This dinosaur’s 50-foot-long neck didn’t have a stretch

Few creatures pushed anatomy to its limits like the sauropods. These massive dinosaurs moved on pole-like limbs that supported an enormous girth, gripped whip-like tails to ward off predators and used long necks to suck up leaves.

While this whole group of dinosaurs is commonly referred to as “long-necked,” Mamenchisaurus, which prowled around what is now China during the late Jurassic period, would have envied other sauropod necks. In a study published Wednesday in Journal of Systematic PaleontologyResearchers estimate that Mamenchisaurus’ neck extended nearly 50 feet. Longer than the average school bus, its neck is the estimated longest of any sauropod. It may be the longest animal neck ever observed.

In 1987, paleontologists discovered the partial skeleton of a sauropod sticking out of the rusty red sandstone of the dinosaur-rich Shishogu Formation in northwest China. The remains were fragmentary, consisting mostly of a lower jaw, pieces of skull and two vertebrae, but they hinted at a massive animal that threatened across the swampy plains 162 million years ago alongside primitive dinosaurs.

The researchers named the dinosaur Mamenchisaurus sinocanadorum and related it to several other long-necked sauropods from East Asia. But Mamenchisaurus’ true size has remained a mystery. No other fossilized remains of sauropods have been excavated, leaving scientists with only those pair of vertebrae to examine.

This was the case for many of the largest dinosaurs, said Andrew Moore, a Stony Brook University paleontologist who studies sauropod anatomy. Moore, who led the new study, said.

So he turned to the fossils of several close relatives of Mamenchisaurus, especially Xinjiangtitan, a slightly older sauropod discovered in northwestern China in 2013. Remarkably, the researchers unearthed the entire backbone of the Xinjiang Titan. At about 44 feet long, it represents the longest complete neck in the fossil record.

“By using these more complete, but smaller specimens, we can expand and provide a very competent estimate of what Mamenchisaurus would have looked like,” said Dr. Moore.

After comparing Mamenchisaurus and Xinjiangtitan, Dr. Moore and his team concluded that Mamenchisaurus possessed a neck about 50 feet long. This would account for nearly half of the estimated total body length and equate to just over eight giraffe necks stacked end-to-end.

To determine how Mamenchisaurus managed its neck as long as it was a truck semi-trailer, Dr. Moore and his colleagues used a computed tomography (CT) machine to analyze the animal’s vertebrae. Instead of being stuffed with heavy marrow and tissue while the dinosaur was alive, the interior of the sauropod’s vertebrae was filled with large air pockets similar to those of modern birds like storks and pelicans. These empty pockets accounted for up to 77 percent of the volume of each bone, which greatly reduced the weight of Mamenchisaurus’ spine.

Neck relief is essential for all sauropods, said Cary Woodruff, a paleontologist at the Frost Museum of Science in Miami who specializes in the study of sauropods. “Having such a long neck is a lot of weight to have to put away from your body,” said Dr. Woodruff, who was not involved in the new study. “If you had to hold a hammer with your arm outstretched, your arm would get tired very quickly.”

Although its vertebrae were hollow, Mamenchisaurus’ neck was far from weak. During the initial excavations, paleontologists discovered a fossilized rod of bone tissue several feet long. It may have been a rigid extension of the vertebra, often called the cervical rib, which would have run the length of the neck, supporting its lightweight bones like a brace. While this reduced the flexibility of its neck, these ribs helped keep the sprawling structure stable.

“Although it had a lot of bones, it wasn’t like a snake that could curl itself back in,” said Dr. Woodruff. “It was basically like a rod.”

With its reinforced spine, Mamenchisaurus likely kept its neck horizontal at a relatively shallow angle above the ground. But due to its long neck, it can still pluck leaves from the tops of many trees. This may have helped the sauropods squeeze into a unique niche in an ecosystem that might have been crowded with other giant herbivores.

According to the researchers, several groups of sauropods appear to have evolved extremely long necks, which may have rivaled the crane-like projections of Mamenchisaurus.

“We don’t really know what the limits are, because they keep pushing them as we make more and more discoveries,” said Dr. Moore. “It should always be our default to assume there is something bigger out there.”