Few creatures have pushed anatomy
to its limits like sauropods. These supersized dinosaurs moved on pillar-like
limbs that supported massive girth, wielded whip-like tails to ward off
predators, and used long necks to vacuum up foliage.
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While this entire group of dinosaurs is
commonly referred to as “long necks”, Mamenchisaurus, which roved around what
is now China during the late Jurassic period, would have given other sauropods
neck envy. In a study published this month in the Journal of Systematic
Paleontology, researchers estimate that Mamenchisaurus’ neck stretched to a
length of nearly 15 meters. Longer than the average school bus, its neck is the
longest estimated of any sauropod species. It may be the longest neck on an
animal ever observed.
In 1987, paleontologists discovered the
partial skeleton of a sauropod poking out of the rusty red sandstone of the
dinosaur-rich Shishugou Formation in northwest China. The remains were fragmentary,
consisting mostly of a lower jaw, bits of skull, and a couple vertebrae, but
they hinted at an enormous animal that thundered across marshy plains 162
million years ago alongside primitive tyrannosaurs.
Researchers named the dinosaur
Mamenchisaurus sinocanadorum and connected it to several other long-necked
sauropods from East Asia. But Mamenchisaurus’ true size remained an enigma. No
other fossilized remains of the sauropod have been excavated, leaving scientists
with only those couple vertebrae to examine.
“What’s particularly tantalizing and frustrating is that oftentimes, the longest necks belong to the things that are the least known in the fossil record for the simple reason that it’s really hard to bury something that large.”
Andrew Moore, a paleontologist at Stony
Brook University who studies sauropod anatomy, said that this was the case for
many of the largest dinosaurs. “What’s particularly tantalizing and frustrating
is that oftentimes, the longest necks belong to the things that are the least
known in the fossil record for the simple reason that it’s really hard to bury
something that large,” Moore, who led the new study, said.
Clues from a buried relativeSo he turned to the fossils of several
close relatives of Mamenchisaurus, especially Xinjiangtitan, a slightly older
sauropod discovered in northwest China in 2013. Remarkably, researchers
unearthed Xinjiangtitan’s entire vertebral column. At nearly 13m long, it represents
the longest complete neck in the fossil record.
“By using these more complete, but
smaller specimens, we can scale up and make a pretty competent estimate of what
Mamenchisaurus would have looked like,” Moore said.
After comparing Mamenchisaurus and
Xinjiangtitan, Moore and his team concluded that Mamenchisaurus possessed a
neck nearly 15m long. This would account for roughly half of its estimated
total body length and is equivalent to just over eight giraffe necks stacked
end-to-end.
To determine how Mamenchisaurus managed a
neck as long as a semitrailer, Moore and colleagues used a CT scanner to
analyze the animal’s vertebrae. Instead of being stuffed with heavy marrow and
tissue while the dinosaur was living, the interior of the sauropod’s vertebrae
were filled with large air pockets similar to those found in modern birds like
storks and swans. These empty pockets accounted for up to 77 percent of each
bone’s volume, vastly decreasing the weight of Mamenchisaurus’ spine.
“Having such a long neck is a large weight that you have to position away from your body. If you have to hold a hammer with your arm stretched out, your arm’s going to get tired pretty quick.”
Cary Woodruff, a paleontologist at the
Frost Science Museum in Miami who specializes in studying sauropods, said that
lightening the neck’s load was essential for all sauropods. “Having such a long
neck is a large weight that you have to position away from your body,” said Woodruff,
who was not involved in the new study. “If you have to hold a hammer with your
arm stretched out, your arm’s going to get tired pretty quick.”
Neck reinforcementEven though its vertebrae were hollow,
Mamenchisaurus’ neck was far from frail. During the initial excavation,
paleontologists uncovered a fossilized rod of bony tissue several meters long.
It may have been a stiff extension of the vertebra, often called cervical ribs,
that would have run the length of the neck, bolstering its lightweight bones
like a brace. While this reduced the flexibility of its neck, these ribs helped
keep the sprawling structure stable.
“Even though it had a lot of bones, it
wasn’t like a snake where it could curl up back on itself,” Woodruff said. “It
was basically like a rod.”
With its reinforced spine, Mamenchisaurus
most likely kept its neck horizontal at a relatively shallow angle above the
ground. But because of how long its neck was, it could still pick leaves off
the tops of many trees. This may have helped the sauropod squeeze into a unique
niche in an ecosystem that was probably crowded with other gigantic 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 continue to push them as we make more and more discoveries,” Moore
said. “Our default should always be to assume that there’s something larger out
there.”
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