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Rare Fossil Embryos Reveal Dinosaur Growth

Stefan Lovgren
for National Geographic News
July 28, 2005
 
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The oldest terrestrial dinosaur embryos ever discovered reveal a strange-looking baby herbivore that was born on four legs, not two, as previously thought.

The embryos, dating back 190 million years, are of Massospondylus carinatus dinosaurs. Related to the giant sauropods (long-necked, plant-eating dinosaurs), the animals grew to be 16 feet (5 meters) long and were the most common dinosaur in South Africa, where the embryos were found.

"As the animal grew from embryo to adult it went from an awkward-looking, big-headed quadruped to a small-headed, long-necked adult that was quite comfortable running around on its hind legs," said Robert Reisz, a vertebrate paleontologist at the University of Toronto at Mississauga, Canada.

The discovery is unique because it is the first time that scientists have been able to chart a dinosaur's growth from embryo to adulthood.

The findings, which are published tomorrow in the journal Science, may provide clues about how the giant sauropod dinosaurs evolved. The research was supported in part by the National Geographic Society's Committee for Research and Exploration.

Complete Skeleton

The embryos were found during a road construction project in South Africa in 1978. But researchers have not attempted to expose them from the fossil eggshells and surrounding rock until now.

"In general, dinosaur eggs are relatively rare, but embryos are even rarer," Reisz said. "This [find] is unique not only because it is the oldest preserved embryo but because the preservation is fantastic."

Reisz's research assistant, Diane Scott, worked on the delicate, 2.4-inch-long (6-centimeter-long) eggs for more than a year. She uncovered two embryos, including a full, 6-inch-long (15-centimeter-long) skeleton curled up inside its egg.

At first, the scientists were not able to identify the animal. But as the work continued they recognized it as a Massospondylus. The species belongs to a group of plant-eaters known as prosauropods, a smaller relative of the giant sauropods.

Since scientists already have numerous juvenile and adult skeletons of the Massospondylus, the newly uncovered embryos enabled researchers to study how these dinosaurs grew from embryos into adults.

"We were able to actually plot various elements of skeleton onto a chart and figure out how particular parts of skeletons grew relative to other parts," Reisz said.

Beanstalk

That growth pattern turned out to be highly unusual. The hatchling had a huge head and forelimbs as long as its hind legs. As the animal grew, its neck stretched dramatically, while its head got increasingly smaller relative to its body. Its hind legs grew more than twice as long as its forelimbs.

"This kind of change hasn't been shown in any other dinosaur to my knowledge," said James Clark, a dinosaur expert at George Washington University in Washington, D.C.

An adult Massospondylus had a head that was only 8 inches (20 centimeters) long. Its upper limbs were only half the size of its thighbones. It grew to be about 16 feet (5 meters) long, with a beanstalk-like neck and an 8-foot (2.4-meter) tail.

The earliest sauropods may have also developed with quadrupedal proportions, like their Massospondylus cousins. But these early sauropods retained their four-footed stance into adulthood.

The growth pattern of the Massospondylus could therefore provide clues about how the giant sauropods evolved.

"These animals are essentially predecessors to those large sauropods," Reisz said.

Parental Care

The scientists also found that both skulls in the embryos completely lacked teeth.

"Put together with the large head and generally awkward proportions of the body, this raises the possibility that these animals were not able to move around efficiently after they were hatched and may have required parental care in the form of [food] regurgitation from the mother," Reisz said.

If this interpretation is correct, it would be the oldest known indication of parental care in the dinosaur fossil record.

"This is clearly speculative," Reisz said. "We would like to go back and examine this in greater detail. There is a lot more to be done on this project."

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