Giant Dinosaurs Got a Head Start on Growth

Growth spurt. By studying fossilized embryonic femur bones at different stages of development, scientists can learn how Lufengosaurus grew up to be a giant.

(top left) D. Mazierski; (center) Debivort/Creative Commons; (right) D. Mazierski and D. Scott From Photos by A. Le Blanc

Nearly 200 million years ago, some of the earliest dinosaurs on Earth laid their eggs in modern Yunnan Province in southern China, only to have one nest after another destroyed by floods. Today, the remains of those lost eggs—and the embryonic dinosaurs that they contained—are helping scientists understand how their relatives grew up to be giants.

The destroyed nests probably belonged to Lufengosaurus, a long-necked, plant-eating dinosaur that lived in the region during the Jurassic period. Although Lufengosaurus and its relatives, called sauropodomorphs, have long been considered to be "those boring dinosaurs," says paleontologist and lead author Robert Reisz of the University of Toronto, Mississauga, in Canada, they do have one trait that makes them stand out—their size: "They were [always] the biggest things that lived in the neighborhood." Lufengosaurus, for example, grew to be about 9 meters long—the largest creature in the region at the time. But because fossils are literally set in stone, paleontologists have had few clues about how these animals grew to such gigantic proportions.

At first glance, the newly discovered bone bed doesn't look like much help. Although it offers some of the oldest embryonic dinosaur bones ever discovered, there are no complete skeletons to be found here; the floods that washed away the nests ripped apart their fragile, unhatched inhabitants and left behind only a few eggshell fragments and a chaotic jumble of hundreds of tiny bones. But it's precisely this mixture of specimens from many different nests—and, therefore, multiple stages of development—that has paleontologists so excited.

"When you get a beautiful little embryo inside an egg, it's gorgeous, … but it's only a glimpse, sort of like a frozen moment in the embryonic life of the animal," Reisz says. "Here, because we have limb bones at various different stages of development, we can actually follow the embryonic life of the organism."

To reconstruct Lufengosaurus's development, Reisz's team focused on the 24 femur bones found in the remains. The first thing the researchers noticed is that the largest of these leg bones were nearly twice the size of the smallest ones, revealing that the creatures grew significantly before they even hatched. And when researchers cut open the femurs to study their structure, they noticed that the spaces in the bones where blood vessels and other tissues would have grown were particularly large. Scientists know that the larger such so-called vascular spaces are, the faster the animal is growing. Judging from the size of their femurs' vascular spaces, Reisz and colleagues concluded that the Lufengosaurus embryos grew faster than all other known dinosaurs and all living birds—"faster than anything we have ever seen," Reisz says. This rapid embryonic growth may be the key to understanding adult sauropodomorphs' towering physiques, the team reports online today in Nature.

But the real surprise that the bones contained didn't reveal itself until the researchers subjected them to the powerful x-rays produced by a synchrotron in Taiwan. When they did, they noticed traces of what they suspect is organic matter inside the bones. "Organic remains [of dinosaurs] have been found before, but this is by far the oldest," Reisz says. If the bones do, in fact, contain complex proteins, he hopes to compare them with proteins in living organisms to learn more about the biology of dinosaurs. Such a possibility "really opens up a new avenue of research for paleontology," he says.

Other researchers are more cautious. "Almost every example of such organic material is hotly disputed," and this one will likely be no different, points out Hans-Dieter Sues, a vertebrate paleontologist at the Smithsonian Institution National Museum of Natural History in Washington, D.C., who was not involved in the study. "You can never really totally rule out contamination."

Still, if the evidence holds up, the find could finally tip the scale in favor of soft tissue preservation. "This [study] lends support to the idea that that some of these organic molecules … may actually be preserved over millions of years," says Luis Chiappe, a vertebrate paleontologist at the Natural History Museum of Los Angeles County in California who was not involved with the research. Regardless, he says, the bone bed itself is "a spectacular find" that's likely to yield many more insights into dinosaur development for years to come.

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