give sauropod spines the strength they needed to withstand their owners’ titanic weights, Fronimos and his co-author, Jeffrey Wilson, a vertebrate paleontologist at the University of Michigan, report this month in Ameghiniana .

Some S. nigerensis sutures are even more complex or zigzagging than others. This, Fronimos explains, suggests different backbones had to deal with relatively more or less stress. For instance, the most complex sutures are in bones behind the shoulders, whereas the sutures closer to the head are less complex. “As you move down the neck, each individual bone has to support more and more weight because there’s more and more neck out ahead of them that it has to support,” he says. “So, there’s going to be more stress applied to that bone until you get to the shoulders, which are bearing the whole weight of the neck.” For Wedel, who was not involved in the work, this pattern of complexity “makes good mechanical sense.”

“I love this paper because it has explanatory power,” Wedel says. It also reveals how a process that’s not an issue during human growth “turns out to be a problem when you weigh 30 tons,” he says.

The next step, says Fronimos, is to look at the backbones of other dinosaurs, to see whether they, too, have the telltale teeth. Sauropods might have been the largest dinosaur lineage, but even smaller species like Tyrannosaurus rex could have weighed up to about 9 metric tons. If they do sport similar treads, Fronimos says, as with the sauropods, it will help paleontologists make “more precise interpretations about how these animals lived and what kinds of stresses their bodies were subjected to.”